Investigation of Polyvinyl Pyrrolidone Synthesized Silver Nanoparticle Ink for use in Medication Compliance Monitoring Applications Heather Jean Meredith In accordance with Glen Flores, Ph.D. Department of Materials Science and Engineering, University of Florida and Convergent Engineering, Gainesville, FL Advisor: Dr. Gerald Bourne Spring 2010 Summa Cum Laude Bachelor of Science in Materials Science and Engineering University of Florida
! Objective : Refine polyvinyl pyrrolidone (PVP) synthesized silver nanoparticle ink to provide a repeatable system with long term stability that has high conductivity at low metallization temperatures, and enhanced printability for the use on a biocompatible electronic pill. Summary : A PVP synthesized silver nanoparticle (SNP) ink formulation was the focus of this research. The overall objective of creating a biocom patible electronic pill requires a conductive antenna printed or attached to a gelatin capsule. The antenna trace is in the form of metalized silver nanoparticles and after ingestion is utilized to transmit radiofrequency signals to an external receiver for monitoring purposes. In the PVP SNP system, PVP is used as a protective agent of the silver ions and must be removed in order to allow the silver to make a complete electrical pathway upon sintering. For this reason, one of the first tasks was to remove over 95% of all residual PVP c ontent from the ink through multiple, vigorous washing steps. Thermal gravimetric analysis (TGA) was used to verify that less th an 5% of the PVP content remained after washing of the SNPs. Once confirmed, a resistivity close to the value of bulk silver ( 1 51 cm ) could be obtained after sintering of the silver nanoparticle ink at a temperature of 200 Â¡ C. 1 The size of the SNPs must be approximately 50 nm in order for effective printability 1 Particle size characterization using the Microtrac Nanotrac at the University of Florida's Particle Engineering Research Center (PERC) was utilized for this analysis of the SNPs. This characterization technique was also used to confirm that there wa s repeatability between identical formulations o f ink. A printing methodology of the silver nanoparticle ink was established for printing of conductive, fine lined antennas. Pad printing of SNPs was investigated for d irect p rinting onto gelatin capsules, which is favored for high manufacturability of t he product. Once the antenna trace has been directly printed onto the gelatin capsule, it must be sintered at a temperature that does not cause brittleness of the pill. To do this, the metallization temperature of the SNPs must be
! # reduced to 100 Â¡ C with a m etallization additive. N,N dimethylformamide (DMF) was researched for this application. As the electronic pill will be ingested, a study of biocompatibility of the SNPs that considers the dosage and form of the silver was necessary. Also if direct printing was not achieved biocompatible substrates that could be printed on and externally attached had to be considered. Introduction/Background: Silver nanoparticles are utilized for high efficiency radiofrequency (RF) applications due to their superior conductivity, printability, and stability. A current application of these silver nanoparticles involves the creation of an electronic pill that will assis t in medication compliance monitoring. For this application, silver nanoparticle based ink will be used to create an external biocompatible radiofrequency identification (RFID) antenna that can be printed or attached to gelatin capsules (Figure 1). Once in gested, the an tenna trace will transmit radiofrequency signals, allowing it to be detected from inside the digestive system by an external receiver. 2 The need for this electronic pill was established due to patient s lack of medication compliance. It is es timated that 25 50% of patients are non adherent to their prescribed medication regimen. Noncompliance results in an estimated 125,000 deaths per year and an overall cost exceeding $100 billion a year. 3 Presently the only way to control noncompliant patients is through lengthy procedures including drug assessments of blood or urine, use of drug markers in medication, direct observation of patients, self reporting by patient, pill counting, and reviews of prescription records. 4 By monitoring medication compliance with this pill, deaths, time, and expenses could be kept to a minimum. The use of metallic nanoparticle based inks is becoming more widespread in electronic applications. Unlike molten metal and numerous polymers that have been previously used in electronics, nanoparticles can be used at room temperature and have improved conductivity. Three main metals have been focused on in the fabrication of electronics using nanoparticle inks and include silver, gold, and copper. In most recent studies, si lver nanoparticle based ink has Figure 1. Prototype displaying RFID antenna attached to capsule
! $ been utilized due to its superior physical and mechanical properties. Silver nanoparticles are also of great interest due to their ability to sinter at temperatures as low as 200 Â¡ C. 1 Sintering of metallic nanoparticles below 200 Â¡ C is scarcely reported therefore further research of reducing the metallization temperature through additives such as N,N dimethylformamide (DMF) must be completed in order for direct printing of antenna traces onto low melting substrates. Silver is a lso cost efficient, which proves to be extremely favorable for production at a large scale. According to Sigma Aldrich, a bottle of silver nanopowder costs $8.10 USD per gram, while gold nanoparticles cost $254.50 USD per gram. Gold nanoparticles end up co sting 31 more times than silver on a per gram basis. 2 Therefore, it can be concluded that silver nanoparticles are more favorable for creating conducting antenna traces for this electronic application. In order to produce conductive antenna traces in an e conomical and rapid fashion, different printing techniques and mediums must be thoroughly researched and developed. Some current printing methods of nanoparticle based inks include drop on demand (DOD) inkjet printing, pad printing, screen printing, microc ontact printing, and Dip Pen Nanolithography (DPN) With these techniques, conductive lines can be drawn onto substrates in one step. The focus of this research will be on pad printing of silver nanoparticles. Because the metallic nanoparticles will be ingested for this electronic application, further research must be done on the biocompatibility of silver. A main concern of ingestion of silver nanoparticle inks is a rgyria, a permanent bluish gray discoloration of the skin. The American Conference of Governmental Industrial Hygienists has limited intake of metallic silver to 0.1 mg/m 3 and 0.01 mg/m 3 soluble c ompounds of silver. 5 Exposure to soluble forms of silver may result in a rgyria and other toxic effects, in cluding liver and kidney damage, irritation of the eyes, skin, respiratory and intestinal tract, and changes in blood cells. Because this silver nanoparticle ink will be in the metallic form, it is expected that health risks will be negligible. 2,5 Technic al Approach: Before a novel procedure was developed for the synthesis of silver nanoparticle s multiple commercially available inks, such as Methode, Acheson, and Creative Materials, were evaluated These inks proved to be inadequate for this in vivo application due to their non biocompatible
! % components, their low conductivity that was too poor for high efficiency applications, their high er sintering temperature, and the difficult ly of printing due to their high viscosity. For these reason s Flores (20 09) developed a chemical reduction method to synthesize nanosized silver colloids in a process that was repeatable, simple, and short (Figure A.1) 2 The specific materials for this synthesis are listed in Appendix A, but due to proprietary issues detailed steps can not be released. For the s ynthesis silver nitrate (AgNO 3 ) was reduced at room temperature by formaldehyde (HCHO) in the presence of p olyvinyl pyrrolidone (PVP). Adsorbed polymers such as PVP can kinetically stabilize silver colloidal systems When ad sorbed polymer coil s on a silver coll oid begin to interact with coils emanating from a second colloidal surface, there is a reduction in the conformational entropy of the coils This results in an increase in free energy therefore creating a repulsi ve force between the surfaces of the colloids. 6 This steric stabilization allows the silver colloids to be well dispersed in solution. 2 The minimal concentration of PVP necessary for SNP protection is PVP/AgNO 3 = 1.5:1. 1 Lack of polymer coverage must be avoided as this results in attractive forces, which therefore leads to flocculation. In order to ensure complete coverage a ratio of 2:1 was used. 2 To complete the rea ction, sodium hydroxide (NaOH) wa s added in order to keep the reaction at a high pH so to assist HCHO in the reduction process. After one hour, the reduction was complete and acetone was added to terminate the reaction. The solution was then washed with acetone and centrifuged in order to precipitate out the silver colloids and remove PVP and any unreacted materials. Due to chain entanglements, it was difficult to remove the PVP from the surface of the colloid surfac es. Rigorous mechanical shaking of the silver nanoparticle ink followed by centrifuging prove d to be the most effective method of removing these polymer chains. This wa shing step was repeated multiple times in order to recover the maximum amount of relatively pure silver colloids. 2 Although the washing steps are not able to remove all PVP from the silver colloids, for high conductivity the residual PVP content should be less than 5%. Because this is such a small percentage, the PVP does not hinder the electrical conductivity after pr inting and heat treatment. 1 The Perkin Elmer Thermal Gravimetric Analysis (TGA) at the Particle Engineering Research Center (PERC) was used to determine the residual PVP amount left on the
! & 0.95 0.96 0.97 0.98 0.99 1 1.01 0 100 200 300 400 500 600 700 800 Temperature ( Â¡ C) surface of the colloid A four hour comprehensive trial using TGA w as initially completed to determine how much mass loss was occurring at different temperatures, so that other samples could be evaluated properly For the initial trial, the temperature was increased to 100 Â¡ C at a rate of 15 Â¡ C/min and held for 30 minutes. The temperature was than increased to 180 Â¡ C at 15 Â¡ C/min for 120 minutes, followed by an increase to 400 Â¡ C at 15 Â¡ C/min for 60 minutes. There was a final increase to 700 Â¡ C at 20 Â¡ C/min for 30 minutes Figure 2 displays the mass loss of PVP at the pr eviously stated temperatures As can be seen from Figure 2, there is a significant mass loss between 300 500 Â¡ C, which correlates with the thermal decomposition of PVP. Using this comprehensive trial, a final trial was established t o determine the mass l os s of PVP for multiple samples. This trial involved increasing the temperature to 300 Â¡ C at a rate of 20 Â¡ C/min and hol d ing for 120 minutes to remove all water from the system The temperature was than increased to 480 Â¡ C at 20 Â¡ C/min for 120 minutes to obtain total mass loss. Figure 2. Thermal gravimetric analysis displaying mass loss of PVP from SNPs as a function of temperature ( 4 hour trial) After the silver colloid s have been recovered, the ink wa s manipulated with propylene glycol and a 10:1 ratio of ethylcellulose: terpineol to create a viscosity that wa s best for printing (20 40 Pa s). 2 Size, dispersion, and stability of the nanoparticles had to be considered when research ing printing methodologies. Clogging due to large particles or the aggregation of the particles had to
! be prevented. For maximum printability, literature states that nanoparticles should have a diameter of 50 nm. 1 It was determined that pad printing would be optimal for printing of the SNP ink to create antenna traces with fine lined printing features (~100 microns), uniformi ty, and quick manufacturability For the pad printing process, a clichÂŽ is created by etching the antenna patterns into a steel plate. A doctor blade is used to apply the ink into the grooves of the clichÂŽ. The nanoparticle ink is then transferred with an elastomeric stamp to a substrate (Figure 3) 2 ,7 Figure 3. Pad printing schematic diagram 7 Kapton, a polyimide from DuPont is currently the substrate that is printed on. The problem is that this material is non biodegradable, and therefore could not be ingested. To avoid this problem the RFID antenna was transferred from the Kapton to an enteric coating. 2 This coating is a p olymethyl methacrylate (PMMA) copolymer, which is biocompatible and could easily be wrapped around the gelatin capsule. In the future, direct printing onto the gelatin capsule or other biocompatible substrates is desired. This currently cannot be done d ue to the fact that at temperatures above 150 Â¡ C these substrates will degrade, melt, or warp. 2 After the RFID antenna was pad pri nted, the silver nanoparticles we re sintered at 200 Â¡ C for 5 15 minutes. The gelatin capsule or biocompatible substrates would not be able to withstand this temperature therefore proving why the RFID antenna must be print ed on Kapton and than heat ed to high temperatures. When curing silver, increasing the temperature causes a sintering effect of nanosized silver colloids, which c auses a decrease in resistivity. When the ink was sintered at 200 Â¡ C for 5 15 minutes the resistivit y of the silver antenna trace was reduced to approximately 10x10 6 cm. 2
! ( A Design of Experiment (DOE) was c ompleted to see the effects of varying PVP molecular weight ( MW= 10, 000, 40, 000, 55 ,000 ), and the amount of PVP, formaldehyde (HCHO), and sodium hydroxide (NaOH) on the properties and size of silver colloids (See Appendix B Table 1 for the 13 different trial s ) The size s of the SNPs listed in Table 1 were determined using the Microtrac Nanotrac at PERC. With the Nanotrac, a dynamic light scattering method is used to determine the size distribution For this method a laser is coupled to the sample through an optical beam splitter in the Nanotrac prob e T he laser is scattered by the particles in the sample, which are constantly moving due to Brownian motion. This shifted light is transmitted through the optical beam splitter to the photodetector. This light can be compared to a reference signal, which is created by the reflection of t he original laser back through the optical beam splitter to the photodetector. The Microtrac s oftware program analyzes the signals, calculates the Doppler shifts and translates this to the particle size distribution. 8 Results and Discussion : The silver nanoparticle synthesis produced a stable ink formulation that could be cre ated in less than three hours. When suspended in water, isopropanol, or ethanol, the ink displayed a long shelf life and remained homogenous withou t any precipitation The repeatability o f the ink formulation was proven with the Microtrac Nanotrac The results of the Design of Experiment show that molecular weight controls the particle size of the silver colloids Figure 4 displays the size distribution output from the Microtrac Nanotrac. PVP protection effectively controls the co lloids' size keeping them in a range of 20 200 nm. The first seven peak s in Figure 4 correspond with those SNPs protected using PVP molecular weight of 10,000 g/mol The size range of these SNPs was from 10 25 nm. T he other peaks are those protected with a PVP molecular weight of 40,000 g/mol or 55 ,000 g/mol and range in size from 40 200 nm, with the majority at Figure 4. SNP size distribution output from Microtrac Nanotrac. Note that each colored line corresponds to one of the 13 trials of the DOE (Table 1 in Appendix B )
! ) 0.97 0.975 0.98 0.985 0.99 0.995 1 1.005 0 100 200 300 400 500 80 nm It is believed that the micron sized particle observed in Trial 3 was an agglomeration as a result of PVP entanglement at this hi gher molecular weight. The results from the Microtrac Nanotrac proved that the size of the silver colloids were suitable for printing. Pad printing of SNP ink resulted in a high resolution antenna trace (Figure 5). The antenna trace displayed conducting, narrow (~100 microns), thick antenna lines with little to no breaks in the trace. This printing method wou l d a ls o a llow for simple direct printing onto curved shapes such as gelatin capsules, which would ultimately lead to high manufacturability of the electronic pill. During sintering PVP hinders SNP interaction and may be difficult to remove at lower metalliz ation temperatures. This prohibits silver from forming a complete electrical pathway upon sintering, which therefore increases resistivity. T hermal gravimetric analysis (TGA) was used to confirm that the majority of PVP was removed from the colloidal surfa ce prior to sintering. It was determined from the c omprehensive four hour tr i a l that the majority of mass loss occurred between 300 500 Â¡ C Therefore for all samples the analysis involved increasing the temperature to 300 Â¡ C at a rate of 20 Â¡ C/min and hol d ing for 120 minutes to remove all water from the system. The temperature was than increased to 480 Â¡ C at 20 Â¡ C/min for 120 minutes to obtain total mass loss Figure 6 shows the total mass loss and proves that less t han 5% of PVP remains. Figu re 5. RFID antenna trace that can be directly printed or attached to the exterior of a gelatin capsule Temperature ( Â¡ C) Figure 6. T hermal gravimetric analysis displaying mass loss of PVP from SNPs as a function of temperature (2 hour trial)
! Because such a minimal amount of PVP re mained on the colloidal surface, high conductivity of sintered SNPs could be obtained 1 When the RFID antenna trace was s intered at 200Â¡C for 5 15 minutes a low resistivity of 10x10 6 cm was obtained which is only one magnitude off from the res is tivity of bulk silver (1.51 x10 6 cm ) 2 Drake et al (2005) states that the dosage and form of the silver must be considered when determining biocompatibility Sintering of the RFID antenna results in the silver being in a metalized form. Metallic silver is not readily absorbed into the ski n and therefore should not cause any adve rse effects. Metallic silver is used for surgical prosthesis and splints, and fungicides. 5 It should also be noted that all solvents used in the synthesis of the SNPs were removed during sintering, therefore the ink is completely biocompatible. Clinical te sts will ultimately have to be conduct ed in order to prove the safety and efficacy of the silver antenna trace in order to gain approval from the Food and Drug Administration (FDA) before commercialization of the product Conclusion : It has been proven that it is feasibl e to develop a radiofrequency identification antenna out of silver nanoparticle ink for a biocompatible pill that will assist in medication compliance monitoring. It can be assumed that because the antenna traces a re in the f orm of metallic SNPs minimal health risks will be present, but before the pill is manufactured, clinical tests will have to be completed to obtain the approval of the FDA. Analysis of multiple formulations of ink proved that the PVP SNP system is highly re peatable and stable The rigorous washing steps of the SNP synthesis, effectively l e ad to the removal of PVP from the surface of the silver colloids. By confirming with thermal gravimetric analysis (TGA) that less than 5% of residual PVP remained high conductivity of sintered SNPs was obtained After a heat treatment of 200 Â¡ C for approximately 5 15 minutes the resistivity of the antenna trace was reduced to within a magnitude of the resistivity of bulk silver ( 1 51 cm ). P ad printing of SNP ink resul ted in fine lined antenna trace s In order to be used as an electronic circuit on low melting temperature substrates (such as gelatin), curing temperatures of the SNPs must be reduced to below 100 Â¡ C so as not to deform or cause any brittleness in the gelat inous material. Future work for this project
! "+ must include r esearch of N,N dimethylformamide (DMF) to reduce the metallization temperature 10 The nScrypt industrial inkjet printer at the F lorida Institute of Sustainable Energy (F ISE ) will be utilized to determine the correct concentration of DMF:silver nitrate needed to dramatically reduce the sintering temperature. The nScrypt contains a 3 part mixing device that can be loaded with the silver nanoparticle ink, dimethylformamide, and any other additives, which can then be combined and printed in one step. 9 If the correct concentration of DMF is unstable in the ink solution, research about stabilizing additives will be completed. Resistivity at these low sintering temp eratures will be m onitored with the expectation that a low resistivity of approximately 1 5 cm will be maintained.
! "" Appendix A Basic Ink Synthesis Materials: Polyvinyl pyrrolidone (PVP) (MW=10,000 and 40 ,000) from Sigma Aldrich Silver nitrate (AgNO 3 ) from Fisher Scientific 30% by mass formaldehyde (HCHO) from Sigma Aldrich 50% by mass sodium hydroxide (NaOH) from Sigma Aldrich Acetone from Fisher Scientific 50 mL centrifuge tubes Figure A.1. Steps of Basic Ink Synthesis
! "# App endix B Table 1 Design of Experiment Run PVP MW PVP NaOH HCHO Ave. Size (g/mol) (g) (mL) (mL) (nm) 1 10000 3.0 0.75 20.00 20.1 2 10000 3.0 2.00 5.00 26.6 3 55000 4.5 1.69 16.25 200.8 4 55000 3.0 0.75 5.00 92.6 5 10000 7.5 1.06 16.25 22.9 6 40000 6.0 1.38 12.50 97.3 7 55000 7.5 1.06 8.75 84.6 8 10000 3.0 2.00 5.00 22.4 9 40000 6.0 1.38 12.50 89.1 10 55000 9.0 2.00 29.00 152.0 11 10000 7.5 1.69 16.25 25.0 12 10000 9.0 0.75 5.00 16.2 13 10000 3.0 0.75 20.00 23.7
! "$ References 1. Lee H, Chou KS, Huang KC. "Inkjet printing of nanosized silver colloids." Nanotechnology. IOP Publishing. 2005; 16: 2436 2441 2. Flores GP. "Bimodal molecular weigh polyvinyl pyrrolidone synthesized silver nanoparticles for use as low curing temperature conductive materials." University of Florida. 2009. 3 Hughes CM. "Medication non adherence in the elderly". Drug Aging. 2004; 21:7 93 811. 4 Nugent C, Finaly D, Davies R, Paggetti C, Taburini E, Black, N. "Can technolo gy improve compliance to medication?" From Smart Homes to Smart Care. IOS Press. 2005; 65 72. 5. Drake P, Hazelwood, K. "Exposure Related Health Effects of Silver and Silver Compounds: A Review. Annals of Occupational Hygiene Oxford University Press. 2 005; 49.7: 575 585 6 Stokes, R., & Evans, D.F. (1997). Fundamentals of interfacial engineering New York, NY: John Wiley & Sons, Inc. pp.309 7. Service Tectonics, Inc. (2004). Pad Print Process: About the Pad Printing Process Retrieved from < http://padprinting.net/proc_about.html > 8. "Nanotrac." Nanotechnology Particle Size Measurement Solutions Microtrac Inc., 2008. Web. 31 Mar 2010. < http://www.microtrac.com/ProductsTechnology/NanotracParticleSizeAnalyzer/NanotracTe chnology.aspx > 9 University of Florida Institute for Sustainable Energy. (2007). Prototype Development & Demonstration Laboratory Retrieved from 10 Pastoriza Santos I, Liz Marzan LM. Formatio n of PVP protected metal nanoparticles in DMF. Langmuir. 2002 ; 18:2888 2894.