Thermally Stable and Sterilizable Polymer Transistors for Reusable

Apr 4, 2016 - ... endure the standard autoclave sterilization for reusable medical devices. ... flexible TFTs in sterilizable/reusable medical device ...
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Thermally Stable and Sterilizable Polymer Transistors for Reusable Medical Devices Aung Ko Ko Kyaw,* Feroz Jamalullah, Loga Vaithieswari, Mein Jin Tan, Lian Zhang, and Jie Zhang* Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Singapore 138634, Republic of Singapore S Supporting Information *

ABSTRACT: We realize a thermally stable polymer thin film transistor (TFT) that is able to endure the standard autoclave sterilization for reusable medical devices. A thermally stable semiconducting polymer poly[4-(4,4-dihexadecyl4Hcyclopenta[1,2-b:5,4-b]dithiophen-2-yl)-alt[1,2,5]thiadiazolo [3,4c] pyridine], which is stable up to 350 °C in N2 and 200 °C in air, is used as channel layer, whereas the biocompatible SU-8 polymer is used as a flexible dielectric layer, in addition to conventional SiO2 dielectric layer. Encapsulating with in-house designed composite film laminates as moisture barrier, both TFTs using either SiO2 or SU-8 dielectric layer exhibit good stability in sterilized conditions without significant change in mobility and threshold voltage. After sterilization for 30 min in autoclave, the mobility drops only 15%; from as-fabricated mobility of 1.4 and 1.3 cm2 V−1 s−1 to 1.2 and 1.1 cm2 V−1 s−1 for TFTs with SiO2 and SU-8 dielectric layer, respectively. Our TFT design along with experimental results reveal the opportunity on organic/polymer flexible TFTs in sterilizable/reusable medical device application. KEYWORDS: organic transistor, semiconducting polymer, thermal stability, moisture-barrier, medical devices

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sterilization).15,16 This condition is necessary for the complete removal of all micro-organisms including spore-forming and nonspore-forming bacteria, viruses, fungi and protozoa−that could contaminate the medical apparatus. The polymer TFT that can be fitted into such application is still challenging because of the fact that the degradation of semiconducting polymer/dielectric layer arises from changes in chemical bonding, crystalline structure and morphology at elevated temperature, or due to mismatch in the thermal expansion of materials stack within the device. Carbazole-based polymer which is stable in air at temperature up to 150 °C and in N2 at temperature up to 350 °C,17 and thiophene-based polymer which resists the breakdown of the device up to 250 °C have been reported a couple of years ago.18 However, their mobility is very low (in the range of 1 × 10−3 to 1 × 10−2 cm2 V−1 s−1), and hence they cannot be practically used. Recently, thermally stable organic TFT was demonstrated for medical application using a small molecule [dinaphtho[2,3-b:2′,3′-f]-thieno[3,2b]thiophene (DNTT)] but the device performance severely degraded at temperatures 150 °C and above even in nitrogen environment.19 Moreover, a relatively thick gold barrier layer (200 nm) used in prior literature is more costly than metal-free barrier layer and is therefore less preferred in industrial application.

rganic thin-film transistors (TFT) have ignited a great deal of research interest because of excellent mechanical flexibility of organic materials, low-cost fabrication and material variety.1 Their unique properties open up a wide spectrum of potential applications spanning from flexible displays, integrated circuits, biosensors to wearable electronics, fashion, textiles, and electronic skins for robotics and prosthesis.2−6 The organic semiconductors including conjugated polymers are, however, well-known to be unstable in air and moisture due to the oxidation, and hence a myriad of research focus on the air stability.7−10 On the other hand, thermal stability is paid less attention in conjugated polymer research.11,12 In fact, they are also generally susceptible to high temperature, being prevented from usage in several applications. For instances, the operation temperature of most integrated circuit used in consumer electronics and industrial electrical systems can be as high as 125 °C.13 In the automotive, aerospace, well-logging, and geothermal energy production industries, the electronic devices and circuits are exposed to temperature higher than 150 °C.13,14 Moreover, because matured silicon technology requires processing at high temperature (>300 °C), conjugated polymers with high-temperature-withstanding capability will lead to fabrication of novel organic−inorganic hybrid devices on traditional silicon platform. If the electronic devices are integrated into reusable medical apparatus, they should be able to endure standard medical sterilizing condition which is heating at 121 °C for 20 min or 134 °C for 5 min in pressurized autoclave (steam © 2016 American Chemical Society

Received: January 5, 2016 Accepted: April 4, 2016 Published: April 4, 2016 9533

DOI: 10.1021/acsami.6b00108 ACS Appl. Mater. Interfaces 2016, 8, 9533−9539

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ACS Applied Materials & Interfaces

the absorption spectra are not observed when the films are annealed up to 200 °C in air. The spectrum is affected only after the exposure to temperature above 200 °C. The absorption spectra suggest that electronic band structure of PCDTPT is still stable after exposure to temperature as high as 350 °C in N2 and 200 °C in air. To examine the stability at high temperature on the chemical bonding in PCDTPT, X-ray photoelectron spectroscopy (XPS) was carried out. The C 1s core level XPS spectra of PCDTPT film annealed at various temperatures in N2 and in air are shown in Figure 2a, d, respectively. The C 1s peak of as-cast film is located at the binding energy of 285 eV with full width half-maximum (fwhm) about 1 eV. The asymmetry in the C 1s peak exhibits the presence of an additional contribution at the higher binding energy side. The spectral feature can be resolved by fitting with three components; one centered at 284.9 eV can be assigned to C−C bonds and another at 285.1 eV can be attributed to C−H bonds, whereas the last one at higher binding energy around 286.1 eV originates from C−N and C− S bonds.21−23 The position and fwhm of C 1s peak does not change after annealing for 15 min at temperature up to 400 °C in N2 and 200 °C in air. After annealing at 300 °C in air, however, the fwhm of C 1s peak broadens to 1.6 eV although the peak position remains unchanged. Figure 2b, e show the N 1s core-level spectra of PCDTPT film annealed at various temperatures in N2 and air, respectively. The spectral feature of N 1s consists of two components because PCDTPT contains N atoms at two different locations within the monomer unit; one in thiadiazole unit and the other in the pyridine unit. The peak at 399.8 eV arises from the N core level in thiadiazole unit while the peak at lower binding energy (∼398.5 eV) can be attributed to pyridine unit.21,24 The position of N 1s peak remains unchanged up to annealing temperature 350 °C in N2 and 200 °C in air. The peaks, however, shift 0.1 eV to lower binding energy after annealing at 400 °C in N2. The changes in spectrum are more obvious after annealing at 300 °C in air. The main peak at 399.8 eV shifts to 399.9 eV while the second peak at 398.5 eV drastically shifts to 399.1 eV and almost vanishes. Moreover, the fwhm of the peak widens to 2 eV from 1.5 eV (see Tables S1 and S2 for details). The S 2p spectra of PCDTPT films after annealing for 15 min in N2 and air are shown in Figure 2c and 2f, respectively. The spectrum can be resolved by fitting it with two components each of which is a doublet (S 2p3/2 and S 2p1/2) with intensity ratio of 2:1 due to the spin−orbit splitting.25−27 Because PCDTPT contains thiophene groups and thiadiazole groups per monomer, two different sulfur signals are indeed expected in the photoelectron spectra. The lower binding energy doublet (S1 species) which is observed at 164 eV (S 2p3/2) and 165.1 eV (S 2p1/2) originates from the core levels of sulfur in thiophene unit (Tables S1 and S2). This agrees well with the binding energy reported for S 2p3/2 in unbound thiophene (164 eV) and in other polymers with thiophene unit.25,28 The higher binding energy doublet (S2 species) is detected at 165.5 and 166.5 eV which can be attributed to the core levels of sulfur in thiadiazole group. This assignment is also consistent with reported binding energy for S 2p3/2 in (benzo)thiadiazole (166 eV).24,25 Again, no changes were detected in the S 2p spectrum with increasing temperature up to 350 °C in N2 and 150 °C in air. Only a slight shift (∼0.1 eV) to higher binding energy is observed even the temperature increases to 400 °C in N2 and 200 °C in air. Only after

In this letter, we report high-mobility polymer TFT (∼1.6 cm2 V−1 s−1) with remarkable thermal and air stability, which can withstand temperature up to 350 °C in nitrogen and 200 °C in air without significant changes in mobility and threshold voltage, by using semiconducting polymer poly[4-(4,4-dihexadecyl-4Hcyclopenta[1,2-b:5,4-b]dithiophen-2-yl)-alt[1,2,5]thiadiazolo [3,4c] pyridine] (PCDTPT) with molecular weight of ∼160 kDa. Integrated with metal-free moisture-resistance packaging, the thermally stable polymer TFT is able to endure the steam sterilizationheating to 121 °C for 30 min in pressurized autoclave (15 psi)which is above the practical safety margin for removal of micro-organisms, validating that it is well-suited for application in reusable medical devices. We also demonstrated sterilizable polymer TFT using biocompatible and flexible SU-8 dielectric layer,20 as an alternative to conventional SiO2 dielectric layer. The SU-8-based TFT exhibits a mobility of 1.3 cm2 V−1 s−1 which is comparable to that of SiO2-based TFT (1.4 cm2 V−1 s−1) and remains stable after exposure to the same sterilizing condition. Figure 1 shows the chemical structure (Figure 1a), UV−vis− NIR absorption spectra of PCDTPT thin film annealed for 15 min at various temperatures under N2 environment (Figure 1b) and in air (Figure 1c). Under N2 environment, the π−π* absorption spectra of the films are still stable after exposure to temperature as high as 350 °C. Similarly, a significant change in

Figure 1. (a) Chemical structure of semiconducting polymer PCDTPT. UV−vis-NIR absorption spectra of PCDTPT thin film on quartz substrate after exposure to elevated temperatures (b) in the N2 environment and (c) in the air. 9534

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Figure 2. XPS spectra of PCDTPT thin films after annealing at various temperatures: (a, d) C 1s core levels, (b, e) N 1s core levels, and (c, f) S 2p core levels. (a−c) Annealing in the N2 environment, (d−f) annealing in the air.

(Figure S1). The fibrillar structures with a fiber width of 20−30 nm and fiber length approaching 500 nm are also observed in the Atomic Force Microscopy (AFM) image of PCDTPT film (Figure S2). The thermal stability of transistor was carefully studied by first measuring the electrical characteristics of the asfabricated TFT at room temperature (RT)(∼25 °C), followed by heating at 100 °C for 15 min. The device was allowed to cool to RT and measured the characteristics. The same TFT was then heated again at 150 °C for 15 min, cooled down to RT and measured. This procedure was carried out until 400 °C in the steps of 50 °C. This set of experiments was conducted in a N2-filled glovebox. Another set of experiments was carried out in air, from 50 to 300 °C in the steps of 50 °C, using a separate device. Figure 3 shows the current−voltage (I−V) characteristics (transfer characteristics) of TFTs after exposure to various temperatures in N2 (Figure 3a) and air (Figure 3b), and also the mobility (Figure 3c) and threshold voltage (Figure 3d) extracted from the I−V curves. The as-fabricated mobility of the TFT which was tested in N2 is 1.6 cm2 V−1 s−1. The mobility is consistent up to exposure to temperature of 300 °C with variation of within ±20%. Even after exposure to temperature up to 350 °C, the mobility of the TFT remains as high as 1.2 cm2 V−1 s−1 which represents only 25% decrease in mobility compared to the as-fabricated value. Only after exposure to temperature of 400 °C, the mobility significantly drops to 0.6 cm2 V−1 s−1. Likewise, no change in threshold

annealing in air at 300 °C, the peaks substantially shift more than 0.5 eV to higher binding energy as well as the fwhm of the S 2p3/2 widens about twice. On the basis of the overall XPS analysis, we can conclude that the electronic structure of PCDTPT film is still stable after exposure to temperature as high as 350 °C in N2 and 200 °C in air. Previous studies suggested some possible factors influence on the stability of conjugated polymers. It is generally accepted that deepening the highest occupied molecular orbital (HOMO) increases the resistance to oxidation and improves the stability in air even at relatively high temperature.17,29 In addition, the rigid backbone structure through the use of more fused aromatic rings,18,30 and the fluorination of molecules (substitution of C−H with C−F on the backbone)31,32 have been proposed to improve the thermal stability of the conjugated polymer. Therefore, we attribute the stability of PCDTPT polymer to the relatively high HOMO level (−5.2 eV)33 and rigid backbone structure with fused aromatic rings. To examine the stability of PCDTPT film for use in device application, we fabricated TFT in bottom-gate and top-contact configuration (refer to the Supporting Information, experimental procedure details). Note that during the transistor fabrication, PCDTPT film was annealed at 200 °C for 8 min in a N2-filled glovebox to obtain an optimum performance. At this optimum condition, (100) peak associated with first order lamellar scattering is observed at 2θ ≈ 4.23° in the out-of-plane XRD spectrum, confirming the crystallinity of the polymer film 9535

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hexylthiophene)(P3HT), decreases more than 2 orders of magnitude after exposure to 200 °C (Figure S3). Our TFT also exhibits thermal stability in air. Even after exposure to the temperature of 200 °C in air, the mobility of TFT decreases to 1.2 cm2V−1s−1 only, from the as-fabricated mobility of 1.6 cm2V−1s−1 (25% decrease). The variation in threshold voltage also maintains within ±2.5 V. Only after the exposure to the temperature of 250 °C, the mobility substantially decreases to 0.7 cm2V−1s−1. From the thermal stability test, it is evident that the TFTs are stable up to 350 °C in N2 and up to 200 °C in air without substantial changes in mobility and threshold voltage. Despite thermal and air stability, the bare TFT without any barrier layer cannot withstand the steam sterilization condition. We observed that mobility of the bare TFT drops to 0.06 cm2V−1s−1 after sterilizing, most likely due to direct moisture attack (Figure S4). To protect against moisture from steam sterilization, we employed a heat-sealable composite film laminates comprising propylene (PP) film (15 μm), polychlorotrifluoroethylene (PCTFE) film (100 μm) and hot-melt adhesive film (30 μm) which are laminated to one another by an adhesive (Figure 4a). PCTFE film which has a relatively low water vapor transport rate (WVTR) of 0.06 g/m2/day (at 37.8 °C, 100% RH) and excellent moisture barrier property,34 serves as primary barrier layer. The top PP film layer imparts strength, dimensional stability and durability to the film laminate. This film layer also assists in rendering the laminate resistance to shrinkage when heated to elevated temperature. It is also worth mentioning that both PCTFE and PP are autoclavable with good heat resistance and can be used safely for several cycles. This whole stack of composite film laminates is attached to TFT by heat seal laminating. It covers all the area of device except the electrode contact. The edges of the laminates are finally sealed with water-resistant silicone.

Figure 3. Transfer characteristics of TFT fabricated with PCDTPT polymer after exposure to various temperatures (a) in the N2 environment and (b) in the air. (c) Mobility and (d) threshold voltage of the TFTs after exposure to various temperatures. The inset in d illustrates the device structure used in the experiment.

voltage was observed up to temperature of 300 °C. The threshold voltage is slightly shifted to −3.7 V and −4.1 V after exposure to 350 and 400 °C, respectively. In contrast, the mobility of TFT made of well-known polymer, poly(3-

Figure 4. (a) Structure of composite film laminates, which is used as a moisture-barrier in the device, and the chemical structure of propylene and polychlorotrifluoroethylene. (b) Device architecture of SiO2-based TFT encapsulated with the composite film laminates. (c) Transfer characteristics of the device before and after sterilization and (d) output characteristic of the device after sterilization. 9536

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Figure 5. (a) Chemical structure of SU-8. (b) Device architecture of SU8-based TFT (laminated-structure TFT) encapsulated with the composite film laminates. (c) Transfer characteristics of the device before and after sterilization and (d) output characteristic of the device after sterilization.

without additional process step. (Henceforth known as laminated-structure TFT). To fabricate this laminated-structure TFT, SU-8 was first spin coated on indium-doped tin oxide (ITO)-coated polyethylene naphthalate (PEN) film, where ITO serves as gate electrode and PEN is used as carrier substrate (See Experimental Section for details). PCDTPT film was separately spin-coated on an intrinsic silicon wafer which is predeposited with source/drain electrodes. Again, herein silicon wafer serves only as a carrier substrate and does not contribute to the functionality of the device. To complete the device, we laminated the PEN/ITO/SU-8 stack on the PCDTPT coated wafer (SU-8 and PCDTPT are in contact with each other) by hot-pressing them against each other while sandwiching between two sheets of heat-sealable composite film laminates (Figure 5b and Figure S8). The edges of the laminates are finally sealed with water-resistant silicone. The IDS−VGS characteristics of laminate-structured TFT before and after steam sterilization (heating to 121 °C for 30 min in pressurized autoclave at 15 psi) are shown in Figure 5c. Like SiO2-based TFT, the transfer curves of laminated-structure TFT before and after sterilization are almost identical and no change in threshold voltage is observed. To calculate the mobility of laminated-structure TFT, the capacitance measurement with frequency from 1 kHz to 10 MHz was performed (Figure S9). The measured capacitance of SU-8 film is ∼5.9 nF/cm2 (at 1 × 104 Hz). The thickness of SU-8 film is ∼500 nm as measured by surface profiler. The dielectric constant of SU-8 was calculated from measured capacitance and thickness, using the formula, C = ε0ε/t where C is the capacitance, ε0 is vacuum permittivity, ε is dielectric constant and t is the thickness of the film. From the calculation, the dielectric constant of SU-8 is determined to be 3.3, which is consistent with the value previously reported.37,38 The mobility of asfabricated laminated-structure TFT is 1.3 cm2 V−1 s−1, which is

The device structure of TFT integrated with protective packaging is illustrated in Figure 4b. Herein, we use a bottomgate, bottom-contact architecture instead of bottom-gate, top contact architecture because the latter causes “non-ideal” transfer characteristic in current−voltage curve after laminating (Figure S5). The IDS−VGS characteristics of encapsulated TFT before and after steam sterilization (heating to 121 °C for 30 min in pressurized autoclave at 15 psi) are shown in Figure 4c. The mobility of encapsulated TFT after sterilization is 1.2 cm2 V−1s −1 which represents only 15% decrease in mobility, compared to its as-fabricated mobility of 1.4 cm2 V−1 s−1. The transfer curves of before and after sterilization are almost identical and no change in threshold voltage is observed. The sterilized device exhibits low hysteresis with the similar current and turn-on voltage on the forward sweep and reverse sweep, suggesting that interface between polymer and dielectric is still stable even after sterilization, and does not cause charge trapping (Figure S6). The output curve of sterilized TFT also shows good saturation even at large gate voltage (Figure 4d) and is similar to that of as-fabricated device (Figure S7). We also examined the reusability of the transistor by sterilizing the transistor for three times. We found that there is no further drop in mobility from the second time onward. We also demonstrated sterilizable polymer TFT using SU-8 dielectric layer (Figure 5a) which has biocompatibility, good thermal stability and high dielectric breakdown voltage,20,35,36 as an alternative to conventional SiO2 dielectric. Furthermore, the usage of SU-8 brings about additional characteristics to device fabrication such as conferring the ability for varying device structure configuration as well as imparts mechanical flexibility as compared to conventional SiO2 dielectric layer. The device configuration chosen is top-gate and bottomcontact geometry using lamination approach, which is in line with lamination of barrier layer during encapsulation of device, 9537

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with a High Conductivity for Electronic Textile Applications. Nat. Commun. 2015, 6, 7461. (6) Mannsfeld, S. C. B.; Tee, B. C. K.; Stoltenberg, R. M.; Chen, C. V. H. H.; Barman, S.; Muir, B. V. O.; Sokolov, A. N.; Reese, C.; Bao, Z. Highly Sensitive Flexible Pressure Sensors with Microstructured Rubber Dielectric Layers. Nat. Mater. 2010, 9, 859−864. (7) Glowacki, E. D.; Apaydin, D. H.; Bozkurt, Z.; Monkowius, U.; Demirak, K.; Tordin, E.; Himmelsbach, M.; Schwarzinger, C.; Burian, M.; Lechner, R. T.; Demitri, N.; Voss, G.; Sariciftci, N. S. Air-stable Organic Semiconductors Based on 6,6[prime or minute]-dithienylindigo and Polymers Thereof. J. Mater. Chem. C 2014, 2, 8089−8097. (8) Lei, T.; Cao, Y.; Fan, Y.; Liu, C.-J.; Yuan, S.-C.; Pei, J. HighPerformance Air-Stable Organic Field-Effect Transistors: IsoindigoBased Conjugated Polymers. J. Am. Chem. Soc. 2011, 133, 6099−6101. (9) Schmoltner, K.; Schlutter, F.; Kivala, M.; Baumgarten, M.; Winkler, S.; Trattnig, R.; Koch, N.; Klug, A.; List, E. J. W.; Mullen, K. A Heterotriangulene Polymer for Air-Stable Organic Field-Effect Transistors. Polym. Chem. 2013, 4, 5337−5344. (10) Zhang, W.; Smith, J.; Watkins, S. E.; Gysel, R.; McGehee, M.; Salleo, A.; Kirkpatrick, J.; Ashraf, S.; Anthopoulos, T.; Heeney, M.; McCulloch, I. Indacenodithiophene Semiconducting Polymers for High-Performance, Air-Stable Transistors. J. Am. Chem. Soc. 2010, 132, 11437−11439. (11) Fukuda, K.; Yokota, T.; Kuribara, K.; Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Thermal Stability of Organic Thin-Film Transistors with Self-Assembled Monolayer Dielectrics. Appl. Phys. Lett. 2010, 96, 053302. (12) Reuveny, A.; Yokota, T.; Koizumi, M.; Kaltenbrunner, M.; Matsuhisa, N.; Sekitani, T.; Someya, T. Ultrathin, short channel, thermally-stable organic transistors for neural interface systems. 2014 Biomedical Circuits and Systems Conference (BioCAS); Lausanne, Switzerland, Oct 22−24, 2014 ; IEEE: Piscataway, NJ; 2014; pp 576− 579. (13) Huque, M. A.; Tolbert, L. M.; Blalock, B. J.; Islam, S. K. A HighTemperature, High-Voltage SOI Gate Driver IC with High Output Current and On-Chip Low-Power Temperature Sensor. International Symposium on Microelectronics; San Jose, CA, Nov 1−5, 2009 ; International Microelectronics Assembly and Packaging Society: Research Triangle Park, NC, 2009; p 220. (14) Greenwell, R. L.; McCue, B. M.; Zuo, L.; Huque, M. A.; Tolbert, L. M.; Blalock, B. J.; Islam, S. K. SOI-based Integrated Circuits for High-Temperature Power Electronics Applications. 2011 Twenty-Sixth Annual IEEE Applied Power Electronics Conference and Exposition (APEC); Fort Worth, TX, March 6−11, 201 ; IEEE: Piscataway, NJ, 2011; pp 836−843. (15) Tao, F.; Miao, J. Y.; Shi, G. Y.; Zhang, K. C. Ethanol Fermentation by an Acid-Tolerant Zymomonas Mobilis under NonSterilized Condition. Process Biochem. 2005, 40, 183−187. (16) Wang, J.; Sun, B.; Cao, Y.; Tian, Y.; Wang, C. Enzymatic Preparation of Wheat Bran Xylooligosaccharides and their Stability during Pasteurization and Autoclave Sterilization at Low pH. Carbohydr. Polym. 2009, 77, 816−821. (17) Cho, S.; Seo, J. H.; Park, S. H.; Beaupré, S.; Leclerc, M.; Heeger, A. J. A Thermally Stable Semiconducting Polymer. Adv. Mater. 2010, 22, 1253−1257. (18) Choi, D.; Jeong, B.-S.; Ahn, B.; Chung, D. S.; Lim, K.; Kim, S. H.; Park, S. U.; Ree, M.; Ko, J.; Park, C. E. Effects of Side-Chain Interdigitation on Stability: An Environmentally, Electrically, and Thermally Stable Semiconducting Polymer. ACS Appl. Mater. Interfaces 2012, 4, 702−706. (19) Kuribara, K.; Wang, H.; Uchiyama, N.; Fukuda, K.; Yokota, T.; Zschieschang, U.; Jaye, C.; Fischer, D.; Klauk, H.; Yamamoto, T.; Takimiya, K.; Ikeda, M.; Kuwabara, H.; Sekitani, T.; Loo, Y.-L.; Someya, T. Organic Transistors with High Thermal Stability for Medical Applications. Nat. Commun. 2012, 3, 723. (20) Nemani, K. V.; Moodie, K. L.; Brennick, J. B.; Su, A.; Gimi, B. In Vitro and In Vivo Evaluation of SU-8 Biocompatibility. Mater. Sci. Eng., C 2013, 33, 4453−4459.

similar to that of SiO2-based TFT. After sterilization, the mobility slightly decreases to 1.1 cm2 V−1 s−1 (about 15% reduction). The laminated-structure device also exhibits low hysteresis with the similar current and turn-on voltage on the forward sweep and reverse sweep even after sterilization (Figure S10). Likewise, the output curve of laminated-structure TFT also shows good characteristic even after sterilization (Figure 5d) and is similar to that of as-fabrication condition (Figure S11). In conclusion, we have demonstrated thermally stable TFTs that are able to endure the standard autoclave sterilization for reusable medical devices. The thermally stable PCDTPT semiconductor is used as channel layer while the biocompatible SU-8 polymer is used as a flexible dielectric layer in TFT, in addition to conventional SiO2. Encapsulating with nonmetallized composite film laminates as moisture barrier, both TFTs using either SiO2 or SU-8 dielectric layer exhibit good stability in sterilized conditions without significant change in mobility and threshold voltage. The mobility decreases only 15% of asfabricated value, whereas the low hysteresis with the similar current and turn-on voltage on the forward sweep and reverse sweep is still maintained after sterilization. Our TFT design along with experimental results shed light on the opportunity on organic/polymer flexible TFTs in sterilizable/reusable medical devices application.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00108. Experimental procedure details; parameters of core level XPS spectra; comparison of thermal stability between P3HT and PCDTPT; I−V characteristics of transistors; measurement of capacitance (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for support from A*STAR SERC TSRP grant (102 170 0137) and IMRE exploratory project fund (IMRE/14-1C0247). The authors also thank Zhang Zheng for XPS measurement.

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REFERENCES

(1) Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 2014, 26, 1319−1335. (2) Gelinck, G.; Heremans, P.; Nomoto, K.; Anthopoulos, T. D. Organic Transistors in Optical Displays and Microelectronic Applications. Adv. Mater. 2010, 22, 3778−3798. (3) Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Flexible Organic Transistors and Circuits with Extreme Bending Stability. Nat. Mater. 2010, 9, 1015−1022. (4) Lin, P.; Yan, F. Organic Thin-Film Transistors for Chemical and Biological Sensing. Adv. Mater. 2012, 24, 34−51. (5) Matsuhisa, N.; Kaltenbrunner, M.; Yokota, T.; Jinno, H.; Kuribara, K.; Sekitani, T.; Someya, T. Printable Elastic Conductors 9538

DOI: 10.1021/acsami.6b00108 ACS Appl. Mater. Interfaces 2016, 8, 9533−9539

Letter

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DOI: 10.1021/acsami.6b00108 ACS Appl. Mater. Interfaces 2016, 8, 9533−9539