• TABLE OF CONTENTS
HIDE
 Title Page
 Copyright
 Dedication
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 Introduction
 Background
 Materials and methods
 Results and discussion
 Conclusion
 Future research
 Appendix
 Reference
 Biographical sketch
 Copyright














Title: Hydrophilic polymer coatings to prevent tissue adhesion
CITATION THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00086024/00001
 Material Information
Title: Hydrophilic polymer coatings to prevent tissue adhesion
Alternate Title: Tissue adhesion
Physical Description: xiv, 172 leaves : ill. ; 28 cm.
Language: English
Creator: Sheets, John Wesley, 1953-
Publication Date: 1983
 Subjects
Subject: Polymers in medicine   ( lcsh )
Biomedical materials   ( lcsh )
Materials Science and Engineering thesis Ph. D
Dissertations, Academic -- Materials Science and Engineering -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis (Ph. D.)--University of Florida, 1983.
Bibliography: Bibliography: leaves 167-171.
Statement of Responsibility: by John Wesley Sheets, Jr.
General Note: Typescript.
General Note: Vita.
 Record Information
Bibliographic ID: UF00086024
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000473800
oclc - 11665750
notis - ACN9009

Table of Contents
    Title Page
        Page i
    Copyright
        Page ii
    Dedication
        Page iii
    Acknowledgement
        Page iv
    Table of Contents
        Page v
        Page vi
    List of Tables
        Page vii
        Page viii
    List of Figures
        Page ix
        Page x
        Page xi
        Page xii
    Abstract
        Page xiii
        Page xiv
    Introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
    Background
        Page 6
        Page 7
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    Materials and methods
        Page 22
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    Results and discussion
        Page 56
        Page 57
        Page 58
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    Conclusion
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    Future research
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    Appendix
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    Reference
        Page 167
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    Biographical sketch
        Page 172
        Page 173
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    Copyright
        Copyright
Full Text










HYDROPHILIC POLYMER COATINGS TO PREVENT TISSUE ADHESION


BY

JOHN WESLEY SHEETS, JR.























A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY



UNIVERSITY OF FLORIDA


1983

























Copyright 1983

by

John Wesley Sheets, Jr.

























To my Mother and Father,

who made it all possible.














ACKNOWLEDGEMENTS


The author would like to acknowledge the following

individuals who were invaluable to him and this research:

Dr. Eugene P. Goldberg, special thanks for his patience and

guidance as my mentor; Drs. Moshe Levy and Shimon Reich, for

their paternal and scholarly assistance; Dr. Moshe Yalon,

for his friendship and rare talents; the faculty and staff,

especially Drs. Stanley R. Bates, Christopher D. Batich,

Charles L. Beatty and John J. Hren, for showing how much fun

science can be; Dr. Larry L. Hench and June Wilson, for

their encouragement and advice; and Drs. Mutaz B. Habal,

Randall Olson, Jeffery Katz and Herbert E. Kaufman, for

their collaboration in the preliminary research.

Also, the author wishes to express his sincere

appreciation of his friends: Thomas C. Saitta (counselor),

William E. Longo (confidant), Walter J. McCracken

(compatriot), Cindy L. Flenniken (colleague), Alan D. Walsh

(crony) and Ronald A. Palmer (coach).

The author is also indebted to the National Institutes

of Health-Eye-Institute and Intermedics Intraocular, Inc.

for grants which provided partial financial support for this

study.














TABLE OF CONTENTS


ACKNOWLEGEMENTS . . . . . . . .

LIST OF TABLES . . . . . . . .


LIST OF FIGURES . . . . . . . . .

ABSTRACT . . . . . . . . . .

1. INTRODUCTION . . . . . . . . .

2. BACKGROUND . . . . . . . . .

2.1 Intraocular Lens Development and Problems .
2.2 Biological Adhesion . . . . .
2.3 Radiation Polymerized Graft Coatings . .
2.4 Plasma Polymerized Graft Coatings . . .

3. MATERIALS AND METHODS . . . . . . .

3.1 Preliminary Studies . . . . ..


Page

" iv

. vii


S. .ix

. .xiii


3.1.1 Prevention of Cornea Endothelium Damage
by Poly(vinyl pyrrolidone) Solutions
3.1.2 Peritoneal Adhesions in Abdominal
Surgery . . . . . . .


3.2 PMMA Substrates . . . ..
3.3 Purification of Monomer (N-VP) .
3.4 y-Radiation Graft Coatings . . .
3.5 Intrinsic Viscosity Molecular Weight of
3.6 RF Plasma Coatings . . . . .
3.7 Contact Angle for Water . . . .
3.8 Critical Surface Tension . . .
3.9 Scanning Electron Microscopy . . .
3.10 ESCA (Electron Scattering for Chemical
Analysis) . . . . . .
3.11 Infrared Spectroscopy . . ....
3.12 Ultraviolet-Visible Spectroscopy . .
3.13 PVP-Iodine Interaction . . . .
3.14 Biophysical Measurements . . . .


. 27
28
. . 29
PVP . 34
. . 36
. 38
S. . 39
41


. . 42
.. . 42
. . 43
. . 43
. . 44


. .

. .









Page


3.14.1 Instrument for Biophysical Measure-
ments . . . . . . . .
3.14.2 Preparation of Material Sample for
Measurement . . . . . .
3.14.3 Preparation of Tissue Samples . . .
3.14.4 Adhesive Force Measurement . . .
3.14.5 Quantitative Cornea Endothelium
Damage by SEM . . . . . .

4. RESULTS AND DISCUSSION . . . . . .

4.1 Preliminary Studies and Overview of Thesis
Research . . . . . . . . ..

4.1.1 Prevention of Cornea Endothelium Damage
by Poly(vinyl pyrrolidone) Solutions
4.1.2 Peritoneal Adhesions in Abdominal
Surgery . . . . . . . .
4.1.3 Other Related Research . . . . .
4.1.4 Overview of Thesis Research . . .


4.2 Intrinsic Viscosity Molecular Weight of PVP
4.3 Contact Angle for Water . . . .
4.4 Critical Surface Tension . . . . .
4.5 Scanning Electron Microscopy . . . .
4.6 ESCA . . . . . .
4.7 Infrared Spectroscopy . . . . . .
4.8 Ultraviolet-Visible Spectroscopy . . .
4.9 PVP-Iodine Interaction . . . . .
4.10 Biophysical Measurements . . . . .
4.11 Structure and Properties of y-Radiation
Graft Coatings . . . . . . .

5. CONCLUSIONS . . . . . . . . .

6. FUTURE RESEARCH . . . . . .

6.1 y-Radiation Graft Coatings for IOL Use .
6.2 RF Plasma Coatings . .. . .
6.3 Further Characterization of Graft Coatings

APPENDIX . . .

REFERENCES . . . . . . .

BIOGRAPHICAL SKETCH . . . . . . .


. 65
S 68
. 77
. 94
. 120
. 122
. 129
. 134
. 136

S. 153

. 158

S. 162

S. 162
. 163
. 163

. 164

. 167

. 172













LIST OF TABLES


Table Page

1. Dog Model for Assessment of Polymer Solutions
to Prevent Adhesions . . . . . .. 58

2. Rat Model-Assessment of PVP Solution to Prevent
Adhesions . . . . . . . . . 58

3. Viscosity Molecular Weight* of PVP. External
Polymer Formed in Solution During y-Radiation
Graft Coating. .. . ...... . . . . 66

4. Contact Angle of Water on y-Radiation Graft
Coating of PVP on PMMA . . . . . . 69

5. Contact Angle of Water for RF Plasma Coatings
on PMMA . . . . . . . . . . 70

6. Contact Angle of Water for Various Biomedical
Polymers . .. ...... . . . . .. 71

7. Critical Surface Tension from Zisman Plots*
for y-Radiation Graft Coatings of PVP on PMMA 85

8. Summary of Zisman Plots for Critical Surface
Tension of RF Plasma Coatings on PMMA . . 93

9. ESCA Results ........ . . . . . 121

10. Summary of Adhesive Force Measurements for
Cornea Endothelium Contact with y-Radiation
Graft Coatings of PVP on PMMA . . .. .. 139

11. Quantitative Cornea Endothelium Damage by SEM
for Contact with y-Radiation Graft Coatings
of PVP on PMMA . . . . . .. . .. 145

12. Summary of Adhesive Force Measurements for
Cornea Endothelium Contact with RF Plasma
Coating on PMMA . . . .. . ..... 148

13. Quantitative Cornea Endothelium Damage by SEM
for RF Plasma Coatings on PMMA . . . .. 149


vi.i:








Table Page


14. Summary of Adhesive Force Measurements for
Cornea Endothelium Contact with Various
Biomedical Polymers . . . . . . ... .151

15. Quantitative Cornea Endothelium Damage by SEM
for Various Biomedical Polymers . . . . 152

16. Summary of Measurements for y-Radiation
Graft Coatings of PVP on PMMA . . . ... 156


viii













LIST OF FIGURES


Figure Page

1. Cross Section of Eye and Cornea: (A) Lense,
(B) Anterior Chamber, (C) Posterior Chamber and
(D) Endothelium . . . . .. ... . 7

2. Scanning Electron Micrograph of Cornea Endothel-
ium (X900) . . . . . . . . 9

3. Scanning Electron Micrograph of Cornea
Endothelium Damaged by Contact to Acrylic Intra-
ocular Lens (X2000) . . . . . . . 9

4. Predominant Reaction Scheme for y-Radiation
Graft Coatings of PVP on PMMA . . . . . 15

5. y-Radiation Effects in Solution . . . . 16

6. Plasma Polymerization Processes . . . . 19

7. Infrared Spectrum of N-VP Monomer (As Distilled). 30

8. Infrared Spectrum of N-VP Monomer (Literature,
Reference 53) . . . . . . .... 31

9. Schematic Drawing of Co-60 Source, Department
of Radiation Biology, University of Florida . 33

10. Schematic Drawing of RF Plasma Apparatus . . 37

11. Tissue-Polymer Adhesion Measurement Instrument 45

12. Acrylic Sample in Holder . . . . . . 46

13. Plexiglass Tank (Filled with Saline) with
Tissue Sample ...... . . . . ..... 48

14. Loading of Acrylic-Endothelium Interface . . 51

15. Deflection of Glass Fiber due to Acrylic-
Endothelium Adhesion with Lowering of Micrometer
Stage . . . . . . . . ... . . 52







Figure Page

16. External Polymer Viscosity Molecular Weight vs.
Monomer Concentration for y-Radiation Graft
Coatings of PVP on PMMA .. . . . .... .67

17. Contact Angle for Water vs. % Monomer for
0.1 Mrad Dose y-Radiation Graft Coatings of PVP
and PMMA . . . . . . . .... . .73

18. Contact Angle for Water vs % Monomer for
S0.25 Mrad Dose y-Radiation Graft Coatings of PVP
S and PMMA . . . . . . . . ... 74

19. Contact Angle for Water vs. % Monomer for
0.5 Mrad Dose y-Radiation Graft Coatings of PVP
and PMMA . . . . . . . .. .. 75

20. Summary of Contact Angle for Water vs. % Monomer
for Various Dose Levels of y-Radiation Graft
Coatings of PVP on PMMA . . . . .. . 76

21. Zisman Plot of Critical Surface Tension (y )
of PMMA . . . .. .. . . . . . 79

22. Zisman Plot of Critical Surface Tension (yC)
of y-Irradiated 0.25 Mrad' PMMA . . ... 80

23. Zisman Plot for Critical Surface Tension (y )
of y-Radiation Graft Coating: 5% N-VP Initial
Monomer Concentration and 0.25 Mrad Dose . . 81

24. Zisman Plot for Critical Surface Tension (y )
of y-Radiation Graft Coating:. 10% N-VP Initial
Monomer Concentration and 0.25 Mrad Dose . . 82

25. Zisman Plot for Critical.Surface Tension (y )
of y-Radiation Graft Coating: 20% N-VP Initial
Monomer Concentration and 0.25 Mrad Dose . . 83

26. Zisman Plot for Critical Surface Tension (y )
of y-Radiation Graft Coating: 30% N-VP IniEial
Monomer Concentration and 0.25 Mrad Dose . .. 84

27. Zisman Plots for Critical Surface Tensions (y )
of 0.25 Mrad y-Radiation Graft Coatings of
Different N-VP Concentrations . . . . . 86

28. Summary of Zisman Plots for Critical Surface
Tensions (y ) of PVP on PMMA vs. % N-VP for
y-Radiation Graft Coatings of 0.25 Mrad Dose 87







Figure


29. Zisman Plot for Critical Surface Tension (y )
of RF Plasma Coating of N-VP on PMMA:
50 Watts(Power)/1000p(Pressure)/10 Minutes
(Duration) . . . . . . . . .

30. Zisman Plot for Critical Surface Tension (y )
of RF Plasma Coating of N-VP on PMMA:
100 Watts(Power)/200p (Pressure)/20 Minutes
(Duration) . . . . . . . . .

31. Zisman Plot for Critical Surface Tension (y c
of RF Plasma Coating of N-VP on PMMA:
35 Watts(Power)/500p(Pressure)/60 Minutes
(Duration) . . . . . . . . .

32. Zisman Plot for Critical Surface Tension (y )
of RF Plasma Coating of HEMA on PMMA:
25 Watts(Power)/500i (Pressure)/15 Minutes
(Duration) . . . . . . .

33. Scanning Electron Micrographs of Scraped
y-Radiation Graft Coatings of PVP on PMMA .

34. Scanning Electron Micrographs of Scraped
RF Plasma Coatings on PMMA . . . . .


Page





89


. 90




. 91


. 114


35. ESCA Spectra for PMMA (Uncoated) (B.E. = Bonding
Energy; N/C = Nitrogen to Carbon Atomic Ratio) 123

36. ESCA Spectra for y-Radiation Graft Coating of
PVP on PMMA: 10% N-VP Initial Monomer Concen-
tration and 0.25 Mrad Dose (B.E. = Binding Energy;
N/C = Nitrogen to Carbon Atomic Ratio) . . 124

37. ESCA Spectra for y-Radiation Graft Coating of
PVP on PMMA: 10% N-VP Initial Monomer Concen-
tration and 0.5 Mrad Dose (B.E. = Binding Energy;
N/C = Nitrogen to Carbon Atomic Ratio) . . 125

38. Infrared Spectra for PMMA and PVP (Literature,
Reference 62) . . . . . . . . . 127

39. Attenuated Total Response-Infrared Spectra of
PMMA, y-Radiation Graft Coating of PVP on PMMA
and PVP Cast Film on PMMA . . . . . . 128

40. Ultraviolet-Visible Transmission Spectra of y-
Radiation Graft Coatings of PVP on PMMA, 0.1 Mrad
Dose level . . .. . . . . . . 130






Figure Page


41. Ultraviolet-Visible Transmission Spectra of
y-Radiation Graft Coatings of PVP on PMMA,
0.25 Mrad Dose Level . . . . . ... 131

42. Ultraviolet-Visible Transmission Spectra of
y-Radiation Graft Coatings of PVP on PMMA,
0.5 Mrad Dose . . ... . . .... 132

43, Ultraviolet-Visible Transmission Spectra of
y-Irradiated PMMA . .. . . . . 133

44. Ultraviolet-Visible Transmission Spectra of
RF Plasma Coatings on PMMA . . . . . 135

45. Adhesive Force vs. Monomer Concentration for
PVP y-Radiation Graft Coatings on PMMA ..... 141

46. Representative Scanning Electron Micrographs
of Cornea Endothelium Damage . . . . . 142

47. Quantitative Cornea Endothelium Damage by SEM
vs. N-VP Monomer Concentration for 0.25 Mrad
Dose y-Radiation Graft Coatings on PMMA . . 147

48. Schematic View of PVP y-Radiation Graft on
PMMA . . . . . . . . . . . 155


xii














Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

HYDROPHILICC POLYMER COATINGS TO PREVENT TISSUE ADHESION

By

John Wesley Sheets, Jr.

April 1983

Chairman: Eugene P. Goldberg
Major Department: Materials Science and Engineering

Adhesion following contact between plastic surgical

materials and endothelium surfaces has been shown to result

in extensive and critical tissue damage in intraocular lens

(IOL) insertion and abdominal surgery. Coatings of

hydrophilic polymer solutions were shown effective in

reducing such damage by acting as lubricants and barriers to

adhesion. The primary emphasis of this study was on the

preparation, characterization and evaluation of permanent

graft coatings of hydrophilic polymers for the prevention of

IOL adhesion to the cornea endothelium because of the

importance of this adhesion problem in IOL surgery.

Gamma-radiation and radio-frequency (RF) plasma

polymerized graft coatings of the hydrophilic polymer,

poly(vinyl pyrrolidone) (PVP) were prepared on the IOL

material, poly(methyl methacrylate) (PMMA). The chemical

and physical properties of these coatings were characterized


xiii








by several techniques, including measurement of the contact

angle for water, electron scattering for chemical analysis

(ESCA), infrared spectroscopy (IR), ultraviolet-visible

spectroscopy (UV-Vis), examination by scanning electron

microscopy (SEM) and measurement of the surface energy.

In addition, a new instrument and technique was

developed for the evaluation of the force of adhesion and

the extent of resultant tissue damage of these coatings and

other plastic materials. These measurements were made for

contact with rabbit cornea endothelium under well controlled

conditions.

Thin graft coatings of PVP on PMMA were found to be

adherent to the acrylic substrate and significantly

increased the hydrophilicity and surface energy as compared

with uncoated PMMA. The altered surface properties of the

PVP graft coated PMMA also resulted in reduction of the

adhesive force and extent of tissue damage, as tested by the

newly devised adhesive force instrument.

Results of this study emphasize the need for develop-

ment of hydrophilic polymer graft coatings for clinical IOL

use. In vivo animal testing of y-radiation graft coatings

of PVP on PMMA are now in progress. The application of the

hydrophilic polmer coating concepts for other surgical

materials and procedures is also under investigation.


xiv












* .


1. INTRODUCTION


SThe use of synthetic polymers for prosthetic and

cosmetic implants and for surgical devices has increased

rapidly over the past three decades. The wide range of

advantageous chemical and physical properties possessed by

polymers has shown them to be uniquely suitable for many

medical applications. Some of the most demanding

requirements to be met by polymers are inertness and

biocompatibility in contact with tissues of the body.

Research on the biocompatibility of polymers (and other

materials) has been focused on short and long term toxicity,

tissue acceptance and thrombogenicity. However, very short

term, temporary and transient contacts between materials and

tissue have not been carefully investigated to date. This

study has therefore emphasized the effects of momentary

contacts between polymer and tissue surfaces, and tissue

damage resulting from such contacts.

This area of research was initiated through an

investigation of post-operative complications following

intraocular lens (IOL) insertion. An IOL is a poly(methyl

methacrylate) lens used to replace the natural crystalline

lense following cataract surgery.







Our preliminary studies showed that contact between an

IOL and the corneal endothelium could result in adhesion of

endothelium cells to the acrylic polymer surface.

Manipulation caused tearing of these adherent cells away

from the tissue surface. Loss of endothelium cells poses a

critical problem since these cells are a non-regenerative

mon6layer and serve an essential function in maintaining

fluid balance and clarity of the cornea. Prevention of this

tissue damage is therefore essential to the success of

ophthalmic IOL implantation surgery.

The major focus of this research has therefore been on

acrylic IOL-cornea endothelium surface interactions and

prevention of tissue damage using hydrophilic polymer

coatings. However, the findings in ophthalmic surgery have

also been extended to other surgical procedures. For

example, the possibility of similar types of tissue damage

in abdominal surgery which results in peritoneal adhesions

was questioned. Manipulations within the peritoneal cavity

were shown to result in tissue adherence to latex rubber

gloves used in surgery. Normal healing of the damaged

tissue produces adhesions between adjacent, highly

regenerative tissue surfaces, with the potential for severe

post-operative complications. Other areas of tissue

adhesion to polymer surfaces with resulting damage were

found in vascular catherization and endotrachea tube

intubation, resulting in desquamation or "stripping" of the








endothelium cells. The resulting tissue damage may lead to

infections following catherization.

The adhesion phenomenon we have discovered appears to

occur extensively in all types of surgery and poses a

significant but yet unappreciated problem for instrument or

device materials in surgery. The results of this study

suggest that modification of the surfaces of biomedical

devices with hydrophilic polymer coatings can reduce or

prevent adhesive tissue damage for IOLs, catheters and

surgical gloves.

The objectives of this research have been to

investigate the adhesion phenomenon, and its prevention, and

especially to develop practical and clinically viable

permanent hydrophilic coatings for safer IOLs. The critical

nature of damage to the corneal endothelium resulting from

adhesion to an IOL and the large number of IOL insertions

currently performed (over 200,000 annually) underscore the

importance of research to modify the IOL surface.

Both temporary and permanent modification with

hydrophilic polymer coatings have been studied. Although

transient soluble polymer coatings were effective in

reducing tissue damage, we chose to emphasize permanent

hydrophilic graft coatings which would be inherently safer

and easier to use clinically. Gamma-radiation graft

coatings of poly(vinyl pyrrolidone) (PVP) on poly(methyl

methacrylate) (PMMA) were studied as well as hydrophilic

polymer coatings produced by radio-frequency (RF) plasma








graft polymerization. Both methods produced cross-linked,

covalently bound graft coatings on the PMMA substrate.

Polymerization conditions were investigated in detail for

y-radiation graft coating.

The chemical and physical nature of these coatings were

characterized. Contact angle measurements were used to

assess the relative hydrophilicity of the surface. Scanning

electron microscopy (SEM) was used to determine surface

morphology. ATR infrared spectroscopy and electron

scattering for chemical analysis (ESCA) were employed to

chemically analyze the surface composition.

A new instrument and technique devised by Dr. S. Reich

was built for the quantitative determination, for the first

time, of the polymer-tissue adhesive force. This method

used the rabbit cornea endothelium as a model endothelium

surface for quantitative measurement of the adhesive force

at the polymer-tissue interface under controlled conditions

of contact. This instrument was also used in conjunction

with SEM to assess the cell damage resulting from polymer

contacts. Adhesive force was correlated with cell damage

and with the chemical and physical nature of the coatings to

understand the adhesion phenomenon and to determine the

coating characteristics necessary to minimize tissue damage.

These studies have shown that the adhesion phenomenon

has broad implications in surgery, although the mechanism is

not fully understood. Hydrophilic polymer coatings for

surgical materials should prove useful to reduce adhesion




5


and tissue damage which may result in post-operative

complications. This study has opened up an interesting new

area of medical materials science that deserves further

study.














2. BACKGROUND


2.1 Intraocular Lens Development and Problems

A cataract is a clouding of the natural crystalline

lense of the eye which necessitates surgical removal of the

lense (Figure 1-A). The optical function of the natural

lense must be replaced if visual acuity is to be restored.

Three alternatives exist for this purpose: spectacle

lenses, contact lenses or intraocular lenses (IOL). An IOL

is placed within the eye, in either the anterior or

posterior chamber, and serves as the optical replacement for

the natural crystalline lense.

The history of the modern intraocular lense began in

1949, when the British surgeon Dr. Harold Ridley implanted a

lens of poly(methyl methacrylate) (PMMA) in an eye following

cataract surgery (1,2). His use of the plastic PMMA had

been suggested by his experiences during the Second World

War, when he found pieces of shattered PMMA aircraft

canopies lodged in the eyes of fliers. He noted that these

fragments did not cause any reaction in the eye and would be

suitable, because of their optical properties, as a

replacement for the natural lense.

The acceptance and use of PMMA as the material for

intraocular lenses grew from that first implantation.













Epithelium

Bowman's
Membrane




Stroma




Descemet's
Membrane
Endothelium


Opticnerve
Figure 1. Cross Section of Eye and Cornea: (A) Lense, (B) Anterior Chamber, (C) Posterior
Chamber and (D) Endothelium.








Although other materials have been suggested, PMMA is the

material which is currently used for nearly all IOLs.* The

style or shape of lens, its placement in either the anterior

or posterior chamber (Figure 1-B & C), the surgical

technique for insertion and the method of fixation within

the eye have been varied extensively since the first

implantation, but the optical portion of the lens has

remained as originally fabricated from PMMA.

Studies have shown that after IOL insertion that there

was an average loss of almost half of the central corneal

endothelial cells (3,4); these cells form a monolayer on the

posterior surface of the cornea (Figure 1-D). These cells

serve an essential function in maintaining fluid balance

within the cornea. Damage to the endothelium will result in

leakage of fluid into the corneal layers and will cause

swelling and blurred vision. This monolayer is nonregen-

erative in man so that the damage is irreversible.

Our initial studies (5,6) revealed that this cell loss

was due to contact with.the acrylic IOL surface at the time

of the insertion surgery (Figures 2 & 3). Others have

confirmed that such contact causes endothelium cell loss

(7,8) and efforts to prevent the contact and damage were

undertaken.



*The only exceptions are poly(propylene) support loops and a
commercially available IOL made of glass with poly(imide)
supports (Lunell, NYC).


































Figure 2. Scanning Electron Micrograph of Cornea Endothelium.
lYQnni


Scanning Electron Micrograph of Cornea Endothelium
Damaged by Contact to Acrylic Intraocular Lens.
(X2000)







When the adhesive damage phenomenon was first observed,

we showed that aqueous solutions of the hydrophilic polymer

poly(vinyl pyrrolidone) (PVP) were effective in preventing

endothelium damage. Since the first publication of our work

on adhesive endothelium damage in 1977 (5,6), research in

many laboratories has resulted in several methods for

protecting the endothelium during IOL insertion.

Surgical techniques have been developed to minimize the

chance for damaging contacts between IOL and endothelium

surfaces. The most widely-used technique employs an air

bubble (9), which is formed and maintained in the anterior

chamber during insertion surgery to keep the endothelium

from collapsing against the lens being inserted. This

technique has been fairly successful but as with any

surgical technique, it is not fool-proof. Another surgical

technique employed a soft contact lens shield to protect the

endothelium (10).

The other methods developed to protect the endothelium

were adaptations of our initial suggestion to use

hydrophilic polymer coatings. Bovine serum albumin,

chondroitin sulfate, human gamma globulin, hyaluronic acid

and whole plasma are among the different natural polymer

solutions studied as protective agents (11-16). Of these,

the sodium salt of hyaluronic acid has found extensive use,

and is marketed under the trademark, Healon (Pharmacia,

Piscataway, NJ) (14-16). More recently, chondroitin sulfate

solutions have been introduced commercially (Cilco,







Huntington, WV) as an alternative to the high-priced and

hard to obtain Healon. However, a major disadvantage of

these solutions is that the surgeon is obligated to use an

additional procedure, application of a protective coating in

the operating room.

Dry, cast coatings of bovine submaxillary mucin (17)

and'poly(vinyl alcohol) (PVA) (18,19) have also been used;

these coatings hydrate and dissolve in situ. While these

transient and soluble coatings have proven effective for

reducing the adhesion and damage from corneal

endothelium-IOL contact, the protection is only temporary.

Damage from contact following the implant surgery, such as

contact occurring by IOL detachment, is not prevented by any

of the temporary coatings. The shortcomings of temporary

protective coatings demonstrate the need to develop

permanently bound coating which can be applied prior to

surgery and provide a protective, hydrophilic surface for

the life of the implant.



2.2 Biological Adhesion
Tissue-materials interface studies to date have

emphasized long-term polymer biocompatibility for

nonthrombogenic and tissue-compatible implants or prostheses

(20). Short-term toxicology and thrombogenicity as well as

long-term tissue acceptance have been major points of

concern (21-25). In the testing recommended for the

evaluation of materials intended for use in contact with







blood and other tissues, issued by the U. S. Department of

Health and Human Services, extensive and exhaustive

protocols for the testing of toxicity and thrombogenicity

are detailed (26,27), but no testing of cell adhesion to

materials (other than blood cell adhesion) is recognized or

recommended. The adhesion of cells and tissue to materials

has-been relatively ignored.

However, some research has been devoted to the study of

cell adhesion to surfaces and reviews are available (28,29).

This research has been relatively limited dealing only with

cell adhesion pertinent to tissue culture (30), studies of

surgical adhesives and dental restoratives (31-33),

bacterial adhesion to otic implants (34), marine bacteria

and barnacle adhesion (35), and changes in cell surfaces

accompanying tumor cell metastasis (21). Few studies have

attempted to measure the actual strength of cell adhesion to

surfaces (36).

In these studies, tissue cultures were grown on

different substrates and then were subjected to shear

forces. Although a number of methods have been used, the

most extensive studies have used the shearing force

generated by a metal disc immersed in the culture medium and

lowered to a measured distance from the cells and then

rotated at a known velocity (36). The percentage of cells

of the tissue culture detached by this constant shear force

for a set period of time was measured. For a shear force of

7.4 dynes/cm applied for twenty seconds, 27.8% of the







cells from a rat fibroblast culture were detached from a

Pyrex glass substrate (37).

The importance of the forces which may be involved in

cell adhesion to a surface has been reviewed (38). Deemed

most important are chemical (electrostatic, covalent and

hydrogen) bonds and van der Waals' interactions of the

London type (hydrophobic bonds) (39). Of these factors, the

hydrophobic-hydrophilic nature of the surfaces and the

resultant interactions with cells has received considerable

attention but there is no general agreement whether cells

adhere better to hydrophilic or hydrophobic surfaces.

However, in the examples of muscle cell adhesion to hydrogel

graft-coated surfaces (24) and with the strength of surgical

adhesives to liver tissue (32), an increased hydrophilicity,

relative to other surfaces, was found to favor reduced cell

adhesion.



2.3 Radiation Polymerized Graft Coatings

Permanent, covalently bound coatings of PVP on PMMA

were produced in this study by Y-irradiation of the

substrate immersed in aqueous solutions of the PVP monomer,

N-vinyl pyrrolidone (N-VP). Gamma-Radiation produces free

radicals in both the PMMA and the N-VP monomer, creating

graft sites in PMMA and initiating N-VP polymerization.

Free radicals in the bulk PMMA tend to degrade PMMA,

since scission predominates over crosslinking for PMMA.

However, free radicals produced on the surface of the PMMA







serve as initiation sites for graft polymerization of the

N-VP (Figure 4).

Free radicals in the N-VP are created with opening of

the vinyl group double bond and homopolymerization of the

N-VP. The PVP chain will continue to grow until termination

occurs, and may react with the acrylic substrate, producing

the- graft polymer. The Y-radiation may also cause

crosslinking of the PVP, by hydrogen extraction and

branching of the PVP chain (Figure 5).

Graft polymerization by Y-radiation of PVP on PMMA was

first studied by Henglein et al. in 1958 (40). However,

these graft coatings were made using the monomer in methanol

following long periods (up to 69 hours) of allowing

diffusion of monomer into the PMMA substrate (with swelling

of the PMMA). This process, along with the high radiation

doses (up to 4 megarads (Mrad)), resulted in severe

distortion of the PMMA (41). While such extreme conditions

would not be suitable for preserving the optical properties

of an acrylic IOL, some penetration of the N-VP into PMMA

may be of interest in future work.

Use of aqueous solutions of N-VP (0-20%) was shown to

reduce diffusion into the substrate when PVP was grafted to

poly(dimethylsiloxane), using high energy (3 MEV) and 1.5 to

4.5 Mrad dose, from a Van de Graff generator (42). These

coatings were shown to be hydrophilic in porportion to the

amount of graft. The irradiation conditions (high doses and

very high dose rates) produced thick coatings, several








CH3 CH3 CH3 CH3
I .1I I I I
CH2-C-CH2 -C -/CH2--C* + *CH2-C
I I I I
SC=O C=-O .C=O C O
I I I I
O-CH3 O -CH CH C H2 O-CH O-- CH3
(PMMA)
CH2 = O


CHj= CH
(N-VP)


CH2-CH2 CH2-CH2

CH2 C=0 CH2 C- O
CH3 N N CH3
I I I I
-- CH2-C--CH2-CH *CH- CH2-CH2---C
I I
C=O C --=
I I
O-CH3 O -CH3
Figure 4. Predominant Reaction Scheme for y-Radiation Graft Coatings of PVP on PMMA.












(1) HOMOPOLYMERIZATION (2) BRANCHING & CROSSLINKING


(1) CH2- CH2
I I
CH2 C=O
N
I
CH,=CH
(N-VP)


CH2- CH,- CH, CH2,- C
CH2- CH2 CH2- CH12 CH2- CH.

CH2 C= CH2 C=O CH2 C= O
SN// N
y I N-VP I I
,vvvvvv- *CH2-CH- *CH2 CH- CH2- CH-


IPVP Chain I......


(2) C 2-C"2 CH-CH, CH2 -CH2 C2- CH2 CH
CH2 C=0 CH2 c=O CH CH2 C=O CH
\a/ \N/ N/ \ N/
NN N N
SI N-VP PVP I
.-CH-CH-* "-.CH-2-CH-1+H*- C2- '+CH2- C C-CH2- CH CH2-

CH2-CH CH2
N-CH
CH2-C
0


Figure 5. y-Radiation Effects in Solution.







mils thick, but low molecular weight graft polymers (less

than 100,000) were found in the coatings.

PVP has also been grafted onto

poly(tetrafluoroethylene) (PTFE) using Y-radiation (43).

Here, the grafting conditions produced a heavy coating (over

70% weight gain for a 50 micron film) and hydrophilic

modification of the surface. Biological testing of the

graft coated surface showed a decrease in the adsorption of

fibrinogen and immunoglobulins from blood serum, but an

increase in albumin absorption.

Extensive research has been conducted by Ratner and

Hoffman on the use of Y-radiation to graft PVP and other

hydrophilic polymers onto silicone rubber (24). Aqueous

solutions of N-VP (20%) were irradiated to 0.25 Mrad using a

20,000 Curie Co-60 source and heavy graft coatings

(1.5 mg/cm2) of PVP were produced. These studies have also

shown the effect of low concentrations (0.005 Molar) of

cupric ion to minimize gelation in the external homopolymer

(44). Biological testing of these surfaces showed a

reduction in the adhesion of chick embryo muscle cells. The

desirability of this low cell adhesion and the aforemen-

tioned reduction in protein adsorption has prompted studies

with radiation grafted hydrogels for a broad range of

biomedical applications (45).

The strategy for our use of Y-radiation graft coatings

was to use low radiation doses (<0.5 Mrad) and low N-VP

monomer concentrations in order to minimize degradation of







the PMMA substrate and crosslinking of the PVP. Swelling

and distortion of the substrate was also minimized by the

use of the low monomer concentrations in aqueous solution.

Diffusion of monomer into the acrylic was limited by

immersion of the substrate in the polymerization medium only

at the time of irradiation. The goal was production of very

thin, uniform adherent PVP coatings on the acrylic

substrate.



2.4 Plasma Polymerized Graft Coatings

In addition to Y-radiation grafting, plasma polymerized

coatings were studied. A plasma is an excited low pressure

gas created by radio-frequency (RF) discharge or a direct

current (DC) electrical glow discharge. The plasma produces

ions, electrons, ion radicals and other excited species

which will readily polymerize. Yasuda has classified the

polymerization reactions of monomers in a plasma into two

categories: plasma-induced polymerization and plasma state

polymerization (Figure 6) (46).

In a plasma-induced polymerization, the vapor of a

monomer is excited to the form in which it would polymerize

by conventional polymerization, maintaining its original

structure. For example, the plasma serves to open the

double bond of the vinyl group in N-VP and promote

conventional radical polymerization.

Plasma state polymerization does not follow

conventional polymerization kinetics and relies on the









(1) Plasma Induced Polymerization
CH2- CH2 CH2- CH2
I I I I
CH2 C=O CH C=O
N /N
N/N/
I Plasma I
CH2=CH -CH2- CH
(N-VP)


CH2- CH2
I I
CH2 C=O
N/

CH2 CH
2


CH2- CH2 CH2-CH2

CH2 C=O CH2 C=O
N N
I I
* CH2- CH CH2- CH.


(2) Plasma State Polymerization
(Possible Reaction)
CH--CH2
I I
CH2 C=O
\N CH2= CH
I Plasma
CH2=CH --aa CH2-CH2-N:
C=O


CH"- CH2


N

CHj CH


CH2-CH2

CH' C=O

CHCH -CHCH
CH2=CH ----CHH


Figure 6. Plasma Polymerization Processes.







highly excited state of the plasma. Here, the plasma

creates reactive species which may be only fragments of the

original monomer. These fragments then react to form a

polymer. The polymer may be formed from molecules which

would not normally react or polymerize. Polymers formed by

these reactions are usually highly cross-linked and are

difficult to characterize because of their complex

structures.

There are a number of advantages to the use of plasma

polymerization in the production of polymer coatings. Most

important is the ability to produce thin and uniform

coatings. Moreover, the coatings can be produced on a wide

range of substrates. By varying the coating conditions,

coatings with a wide range of properties can be produced

from a single starting material.

A large body of literature exists about the use of

plasma polymerization for the production of coatings (see

review by Shen and Bell) (47), and a number of studies are
o
relevant to this study. Thin (less than 1000A) coatings

have been produced on contact lenses by RF plasma

polymerization of a mixture of acetylene, water vapor and

nitrogen (48). The surfaces produced were shown to be

hydrophilic (contact angle for water at 380) and reduced

both protein adsorption and cell adhesion.

In other biomedical applications, RF plasma polymerized

films of poly(ethylene), poly(styrene) and poly(chloro-

trifluoroethylene) have been shown to produce no change from







uncoated silicone rubber in biocompatibility studies done

with intramuscular implants of up to two years (49). Other

studies have shown that RF plasma polymerized films of

hexamethylcyclotrisiloxane on poly(propylene) and silicone

rubber membranes reduce the adhesion of platelets and

leucocytes in in vitro canine testing (50).

SThese different examples of the use of RF plasma

polymerized films to coat a variety of different materials

for a range of biomedical applications demonstrate the

usefulness of this technique in producing thin and uniform

coatings for biomedical applications. In this study, we

have shown that RF plasma grafting may be an alternative

method for producing hydrophilic polymer coatings on

intraocular lenses.














3. MATERIALS AND METHODS


3.1 Preliminary Studies

Although the emphasis of this research has been on the

development and characterization of permanent hydrophilic

polymer graft coatings for intraocular lenses (IOLs),

preliminary experiments were conducted which first

demonstrated the tissue-materials adhesion phenomenon and

the effectiveness of hydrophilic polymer solutions to

prevent tissue damage. The two most significant experiments

are describe here and elsewhere (5,6,51); the remainder of

Part 3, deals with the subsequent research based on findings

of these initial experiments.



3.1.1 Prevention of Cornea Endothelium Damage by
Poly(vinyl pyrrolidone) Solutions

In these experiments, cornea endothelium from rabbit

and human eye bank eyes were contacted to acrylic

hemispheres to simulate IOL insertion (5). These

hemispheres were machined and highly polished to a radius of

curvature of 7.3 mm; their visually smooth surface and

curvature facilitated total contact between the acrylic and

endothelium surfaces.







To obtain the tissue samples, adult New Zealand albino

rabbits were sacrificed by an intravenous overdose of sodium

pentobarbital and the eyes were enucleated immediately upon

death. The corneas were removed with a 3 mm rim of sclera

and the lense-iris diaphragm was peeled from the cornea.

The corneas were placed endothelium-side up on a teflon

bldck and the central areas were punched out with a 7 mm

trephine. An identical procedure was employed in isolating

7 mm cornea buttons from fresh human eyes. Only paired (two

eyes from the same donor) human eyes were used, so that one

could serve as a control for the other. The corneas were

then placed on the acrylic surfaces, described as follows.

Endothelium damage was determined by either optical

microscopy with nitro-blue tetrazolium (NBT) staining or

scanning electron microscopy (SEM).

The cornea buttons were contacted to acrylic hemi-

spheres which had been dipped in either a balanced salt

solution or a 40 weight % solution of poly(vinyl

pyrrolidone) (PVP) (GAF Corporation, New York; PVP K 29-32,

40,000 MW) in a balanced salt solution. The acrylic

hemisphere was held upright following dipping and the cornea

button was placed, endothelium side down, on the acrylic

surface. Contact time was varied from 1 to 60 seconds.

NBT staining (52) was performed by incubating the

corneas, following contact to an acrylic surface, at 370C

for 15 minutes with a drop of NBT stock solution and a drop

of reduced diphosphopyridine (DPNH) (0.3 mg/ml in water) on







the endothelium surface. Endothelium damage was then

estimated by observation with low-power optical microscopy.

Corneas were prepared for SEM by fixation in 2.5% cold

glutaraldehyde for one hour and osmium tetroxide for

90 minutes, both prepared with Millionig buffer. Corneas

were then dehydrated in serial baths of increasing

concentration of ethanol in water solutions. The samples

were then critical point dried using a Bomar SPC-900/EX

critical point apparatus (Bomar, Tacoma, WA) and then coated

with palladium-gold. Samples were then viewed with a Zeiss

Novoscan 30 scanning electron microscope (Carl Zeiss, Inc.,

New York).



3.1.2 Peritoneal Adhesions in Abdominal Surgery

The experimental plan was to perform exploratory

manipulations within the peritoneal cavity on rat and canine

animal models under simulated operating room conditions in

which the surgeon performs a clean, noncontaminating surgery

and yet obtains massive intra-abdominal adhesions, a major

post-operative complication (51). Such adhesions are

defined as the fibrous collagenous connective tissue that

develops after serosal trauma; in severe cases, these

adhesions can result in mechanical constriction of bowel

function, with potentially fatal consequences. Our intent

was to compare the conventional clinical technique with

procedures wherein hydrophilic polymer solutions of PVP and







dextran were applied to tissue surfaces, surgical gloves and

sponges prior to contact with internal organs and tissue.

Female mongrel dogs (12-20 kg) were anesthesized

initially with thiamylal sodium (Surital, Parke-Davis,

Morris Plains, NJ) (20 mg; 0.5 cc/kg of dog's weight) and

maintained on halothane with controlled ventilation

following endotracheal tube insertion. Abdomens were

prepped with a PVP-iodine (Betadine, Purdue Frederick,

Norwalk, CT) solution and draped for a sterile

intra-abdominal procedure. A midline incision was made with

a cold knife and bleeding vessels were electrocoagulated.

Dogs were divided into the following 4 groups of 4 dogs

each, and all were subjected to a complete exteriorized

bowel exploration as follows:

I) Organs manipulated with dry gloves and dry sponges.

II) Manipulation with dry gloves but using saline-wet

sponges and organs wet with sterile saline.

III) Gloves and sponges wet with 25 weight % soluble

dextran (Sigma, St. Louis; 200,000-300,000 MW) in

saline solution. Immediately after opening, 50 ml

dextran solution was sprayed on exposed organs

before manipulation. Just before closure 50 ml

dextran solution was again sprayed on organs.

IV) Same as III using 25 weight % PVP (GAF, New York;

40,000 MW) in saline.

A standardized procedure for organ manipulation was

used for each dog involving palpation of all organs,







exteriorization of bowel, bimanual palpation, replacement of

bowel, and the abdominal wall was closed in layers.

Experimental animals were housed separately and fed routine

diets. Surviving animals were sacrificed and examined

8 days after initial surgery and adhesions of the wound and

small intestine graded on a numerical scale based upon

degree and extent of adhesion bands..

Additional experiments using rats were conducted for

further evaluation of PVP solution. Twenty female

Sprague-Dawley rats (185-200 g) were anesthetized with

pentobarbital and divided into 2 groups. For the control

group of 10, the abdominal wall was opened and all

intra-abdominal organs bimanually examined. Exteriorization

was necessary to simulate clinical conditions for

exploratory surgeries. Abdominal contents were exposed,

gently explored and manipulated using dry gloves and sponges

for 5-10 minutes, the abdominal wall was closed in layers,

and the incision sutured. For the second group, the

procedure was repeated except that immediately after the

abdominal wall incision, 5 ml of 25 weight % PVP solution in

saline was sprayed into the abdominal cavity and all

manipulations were performed using gloves and sponges wet

with the PVP solution. Just before closure, an additional

5 ml of PVP solution was sprayed over the exposed organs.

Rats were maintained in individual cages for observation.

Eight days after surgery surviving rats were sacrificed and

intestines and organs examined for adhesions. The presence









of intra-abdominal adhesions was evaluated using a numerical

scale similar to that for the dog experiments.



3.2 PMMA Substrates

Acrylic test piece of several geometries were required

for the various methods of testing studied. Two test pieces

used most were "stubs" from 1/8" diameter rod, lathe cut to

1/8" length (PMMA from Modern Plastics, Orlando, Florida;

140,000 MW) and "slabs" (1/2" x 7/8" x 1/8"), saw cut from

Perspex CQ sheet (Medical grade PMMA from ICI, England;

2,400,000 MW). The stubs were used for characterization of

coatings by contact angle (for water), electron scattering

for chemical analysis (ESCA), examination by SEM, and

biophysical testing of adhesive force and SEM assessment of

damage from contact to cornea endothelium tissue. The slabs

were used in measurement of critical surface energy and

ultraviolet-visible (UV-Vis) and infrared (IR) spectroscopy.

Following lathe cutting to 1/8" lengths, the stubs were

individually end-polished in two steps. Coarse polishing

was done with water-wet 600 grit silica carbide paper

(Carbimet, Buehler Ltd., Evanston, ILL). A final dry

polish, on a napped cloth wheel (Microcloth, Buehler Ltd.)

using 40 p diamond paste (Astro-Met, GCA-Precision

Scientific, Chicago) was performed until visual inspection

revealed no scratches or surface irregularities. The

polishing was followed by an ultrasonic cleaning procedure:

five minutes in a soap water bath and then two five-minute







rinses in distilled water. Examination of the polished

surfaces by SEM was performed on random samples to assure

smooth surfaces.

The slabs were cut from medical grade acrylic sheet and

were rinsed with distilled water before use.



3.3 Purification of Monomer (N-VP)

The N-vinyl pyrrolidone (N-VP) monomer was obtained

from Eastman Kodak Chemicals (Rochester, NY) and contained

0.1% NaOH as an inhibitor to prevent polymerization. The

monomer was purified by distillation under vacuum prior to

use to remove this additive and any other impurities.

Vacuum distillation was performed in a batch process; a

1000 ml Erlenmeyer flask with a teflon-coated magnetic

stir-bar was charged with approximately 500 ml of the

as-received N-VP. The flask was heated by an electric

mantle and supported by a magnetic stirrer. The flask was

connected by a Claisen tube with a 6" high sidearm (which

held a thermometer to register distillation temperature) to

a water cooled Leibig condenser (40 cm jacket length). This

condenser was lined by a connection joint to a 500 ml

Erlenmeyer receiving flask in an ice bath and to the vacuum

source. Vacuum was supplied by a mechanical pump (Model

D-150, Precision Scientific, Chicago) via a liquid nitrogen

cold trap. The vacuum attained was approximately

100 micron.







Distillation was conducted until a stable,

constant-boiling temperature was achieved typically at

66-800C depending on the vacuum achieved. The first 100 ml

of distillate collected after this point was discarded,

along with any previous distillate. The next 250 ml of

distillate was collected and preserved for use. The

remaining distillant was discarded and the system was

recharged for continued distillation following the same

procedure.

Purity of the distillate was checked by index of

refraction and infared spectroscopy. The index of

refraction was measured by an Abbe-type refractometer

(accuracy 0.0001; Fisher Scientific, Pittsburgh, PA) and

was found in all cases to be within 0.0001 of the literature
D
value of n25 = 1.5120 (53).

Infrared spectra of the distilled N-VP were recorded

with a Perkin-Elmer Model 283B infrared spectrophotometer

under standard operating conditions (Figure 7). These

spectra matched the spectrum for N-vinyl pyrrolidone found

in the literature (Figure 7) (53).

The monomer was stored in sealed flasks at < 0C in a

freezer until use.



3.4 y-Radiation Graft Coatings

Gamma-Radiation graft polymerization coatings were

prepared by irradiation of the PMMA substrate in an aqueous

solution of the N-VP monomer with y-radiation from a 600












Wavenumber (cm" )
1600 1400


200


1 4.0 5.0 6.0 7.0 8.0 9.0 10 12 14 16 1820 25 30 4050
Wavelength in microns

Figure 7. Infrared Spectrum of N-VP Monomer (As Distilled).


3500














Wavenumber (cm'')


2 3 4 5 6 7 8 9 10 11 12 13 14
Wavelength in microns

Figure 8. Infrared Spectrum of N-VP Monomer (Literature, Reference 53).







Curie Co-60 source (Department of Radiation Biology, Univer-

sity of Florida) (Figure 9). Radiation doses used were from

0.1 to 0.5 megarads (Mrad), as measured by Fricke dosimetry,

(54) performed by the Department of Radiation Biology. To

minimize the irradiation times, and also maximize the number

of samples irradiated, the irradiations were usually made at

a distance of two inches from the source. The dose rate at

this distance was 1360 rads per minute.

Solutions of N-VP in water were prepared volumetrically

using freshly distilled water and distilled monomer (pre-

pared as previously described). The solutions were prepared

immediately prior to irradiation and were not degassed.

Different irradiation containers were used for the two

kinds of PMMA substrates which were coated. The short rod

"stubs" were held in a 1/8" teflon sheet which fit in the

slots of a Coplin staining jar (Volume 90 ml; Fisher

Scientific, Pittsburgh); the "stubs" were held such that

both ends were exposed to the polymerization media. The

rectangular PMMA "slabs" were placed for coating in

borosilicate glass test tubes (16 x 125 mm, volume v 18 ml)

(Fisher Scientific, Pittsburgh). The test tubes were then

placed in a specially constructed carousel which held the

tubes upright at a distance of two inches from the source

during the irradiation. The Coplin jars were free-standing

within the Co-60 source and were placed so that the source

would be two inches from the PMMA "stubs." In all cases,










Control Handle
(for lowering & raising source)


Door
Support
Track


Positioning Board
Figure 9. Schematic Drawing of Co-60 Source, Department of Radiation Biology,
University of Florida.







the PMMA substrates were not placed in the irradiation

containers (filled with the polymerization media) until

immediately prior to irradiation.

Following irradiation, the PMMA substrates were removed

from the irradiation containers and holders and immersed

immediately in distilled water. The wash water was changed

several times until all external homopolymer was removed.

The completion of the cleaning procedure was checked by

allowing the samples to air-dry and then inspecting for

debris of external homopolymer. Cleaning by washing and

soaking in distilled water was continued until all

homopolymer had been removed. The cleaned samples were

stored until use in polyethylene beakers filled with

distilled water. The homopolymer and solution remaining

following irradiation were removed from the irradiation

container and saved in capped, sealed amber glass vials.



3.5 Intrinsic Viscosity Molecular Weight of PVP

The molecular weight of the PVP homopolymer formed in a

solution by Y-radiation induced polymerization was

calculated by measurement of intrinsic viscosity and use of

the Mark-Houwink-Sakurada equation. The polymer was

obtained by precipitation of polymer formed under conditions

identical to those used for graft coating of PMMA, but

without the acrylic substrate present.

Solutions of distilled N-VP (5-30%, distilled as

previously described) and freshly distilled water were








placed in test tubes (16 x 125 mm; volume 18 ml) sealed with

aluminum foil and rubber stoppers. No attempt was made to

de-gas the solutions or containers. These test tubes were

irradiated at the dose levels studied (0.1 to 0.5 Mrad)

while held in a specially constructed carousel.

The homopolymer of PVP formed for each set of

conditions was precipitated with 100 ml of acetone in a

small glass container (volume 500 ml) on a blender (Waring,

New Hartford, CN) at high speed. The precipitated polymer

was washed twice with 150 ml of acetone in the blender at

high speed. The polymer was removed from the blender and

loosely sealed in aluminum foil packets. The polymer was

air dried for twelve hours and then dried overnight in a

vacuum oven at 500C and less than 150 torr.

Using the method of the Appendix, solutions for a

viscometry were prepared with methanol as the solvent, and

were filtered twice through Gelman spectroglass filters

(#934-AH, Gelman Instrument Co., Ann Arbor, MI). An

Ubbelholde viscometer, size OB (Fisher Scientific,

Pittsburgh, PA) was used for viscosity measurement, at 300C,

(0.1C) maintained in a water bath. Four dilutions were

used to obtain a In nrel / vs. concentration plot. The

intercept at zero concentration was found via a best-fit

linear plot with a HP-35c calculator (Hewlett Packard,

Corvallis, OR). Coefficients* for the Mark-Houwink-Sakurada


*These values, for the equation [n] = KMa, were K = 23 x
10 ml/g and a = 0.65.








equation were taken from the Polymer Handbook (55). The

molecular weight of PVP was determined for all radiation

graft conditions that produced a soluble homopolymer.



3.6 RF Plasma Coatings
Modification of the PMMA substrate was performed by

graft coating with hydrophilic polymers formed by

radio-frequency (RF) plasma polymerization of N-VP and

2-hydroxyethyl methacrylate (HEMA). A vertical, "bell jar"

reaction chamber was constructed for this procedure; the

schematic diagram shows the apparatus and its set-up

(Figure 10). The plasma was created by induction of a

13.56 MHz signal of a 100 watt RF generator (Tegal,

Richmond, CA) through a ten-turn coil of copper tubing. The

output load was adjusted for optimal conditions and the

power level was controlled by a matching network (Tegal) and

a SWR meter (Heath, Benton Harbor, MI).

PMMA samples (both "stubs" and "slabs") to be coated

were supported on a stand 3.5 inches directly below the

plasma generation region (the plasma was generated in the

tube surrounded by the copper tubing coil); the samples were

secured with double-sided adhesive tape. The chamber was

evacuated to 100 micron pressure by a mechanical pump (Model

D-150, Precision Scientific, Chicago) connected with a

liquid nitrogen cold trap. The vacuum was monitored by a

thermocouple vacuum gauge (Series 270, Granville Phillips,

















Micrometer Valve


- To Line





100 Watt RF
Generator


To Liquid N2
Trap & Vacuum


To Thermocouple
Gauge Control


Figure 10. Schematic Drawing of RE Plasma Apparatus.








Boulder, CO), and maintained until residual water vapor was

eliminated from the system.

Monomer vapor from monomer held in a 20 ml long-necked

Erlenmeyer flask was then bled into the system and adjusted

by'a micro-metering valve (SS-22RS4, Whitey Co., Highland

Heights, OH) until the desired pressure was achieved. The

monbmers, N-VP (Eastman Kodak Chemicals, Rochester, NY) and

HEMA (Aldrich Chemical Co., Milwaukee, WI) were used as

received. The monomer vapor available was sufficiently

pure, and the monomers, as received, contained inhibitors

which prevented polymerization when the monomer was heated

(by a hot air gun) to achieve the desired vapor pressures.

After the desired pressure was achieved, the RF power

was switched on and adjusted to the selected power level, as

monitored by the SWR meter. Monomer vapor pressure and RF

power were maintained at constant levels over the duration

of the plasma polymerization and deposition of coating.

Following the completion of the coating procedure, the

chamber was opened to the atmosphere, and the samples were

cleaned by rinsing with distilled water. The samples were

stored with the side toward the plasma generation region

facing up; testing and characterization was performed only

on this face of the sample.


3.7 Contact Angle for Water

Contact angle measurements were made to assess the

relative water wettability of the different materials' and








coatings' surfaces. These measurements were made with a NRL

contact angle goniometer (Rame'-Hart, Mountain Lakes, NJ) at

ambient temperature and humidity (usually 23C and 60%

relative humidity).

The Y-radiation and RF plasma graft coated and uncoated

acrylic samples were equilibrated overnight in distilled

water; water droplets adhering to the surface were shaken

off prior to testing. All other materials tested were used

as received.

Using a Gilmont micrometer syringe (Gilmont, Great

Neck, NY), a 0.002 ml drop of freshly distilled water was

deposited on the plastic surface. The contact angle on each

side of the drop was measured to the nearest degree and

recorded. The surface was then dried by touching a small

piece of filter paper to the periphery of the drop; any

remaining water was blown from the surface with compressed

gas (Manostat, NYC). Five drops were used on each sample,

yielding ten measurements of the contact angle.



3.8 Critical Surface Tension

The critical surface tensions of both the y-radiation

graft coatings andthe RF plasma coatings were determined by

the construction of a Zisman plot of the cosine of the

contact angle for liquids of different surface tensions

versus the surface tensions of the liquids (56). The

intercept at cosine of the contact angle equal to one (as

determined with a best-fit, linear plot, using a HP-35c







calculator (Hewlett Packard, Corvallis, OR)) was the

critical surface tension for that surface.

The liquids used were selected from a wide range of

liquids used for such measurements (57). The liquids used

were chosen on the basis of their being non-solvents for

both PMMA and PVP and their non-spreading behavior

(formation of a finite contact angle) on both polymers'

surfaces. The value of the surface tensions for these

liquids was taken as the value for 200C, although the

measurements were made at ambient conditions. This was

possible since the change in surface tension with

temperature for organic liquids is only "0.1 dynes/cm/oC.

The five liquids used and their surface tensions are water

(72.6 dynes/cm), glycerol (63.4 dynes/cm), formamide

(58.2 dynes/cm), 2.2'-thiodiethanol (54.0 dynes/cm), and

methylene iodide (58.2 dynes/cm).2

The larger surface area acrylic slabs were used for

these measurements; the sample was stored at ambient

conditions prior to testing. The contact angle measurement

for each liquid was made according to the same procedure as

for the contact angle for water measurements, except two

drops, yielding four angles for measurement, were used for

each liquid. A fresh, clean area of the sample surface was

used for each drop.

Following the measurements of these angles for the five

liquids, the average for each liquid was calculated. These

values, along with the surface tensions of the five liquids







(above), were used in the construction of a Zisman plot for

each surface tested.



3.9 Scanning Electron Microscopy

Scanning electron microscopy was used to observe the

surface morphology of the different coatings and their

thickness. Coated stubs were scraped with a "chisel" made

from the same PMMA rod stock as the coated substrate. These

specimens were then prepared for microscopy by fixing the

sample to aluminum stubs with colloidal carbon paint

(Structure Probe, Inc., West Chester, PA) to assure mounting
0
and to provide an electrical ground. Gold-palladium (200 A)

was deposited on the surface by a Hummer V sputter coater

(Technics, Alexandria, VA).

Following these preparations, the specimens were

examined with a JEOL JSM-35c SEM (JEOL, Boston, MA). The

scratch made by the PMMA "chisel" was photographed at 500X

and 2000X to show both the appearance of the coating and to

reveal the coating thickness. For these examinations, the

SEM conditions were 15 kV accelerating voltage, condenser at

the 12 o'clock position and the sample held at 0 tilt.

Photographs of the scraped area were taken at 500X, 2000X

and 5000X magnifications to reveal both surface morphology

and the coating thickness.








3.10 ESCA (Electron Scattering for Chemical Analysis)

Samples for ESCA were sent to Dr. Christopher Batich*

of Dupont Central Research (Wilmington, Delaware) for

analysis. These samples were mounted to copper wire to

eliminate any background interference in the spectra. A

Kratos ES-300 (Kratos, Manchester, England) ESCA

spectrometer was used, with aluminum (Ka) as the x-ray

source. Data for each peak were collected for twenty

minutes.

Analysis of the raw data was performed by Dr. Batich.

Peak heights and areas were determined for C, N, O, Cl, Cu

and Si. The ratio relative to carbon was then calculated

for each element.



3.11 Infrared Spectroscopy

Surface analysis of the coated surfaces was made by

infrared spectroscopy using a Perkin-Elmer Model 283B

spectrometer (Perkin-Elmer, Stamford, CN). The analysis was

made on the sample surfaces by use of an attentuated total

reflectance (ATR) accessory (No. 185-0382, Perkin-Elmer).

Maximum sensitivity was obtained using KRS-5 crystals as the

prism elements at angles of 600 and 45. Samples, in the

form of coated acrylic "slabs," were placed on both sides of

the element to maximize the spectra's strength. The

greatest slit width available was used, along with a




*Present address, Dept. of Materials Science, University of
Florida.








reference beam attenuator, to compensate for the loss of

signal due to the ATR accessory.

Coated samples were air-dried and stored at ambient

conditions before spectra were obtained. For comparison of

spectra for these graft coated samples, cast films of PVP

were made and analyzed by ATR-IR. These films were made

from 5 and 10% solution of PVP (MW 40,000) (GAF, NYC) in

methanol with a doctor blade. These films were cast on the

acrylic "slabs" and were dried in a vacuum oven at 500C and

less than 150 torr prior to testing. The spectra were

analyzed as above, and were used to aid in resolving

spectral peaks and analysis of the spectra for the graft

coated samples.



3.12 Ultraviolet-Visible Spectroscopy

The ultraviolet-visible (UV-Vis) spectra of the PMMA

and the coated samples were taken from 900 to 190 nanometers

with a Model 552 Perkins-Elmer spectrophotometer

(Perkin-Elmer, Stanford, CN). In addition, the effect of

y-radiation alone was analyzed spectroscopically at

radiation doses of 0.25, 2.5 and 5.0 Mrad.



3.13 PVP-Iodine Interaction

Samples of PMMA and y-radiation graft coated PMMA were

equilibrated overnight in saturated solutions of iodine in

water. Both UV-Vis and ATR-IR spectra were obtained, as

previously described, to use the completing of the iodine by




44


PVP as a measure of the amount of PVP present in the

coating.



3.14 Biophysical Measurements



3.14.1 Instrument for Biophysical Measurements

The instrument was constructed from a design developed

at the Weizmann Institute, Rehovot, Israel by visiting

Professor Dr. Shimon Reich. The instrument functioned to

allow both the measurement of adhesive force and to provide

quantification of damage from contact of endothelium to a

material's surface.

The instrument consisted of two parts: a mechanism

mounted on a scavenged microscope and a measuring microscope

with a micrometer-controlled crosshair, mounted in its

ocular. Figure 11 shows the mechanism mounted on the

scavenged microscope. A detailed description of the

instrument and its operation follows.



3.14.2 Preparation of the Material Sample for Measurement

Three forms of materials (coated and uncoated acrylic,

and other polymers) were tested, but the holder design

required a sample in the form of a 1/8" x 1/8" rod or

"stub." Acrylic stubs were prepared from rod as discussed

previously and were friction-fit into a conical holder at

the point of the cone (Figure 12). Materials not in

suitable rod form were cut from sheet into 1/8" discs and








Water Immersion Test Cell


Adjusted Support
Stage for
Tripod Weight


Rack and Pinion


Cornea Endothelium


Polymer Sample


Tripod Weight

Glass Fiber



Material Holder


Transparent
Plexiglass
Tank


Lateral View of Section (A)

X-Y Micrometer Stage
(for centering of material
sample over endothelium)


Microscope
Stage


- Microscope Base


Tissue-Polymer Adhesion Measurement Instrument.


Figure 11.





































Figure 12. Acrylic Sample in Holder.







attached with cyanoacrylate glue (Loctite, Cleveland, OH) to

acrylic stubs for use.

The holder was suspended, cone pointing down, from a

drawn glass fiber by a fine gold chain. Above this holder a

tripod weight of 17 grams was freely held, such that the

flat base of the cone would support the weight when contact

with the tissue sample was made. Therefore, contact would

be made while this weight was loading the interface with a

known pressure.

The prepared tissue sample was held on the stage of the

microscope in a acrylic tank filled with physiological

saline (0.8%) (Figure 13). A micrometer stage allowed fine

adjustment of the vertical position of acrylic tank and the

tissue sample enclosed. An X-Y micrometer stage allowed

precise centering of the tissue beneath the conical stub

holder. By filling the acrylic tank with saline, surface

tension effects in adhesion were eliminated and tissue

freshness was maintained during the measurements.



3.14.3 Preparation of Tissue Samples

Two different forms of tissue samples were used in the

measurements of adhesive force and for damage

quantification. However, the type and source of the tissue

was the same: Cornea endothelium obtained from adult male

New Zealand albino rabbits. The rabbits were sacrificed by

an intravenous overdose of pentobarbital and the eyes were





































Figure 13. Plexiglass Tank (filled with Saline) with
Tissue Sample.








then enucleated. The eyes were used less than two hours

postmortem.

For tissue damage measurements, the corneas were

removed from the eyes and placed in saline. Then an 8 mm

corneal trephine (Storz, St. Louis, MO) was used to punch

out a disc from the central portion of the cornea. This

cornea "button" was then glued, endothelial side up, to an

aluminum SEM stub with cyanoacrylate glue. A flat, uniform

surface of endothelium was formed that would allow precise

contact with a material surface. This sample was then

positioned and held within the saline-filled acrylic tank

for testing.

In measurement of the adhesive force, a sample holder

was constructed so that multiple (up to six) measurements

could be made from a single cornea. Corneas were removed

from the enucleated rabbit eyes and trimmed to leave a

narrow (1 mm) scleral rim. The corneas were then inverted

and placed on a supporting plastic hemisphere of matching

curvature. The cornea was held at the limbus with a metal

collar, secured with a screw plate. In this manner, a

"corneal hemisphere" was created; by holding the hemisphere

tilted within the acrylic tank, rotation of the hemisphere

on its axis would provide multiple contact points along a

latitudinal line. Contact was made normal to the surface by

the test piece held above.







3.14.4 Adhesive Force Measurement

Operation of the instrument to measure adhesive force

was made after the freshly prepared tissue sample was

positioned within the acrylic tank and it was placed on the

micrometer-controlled stage. The material test piece was

friction-fit into its holder and suspended above the tissue

tank.

The rack and pinion was used to lower the test piece to

within a few millimeters of the tissue surface. At this

point, the crosshair of the measuring microscope was zeroed

and aligned with a reference point of the glass fiber. This

was the resting position of the glass fiber, which was

stressed only by the weight of the test piece and holder.

Contact between the test piece and the tissue sample was

then made.

The stage supporting the acrylic tank and the tissue

sample was slowly raised by operation of its micrometer

control. As the tissue sample contacted the test piece

surface, the elevation was continued until the tripod weight

was supported by the test piece holder. This loading was

maintained for thirty seconds.

The micrometer-controlled stage was then lowered

slowly. Adhesion between the two surfaces caused the test

piece to accompany the tissue in its downward travel. This

movement was simultaneously tracked by the micrometer

crosshairs of the measuring microscope (Figure 15).















































Figure 14. Loading of Acrylic-Endothelium Interface.






































Figure 15. Deflection of Glass Fiber due to Acrylic-
Endothelium Adhesion with Lowering of
Micrometer Stage.








This crosshair monitored the deflection of the glass

fiber until the adhesive force at the tissue-material

interfaced was exceeded by the flexural stresses in the

glass fiber. When that occurred, the crosshair micrometer

was read and recorded, since the interface was broken and

two surfaces had parted.

The "corneal hemisphere" was then rotated approximately

600 to bring a fresh area of tissue into position for

measurement. The process was then repeated and continued

until all six positions on the tissue were utilized for

measurements.

The reading recorded during these measurements was

converted into force measurements by a calibration curve

prepared for the glass fiber in use. Such curves were

prepared for each glass fiber; the deflection of each fiber

was measured with known weights hung from the fiber. The

weights were hung on the glass fiber at the same point at

which the gold chain (which suspended the test pieces

experimentally) was located. Then, just as in the procedure

for the measurement of the adhesive force, the micrometer

crosshair of the measuring microscope was used to measure

the deflection for that weight.

These values yielded a linear plot of force versus

deflection. The values for deflection in the adhesive force

measurements were converted into force by use of linear best

fit program and a HP-35c programmable calculator

(Hewlett-Packard, Corvalis, OR). Then, assuming complete








contact over the entire 0.079 cm2 of the test pieces, the

adhesive force in terms of milligrams/cm2 was calculated.



3.14.5 Quantitative Cornea Endothelium Damage by SEM

The procedure for measuring the tissue damage resulting

from a material contact began by contacting the material

sample with the endothelium just as in the measurement of

adhesive force. However, only one contact was made on each

tissue sample and following that single touch, the tissue

was removed from the instrument. The measurement made by

the micrometer crosshair was noted, but was used only to

confirm that proper contact had been made.

The tissue sample was then immediately prepared for

scanning electron microscopy, while still glued to the

aluminum stub. The tissue was fixed for 1-3 hours in 2.5%

glutaraldehyde in Millionig buffer, and was then placed in a

buffer rinse for equal period of time. The tissue was then

dehydrated by a series of increasingly concentrated

ethanol-water baths. The samples were held in baths of 50,

70, 80, 90, 95 and 100 percent ethanol solutions for

ten minutes each. Following an additional ten minutes fresh

ethanol, the samples were then critical point dried using a

Tousimis Samdri PVT-3 (Tousimis, Rockville, MD).

The tissue specimens were then reglued to SEM stubs

with colloidal carbon paint, and after sufficient drying,
the samples were then sputter-coated with 200 A of
the samples were then sputter-coated with 200 A of







gold-palladium. The samples were then viewed with a JEOL

JSM-35C SEM.

SEM photomicrographs at 20-30X were taken of each

specimen and evaluated. The initial assessment of the

extent of damage was done by averaging two independently

made estimates. The accuracy of these assessments was

confirmed by grid counting; this technique employed a grid

and the number of squares within the areas of damage were

compared to the number of squares within the entire area of

contact. This grid counting technique was used on a

selected sample of photomicrographs and percent of tissue

damage obtained by these measurements matched those made by

experimenters' assessments.














4. RESULTS AND DISCUSSION


4,.1 Preliminary Studies and Overview of Thesis Research

In the preliminary studies the tissue-materials

adhesion phenomenon was discovered and the use of

hydrophilic polymer solutions to reduce adhesion was

demonstrated (5, 6, 51). The results of these and related

studies are presented and discussed here since they

established the research goals and plan for this

dissertation research.



4.1.1 Prevention of Cornea Endothelium Damage by Poly(vinyl
pyrollidone) Solutions

The discovery of the tissue-material adhesion

phenomenon dealt with the intraocular lens-cornea

endothelium interface (5). In these studies, rabbit, and

human cornea endothelium was examined by optical microscopy

with nitro-blue tetrazolium staining and scanning electron

microscopy to reveal damaged cells following contact to

poly(methyl methacrylate) (PMMA) surfaces, dipped in either

balanced salt solution or solutions of the hydrophilic

polymer, poly(vinyl pyrrolidone) (PVP).

Examination by both methods showed that 30-35% of the

endothelium cells were damaged by 60 second contact with








acrylic surfaces dipped in balanced salt solution. Even a

one second contact produced extensive damage (approximately

20-25%). However, when the'acrylic surface was dipped in a

PVP solution (40 weight %), examination revealed almost no

damage to the endothelium, regardless of the time of

contact. Balanced salt solution would not wet the acrylic

surface while the PVP solution produced an adhesion

barrier-lubricating boundary layer.



4.1.2 Peritoneal Adhesions in Abdominal Surgery

These experiments evaluated the ability of solutions of

the hydrophilic polymers PVP and dextran to prevent

peritoneal adhesions resulting from trauma due to surgical

manipulations.51 Tables 1 and 2 show the results for the

canine and rat models, respectively.

A numerical rating scale was used to derive objective

data from subjective observations. Ratings were based upon

examination of the wound and small intestine and scored on

the.following scale:

0 No adhesions

1 Very slight evidence for adhesions

2 Slight to moderate number of adhesions

3 Moderate to extensive adhesions

4 Massive and extensive adhesions.

For rat experiments, the score for each animal

represented a consensus of 3 surgical observers. The dog

experiments were scored by 2 surgical observers. Scores








Table 1. Dog Model for Assessment of Polymer Solutions to
Prevent Adhesions.



Wound Small Intestine
Avg. Score Rel. Score Ave. Score Rel. Score


Drya 3.0 10 2.5 8

Salinea 3.0 10 1.5 5

PVPb 2.0 7 0.3 '1

Dextranb 2.0 7 1.3 4


aAverage of 4

bAverage of 3


surviving dogs.

surviving dogs.


Table 2. Rat Model-Assessment of PVP Solution to Prevent
Adhesions.



Average Score/Animal Relative Score


Control 3.5 5.0

PVP Coatingb 0.7 1.0


aAverage of 4 animals surviving anesthesia and 8 day
maintenance with healed wounds.
Average of 3 surviving animals.








were averaged for each test group; the lowest average score

was assigned a value of 1.0 and higher scores assigned

appropriate relative values compared to the base low score.

In both the dog and rat experiments, the controls

showed moderate to extensive adhesions to the wound and of

the small intestine. Comparative results with PVP coating

were highly meaningful in both cases in that virtual

elimination of adhesions of the small intestine was

demonstrated. The dextran-treated dogs also showed some

reduction in wound adhesions. However, the saline-wet

control and the dextran-treated group both tended to have

less severe intestinal adhesions as compared with the dog

control group but were still much inferior to the

PVP-treated group.

The peritoneal adhesions were eliminated by the action

of the PVP solution to prevent serosal rubber-glove damage

during surgical manipulations. Rubbing of an intestinal-

surface was shown by SEM to produce considerable serosal

abrasion; virtually no tissue damage was observed following

rubbing by rubber gloves wet with PVP solution. As with the

acrylic-cornea endothelium contact, tissue damage was

eliminated by the interposition of nonadhesive, lubricating

boundary layer in the form of a surface-coating hydrophilic

polymer solution.

These experiments are significant in demonstrating for

the first time that surgical tissue damage and resulting

adhesions may be substantially reduced by application of








protective polymer coatings prior to manipulation of tissue

and organs in surgery.



4.1.3 Other Related Research

Unpublished research has shown the occurrence of the

tissue-materials adhesion phenomenon in the use of

endotracheal catheters. In these experiments, inflated

catheter cuffs were contacted with sections of trachea,

which had been removed from ferrets and then laid open

longitudinally. This contact was made with a loading force

which would simulate the clinical inflation of the cuff

within the trachea. These contacts, using cuffs dry and wet

with a 25 weight % solution of PVP in phosphate-buffered

saline, showed that desquamation of tracheal endothelium was

reduced by the use of the PVP solution. Quantitative data

were not obtained because the area of contact was difficult

to assess.

Quantitative measurement of the IOL-endothelium inter-

action was also difficult; attempts to measure the adhesive

force at an acrylic-cornea endothelium interface using an

Instron tensile test machine with a one gram full-scale load

cell were unsuccessful. The results of these tests were not

reproducible, because of difficulty in mounting the tissue

and dehydration of the tissue during the time required for

measurement. Loading of the interface was difficult and

extremely variable as was estimation of the actual area of

contact. Similarly, attempts to measure cornea endothelium








damage quantitatively by optical microscopy with NBT

staining were unsuccessful; counting of damaged cells at

high magnification was unable to distinguish between

materials of widely different tissue adhesion properties.

The. failure of these experiments demonstrated the need for

an in vitro method for evaluation and accurate measurement

of the tissue-material adhesion phenomenon.



4.1.4 Overview of Thesis Research

As described above, surface modification by solutions

of hydrophilic polymers was shown effective in reducing or

eliminating tissue-material adhesion in several different

areas of surgery. However, a clinical need for preventing

IOL-cornea endothelium adhesion and its damaging conse-

quences existed and efforts were focused on this problem.

Although solutions of PVP used clinically showed promise

(5), a permanent modification of the IOL surface eliminates

the need to apply coating by the surgeon in the operating

room. Greater safety would also be assured by permanent

modification if the IOL became detached following

implantation, when damage to the endothelium from contact,

just as produced during the original insertion, might occur.

Cast films of a hydrophilic polymer would eliminate the

application of the coating by the surgeon, but would not be

a permanent means to prevent any post-surgical occurrence of

cornea endothelium-IOL adhesion.








PMMA is very difficult to modify by conventional

wet-chemistry; attempts in this laboratory to perform

hydrophilic surface modifications by such means were

unsuccessful. For example, the acrylic surface appeared

unaltered by exposure to a boiling aqueous solution of

concentrated sodium hydroxide. Gamma-radiation graft

polymerization was chosen as a method to produce hydrophilic

surfaces by polymerization on the surface of the PMMA. This

investigation emphasized the use of y-radiation to form

graft coatings of PVP on the PMMA surface and so produce a

hydrophilic modification of the acrylic lens surface.

A study of radiation dose and monomer concentration was

undertaken for Y-radiation graft polymerization of PVP on

PMMA. Low radiation doses (<0.5 megarad (Mrad)) and low

monomer concentrations were chosen to minimize PMMA

degradation and gelation in the external PVP homopolymer.

Distortion of the PMMA by solvent swelling was also

minimized by using monomer concentrations of < 30 volume %

in aqueous solution and immersion of the PMMA in the

polymerization medium only at the time of irradiation.

Hydrophilic surface modification of PMMA was also

performed by radio-frequency (RF) plasma coatings. These

coatings were produced from the monomers of PVP and

poly(hydroxyethyl methacrylate) (PHEMA) to show an

additional possible method of surface modification of

hydrophilic polymer graft coating.








The hydrophilic polymer graft coatings were

characterized to determine the nature of the coating, most

particularly the properties which might influence the

biophysical behavior. The different tests which were

performed are discussed in the following sections.

Measurement of the intrinsic viscosity of the external

polymer formed in solution in y-radiation graft coating was

made to understand the relative amounts of polymer formed

under the different conditions in the study of radiation

dose and monomer concentration. The molecular weight of the

external polymer determined by these measurements would be a

relative measure of the extent of polymerization for the

graft coatings themselves and could be correlated to the

biophysical behavior and other properties of the graft

coating.

The contact angle of water was a surface property

measured to show the water wettability of the coatings and

their hydrophilicity, relative to PMMA. Contact angle was

measured for hydrated samples. Closely related to the

measurement of contact angle for water, the critical surface

tension for the graft coated surfaces was determined. It

was felt that the surface energy of the material might have

a significant influence on the contact adhesion resulting

from contact to an endothelium surface.

Scanning electron microscopy was used to examine the

surfaces and detect and record any changes in the surface








morphology by the graft coating. The examination by SEM

also was used to reveal the coating thickness, by scraping

the coating away and viewing the border of the scrape.

Three different spectroscopic techniques were used to

detect the presence of PVP graft coating on the PMMA

surface. Electron scattering for chemical analysis (ESCA)

and infrared spectroscopy (IR) were used for this purpose.

Ultraviolet-visible (UV-Vis) spectroscopy was used as a

possible means to show the coating presence but also to show

if the coating and the coating process had any effect on the

light transmissivity of the PMMA. Difficulties in detecting

the PVP by IR and UV-Vis lead to use of iodine to complex to

the PVP and enhance detection; the results of these

experiments will also be discussed.

In addition to the preparation and characterization of

hydrophilic polymer graft coatings on PMMA, an instrument

and technique for measurement of adhesive force and

endothelium damage were developed out of necessity; as

previously described, attempts to quantify tissue-materials

adhesion had been unsuccessful and such a means was

necessary for evaluation of the biophysical properties of

hydrophilic polymer graft coatings. The results of the

biophysical testing of adhesive force and quantitative

endothelium damage measurement of these materials will be

presented and the correlations to the other characteriza-

tions described above will be discussed.








The tissue adhesion properties of several common

biomedical polymers were also evaluated for comparison to

the graft coated acrylic system. The contact angle for

water of these materials was also measured for the same

purpose.



4.2 Intrinsic Viscosity Molecular Weight of PVP

The molecular weight of the external polymer formed in

solution during Y-radiation graft coating gave an indication

of the relative molecular weight of the polymer (PVP)

actually grafted to the PMMA substrate; the molecular

weights of PVP formed under Y-radiation graft coating

conditions are presented in Table 3. The linearity of the

molecular weight versus initial N-VP concentration plot,

shown in Figure 16, coincides with previously reported data

(58), as did results for 0.1 and 0.25 Mrad dose levels. The

molecular weights for a given initial monomer concentration

were consistently higher for the higher dose level. The

difference became larger with increasing concentration.

In addition, the insolubility of the precipitated

homopolymer of the 5% and 10% N-VP concentrations at the

0.5 Mrad dose level in methanol, the solvent used for the

viscometry, indicated, along with the gelation of the

external homopolymer at 20% and 30% monomer concentrations

for that radiation dose level, the tendency for increased

crosslinking at the 0.5 Mrad dose level. Gelation of PVP in

solution has previously been shown to be a function of both








Table 3. Viscosity Molecular Weight* of PVP.
External Polymer Formed in Solution During Y-Radiation Graft.Coating.


% N-VP (Initial Concentration in
Polymerization Media)/Y-Radiation Dose (Mrad)


5%/0.1

10%/0.1

20%/0.1

30%/0.1


5%/0.25

10%/0.25

20%/0.25

30%/0.25


309,470

693,473

1,028,743

1,307,795


577,992

714,313

1,371,339

2,278,969


The external polymers formed for 0.5 Mrad radiation doses were insoluble and
therefore unfit for intrinsic viscosity measurement.



*Calculated by Mark-Houwink-Sakurda equation from intrinsic viscosity in
methanol at 300C.


















/
/
/
2.0 /




S1.5 /


-o /


S1.0 -



Coefficient of

/ 0.1 Mrad r2 = 0.96

0.25 Mrad r2 = 0.98
//


I I I I
5 10 20 30
%N-VP(Initial Concentration in Polymerization Media)


Figure 16. External Polymer Viscosity Molecular Weight
vs. Monomer Concentration for y-Radiation
Graft Coatings of PVP on PMMA.
Graft Coatings of PVP on PMMA.


I








radiation dose and concentration (59). Experiments using

cupric and ferrous ions as chain transfer agents to reduce

gelation (as reported (44)) were performed but not pursued

because of possible toxicity of the cupric ion and

difficulty in removing the ferrousion.



4.3 Contact Angle for Water

The measurement of the contact angle of water for the

different surfaces was important since it was felt that it

was the hydrophobic nature of the PMMA that made it adherent

to the endothelium of the cornea (Tables 4-6). Measurement

of the advancing angle of water for hydrated samples would

be valuable in correlating the hydrophilic nature of the

coatings with their biophysical behavior in the adhesive

force testing and the endothelium damage produced (and

evaluated by SEM).

The measurements revealed a significant decrease in

contact angle and therefore an increase in hydrophilicity

was achieved using Y-radiation- graft coatings of PVP on the

PMMA substrate. Uncoated PMMA showed a contact angle of 720

and the y-radiation graft coatings showed a decrease in

angle down to 300 for a coating prepared at 30% N-VP

concentration and 0.1 Mrad dose.

As with the molecular weight for the external

homopolymer formed in solution, at 0.1 and 0.25 Mrad doses

the change in contact angle for water was proportional to

increasing monomer concentration for a given dose level.

This linearity was found not only for the 0.1 and 0.25 Mrad








Table 4. Contact Angle of Water on Y-Radiation Graft Coatings of PVP on PMMA.



Polymer Average Contact Angle (0) Standard Deviation (O)


PMMA* 72 8

5%**/0.1 Mrad*** 61 3
10%/0.1 57 2
20%/0.1 40 3
30%/0.1 30 1

0%/0.25 65 2
5%/0.25 59 6
10%/0.25 59 7
20%/0.25 .44 3
30%/0.25 37 5

5%/0.5 46 3
10%/0.5 38 7



*Uncoated; agrees with literature value of 780 (57).

**Initial monomer concentration in polymerization media.

***Radiation dose in megarad (Mrad).








Table 5. Contact Angle of Water for RF Plasma Coatings on PMMA.


Plasma Conditions
Power*/Pressure**/Time*** Average Contact Angle (0) Standard Deviation (O)
(Monomer)****


PMMA 72 8

50W/1000p/10min.(N-VP) 40 6

100W/200p/20min.(N-VP) 26 9

35W/500p/60min.(N-VP) 60 3

25W/500p/15min.(HEMA) 42 14



*RF power to plasma (Watts).

**Pressure in reactor (Microns).

***Time of exposure to plasma.

****Monomer vapor used.








Table 6. Contact Angle of Water for Various Biomedical Polymers.



Polymer Average Contact Angle (0) Standard Deviation (o)


Urethane 37 7

Silicone Rubber 101 2

Thermoplastic Elastomer 90 2

Teflon 105 3



*Pellthane TM, Upjohn Corporation, Kalamazoo, MI.

**Silastic 500-5 TM, Dow-Corning, Midland, MI.

***Proprietary hydrocarbon block copolymer, C-flex TM, Concept Inc., Clearwater,
FL.

****TM, FEP, E.I. DuPont de Nemours & Co., Wilmington, DE.








dose levels but also for the 0.5 Mrad dose level coatings

(Figures 17-20). The 0.5 Mrad coatings showed a much

greater decrease in contact angle, although the gelation of

the external homopolymer prevented measurement of the

contact angle at 20% and 30% N-VP concentrations.

Comparison of this linear relationship between N-VP

concentration and contact angle for the different dose

levels showed this greater decrease in contact angle at the

5 and 10% N-VP concentrations for the 0.5 Mrad coatings

(Figure 20). The unexpected result that the 0.1 Mrad

coatings show more of a decrease in contact angle than do

the 0.25 Mrad coatings can be explained only on the basis of

the standard deviation of the contact angle measurements.

The standard deviation for this measurement shows that the

values for the 0.1 and 0.25 Mrad coatings do overlap and the

difference is not significant.

Measurement of the contact angle for the RF plasma

coatings shows that these coatings are hydrophilic, like the

Y-radiation graft coatings (Table 5). Only a preliminary

study of RF graft coating was made. Correlations between

reaction conditions and contact angle were not developed.

However, the results of these experiments, along with the

results of.biophysical tests, will be discussed later.

Contact angles for a number of common biomedical

polymers were also measured and compared with graft coated

PVP on PMMA (Table 6). The majority of these materials

possess a surface which is more hydrophobic than the















































< 0 10 15 20 30
S % N-VP (Initial Concentration in Polymerization Media)
a.


Figure 17.


Contact Angle for Water vs. % Monomer for
0.1 Mrad Dose y-Radiation Graft Coatings of
PVP on PMMA.








































II I I I I


5 10 20 30
% N-VP (Initial Concentration in Polymerization Media)


Figure 18.


Contact Angle for Water vs. % Monomer for
0.25 Mrad Dose y-Radiation Graft Coatings of
PVP on PMMA.


60-

50-

40

30


< 0
,'





































5 10


20*


% N-VP (Initial Concentration in Polymerization Media)
*Uncleanable gel formed at monomer
concentrations of 20% and greater.


Figure 19.


Contact Angle for Water vs. % Monomer for
0.5 Mrad Dose y-Radiation Graft Coatings of
PVP on PMMA.


70 -

60 -


rI
E


0


<


























Legend


Coefficient of
Determination


.. 0.1 Mrad r2 = .99

.a. 0.25 Mrad r2 = .95
......... 050 Mrad r2 = .91


0 5


C %%N-VP
(Initial Concentration in Polymerization Media)
*Coefficient of Determination


Figure 20. Summary of Contact Angle for Water vs. %
Monomer for Various Dose Levels of y-Radia-
tion Graft Coatings of PVP on PMMA.








uncoated PMMA and therefore furnish information about

materials with surfaces much different than the hydrophilic

coatings prepared by Y-radiation graft and RF plasma

polymerizations. These materials are all polymers which are

important or potentially important for biomedical use in

applications in which the contact adhesion phenomena could

play a significant role.



4.4 Critical Surface Tension

Critical surface tension measurements were made to

additionally characterize the 0.25 Mrad Y-radiation graft

coating series and the RF plasma coatings for correlation to

the biophysical measurements and the other surface

characterizations. However, the high surface energies of

these surfaces made the results of these measurements

difficult to interpret.

In order to measure the contact angle over a range of

different surface tension liquids, liquids which showed a

finite contact angle and would not spread (contact angle of

zero) were required. These liquids were all of relatively

high surface tension (greater than 50 dynes/cm) and the

majority of which are polar in nature. The result is polar

interaction with the hydrophilic PVP graft coated surfaces.

The Zisman plot for PMMA, tested at ambient conditions,

showed a critical surface tension of 38 dynes/cm, which is







extremely close to the literature value of 39 dynes/cm (57)

(Figure 21). Measurement of the critical surface tension

for the 0.25 Mrad radiation graft series begins with a

slight decrease in the surface tension for the 5% N-VP

concentration graft coating and then begins to increase to

greater critical surface tension for the 10% and greater

N-V'P concentration graft coatings (Figures 22-26).' However,

the surface energies obtained for the higher concentrations

are not realistic, as the numbers approach and exceed

100 dynes/cm (Table 7).

The Zisman plots for the 20% and 30% N-VP concentra-

tions show a change in slope from the normal, negative slope

to a positive slope (Figures 25-28). The coefficient of

determination for these two coatings' plots is also

considerably less than for the other plots. The polar

interactions that result from the use of polar liquids on

these increasingly hydrophilic surfaces disrupt the

usefulness of the Zisman plot as an accurate measurement of

the critical surface tension of these coatings.

In an attempt to accurately calculate the critical

surface tension for these surfaces, the contact angle data

for the different liquids were treated by the method of

Baszkin and Lyman (60). This method separates the

dispersive and nondispersive (polar) interactions between

the liquid and the. solid and has been performed on surfaces

Sof high critical surface tension. The polar contribution to

the critical surface tension has been reported as high as



















0.8 -

Yc = 38 dynes/cm
r2 = 0.92
c 0.6
0
C.,
*

0.4




0.2



I I I I I I
30 40 50 60 70 80.
Surface Tension (Dynes/cm)


Figure 21. Zisman Plot of Critical Surface Tension (y ) of PMMA.
C^











1.0




0.8 -

Y = 43 dynes/cm
r = 0.92
0.6
S*
0
0 0

0.4-




0.2-


I I I I I I

30 40 50 60 70 80

Surface Tension (Dynes/cm)

Figure 22. Zisman Plot of Critical Surface Tension (y ) of y-Irradiated
(0.25 Mrad) PMMA.
















0.8 -
S Y = 28 dynes/cm
r2 = 0.92

0.6

0


0.4




0.2



I I I I I I
30 40 50 60 70 80
Surface Tension (dynes/cm)

Figure 23. Zisman Plot of Critical Surface Tension (y ) of y-Radiation
Graft Coating: 5% N-VP Initial Monomer Concentration and
0.25 Mrad Dose.























<,c "V" N




0.4




0.2





30 40 50 60 70 80
Surface Tension (Dynes/cm)

Figure 24. Zisman Plot of Critical Surface Tension (y c) of y-Radiation
Graft Coating: 10% N-VP Initial Monomer Concentration and
0.25 Mrad Dose.





































30 40 50 60 70 80

Surface Tension (Dynes/cm)


Figure 25.


Zisman Plot of Critical Surface Tension (y ) of y-Radiation
Graft Coating: 20% N-VP Initial Monomer Concentration and
0.25 Mrad Dose.


0.8




e 0.6
o
0



0.4




.0.2

















0.8




A 0.6
o
0



0.4




0.2









Figure 26.


50 60 70 80 90 100
Surface Tension (Dynes/cm)


Zisman Plot of Critical Surface Tension (y ) of y-Radiation
Graft Coating: 30% N-VP Initial Monomer Concentration and
0.25 Mrad Dose.









Table 7.


*
Critical Surface Tension from Zisman Plots for Y-Radiation Graft
Coatings of PVP on PMMA.


% N-VP (Initial Concentration in Surface Energy
Polymerization Media) (Dynes/cm)



PMMA 38

0% 43

5 28

10 46

20 68

30 104



*Using water, formanide, glycerol, thiodiethanol and methylene iodide as
solvents. Instantaneous contact angles were measured.











38 43 46 68


0.4




0.2




30


Figure 27.


40 50 60 70 80 90 100
Surface Tension (Dynes/cm)

Zisman Plot of Critical Surface Tension (y ) of 0.25 Mrad
y-Radiation Graft Coatings of Different N-UP Concentrations.


Yc(dynes/cm) 28




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