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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 43004−43012
Human Hair Keratin for Biocompatible Flexible and Transient Electronic Devices Jieun Ko,† Luong T. H. Nguyen,‡ Abhijith Surendran,† Bee Yi Tan,‡ Kee Woei Ng,‡,§ and Wei Lin Leong*,†,∥ †
School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore ‡ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore § Environmental Chemistry and Materials Centre, Nanyang Environment & Water Research Institute, Nanyang Technological University, Singapore 637141, Singapore ∥ School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, Singapore 637459, Singapore S Supporting Information *
ABSTRACT: Biomaterials have been attracting attention as a useful building block for biocompatible and bioresorbable electronics due to their nontoxic property and solution processability. In this work, we report the integration of biocompatible keratin from human hair as dielectric layer for organic thin-film transistors (TFTs), with high performance, flexibility, and transient property. The keratin dielectric layer exhibited a high capacitance value of above 1.27 μF/cm2 at 20 Hz due to the formation of electrical double layer. Fully solution-processable TFTs based on p-channel poly[4-(4,4dihexadecyl-4H-cyclopenta[1,2-b:5,4-b]dithiophen-2-yl)alt[1,2,5]thiadiazolo[3,4-c]-pyridine] (PCDTPT) and keratin dielectric exhibited high electrical property with a saturation fieldeffect mobility of 0.35 cm2/(Vs) at a low gate bias of −2 V. We also successfully demonstrate flexible TFTs, which exhibited good mechanical flexibility and electrical stability under bending strain. An artificial electronic synaptic PCDTPT/keratin transistor was also realized and exhibited high-performance synaptic memory effects via simple operation of proton conduction in keratin. An added functionality of using keratin as a substrate was also presented, where similar PCDTPT TFTs with keratin dielectric were built on top of keratin substrate. Finally, we observed that our prepared devices can be degraded in ammonium hydroxide solution, establishing the feasibility of keratin layer as various components of transient electrical devices, including as a substrate and dielectric layer. KEYWORDS: keratin, high capacitance, gate insulator, transient electronics, thin-film transistors, biocompatible
1. INTRODUCTION The race against global climate change and electronic pollution can be eased substantially with the development of green electronic devices that are composed of naturally abundant materials or biomaterials for full recyclability.1−3 In addition, natural materials isolated from the native environment and repurposed into thin films can be useful in building a form of biocompatible electronics for applications such as sensors for food monitoring, noninvasive diagnostic systems to improve patient care, as well as bioresorbable, transient electronics, which completely disappears in a controlled fashion after fulfilling its duty, paving the way for electronics to be everywhere, including on the skin or even within our bodies.4−6 In particular, the solution processability of these biomaterials is well suited for the production of flexible or organic electronic devices, such as organic memory devices, chemical sensors, energy storage devices, and organic thin-film transistors © 2017 American Chemical Society
(TFTs), to achieve sustainable, low-cost, and large-scale production. One major advantage is that these biomaterials can be processed from environmentally friendly solvents (such as water and alcohol), which also allows facile deposition of natural compounds atop the subsequent active layers with orthogonal solubility. Specifically, natural biopolymers can be leveraged as biodegradable dielectrics in organic TFTs, including gelatin,7,8 chitosan,9−11 albumin,12−15 and silk fibroin.16−18 To also achieve high-performance organic TFTs, which operate at low voltages, it is essential for the gate insulators to possess high capacitance values. Previous work has shown that improved capacitance/dielectric constants of the gate insulator could Received: October 27, 2017 Accepted: November 21, 2017 Published: November 21, 2017 43004
DOI: 10.1021/acsami.7b16330 ACS Appl. Mater. Interfaces 2017, 9, 43004−43012
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematics of keratin from human hair. The structure and various chemical bonds in keratin are also illustrated. For instance, hydrogen bond forms between −NH and CO. Note the carbon and nitrogen of CO and −NH are in the main chain of keratin. (b) Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrum of keratin powder.
enhance the field-effect mobility of the TFTs.19,20 Biopolymers with high densities of aromatic and polar groups can serve as high-capacitance gate insulator for logic devices, serving as the foundation for more complex devices. The biopolymers typically showed high capacitance values, in the range of ∼nF/cm2 to ∼μF/cm2 (Table S1). Keratins are sulfur-containing fibrous structural proteins found in hair, wool, feathers, and nails that have robust physical properties.21 In comparison to other animal sources, keratin from human hair (Figure 1a) has distinct advantages of having an abundant supply and a realistic source of autologous proteins for biocompatible electronic devices.22,23 In addition, soluble human hair keratins can be easily obtained through a concoction of reducing agents under acidic or alkaline condition. The extracted human keratins are solutionprocessable and have been fabricated into various forms of coatings,24−26 fibrous matrices,27 hydrogels,23 and sponges.28 However, up to now, most of the keratin research has focused on biological applications, such as porous scaffolds for tissue engineering or substrates for in vitro studies. Here, we report, for the first time, keratin from human hair as a high-capacitance dielectric layer as well as a supporting substrate for organic TFTs. The keratin dielectric layer exhibited good solution processability and high capacitance values of above ∼1.27 μF/cm2 at 20 Hz due to its high dipolar nature and hygroscopicity. We have successfully utilized the keratin dielectric layer as a gate insulator by fabricating organic thin-film transistors based on p-channel donor−acceptor copolymer poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4b]dithiophen-2-yl)-alt[1,2,5]thiadiazolo[3,4-c]-pyridine] (PCDTPT). The PCDTPT/keratin TFTs exhibited good charge-transport properties with a field-effect mobility of 0.35 cm2/(Vs) at a low gate voltage bias of −2 V. Flexible PCDTPT/keratin TFTs fabricated on a polymer substrate also maintained the electrical properties before and after bending at 20 mm bending radius. Additionally, an artificial electronic synaptic PCDTPT/keratin transistor was realized and exhibited high-performance synaptic memory effects via simple operation of mobile ion conduction in keratin dielectric. An added functionality of using keratin as a substrate was also demonstrated, where similar organic TFTs were built on top of keratin substrate. Finally, the transient nature or disintegrability of keratin layer was investigated with PCDTPT/keratin TFT on keratin substrate, where we observed that our prepared devices can be degraded in alkaline solution, further
demonstrating their potential for biocompatible and bioresorbable electronics.
2. EXPERIMENTAL SECTION 2.1. Materials. Human hair was obtained from local hairdressers. Poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b]dithiophen-2-yl)alt[1,2,5]thiadiazolo[3,4-c]-pyridine] (PCDTPT) was purchased from 1-Material (CAS#1334407-47-4). Ammonium hydroxide solution in water (28−30%) was purchased from Fisher Scientific. 2.2. Extraction of Human Hair Keratin. The keratin used in this study is extracted from human hair through a reduction process, which breaks the disulfide bonds in the original hair structure. The resulting extracted keratin therefore consists predominantly of keratin monomers. Briefly, the extraction of keratin from human hair is described as follows. Human hair samples were first washed with soap to remove dirt and grime. After rinsing, the hair was soaked in ethanol and left to dry at room temperature in a fume hood. Delipidization of the hair, where all lipids present in the top layer of hair are removed, was subsequently conducted by soaking the dried hair into a mixture of chloroform and methanol (2:1). After delipidization, the dried hair was cut to lengths of 1−2 mm with scissors. Keratin was then extracted by immersing the cut hair in 0.125 M sodium sulfide nonahydrate (Na2S· 9H2O) (purity >98%, ACS reagent, ACROS Organics) aqueous solution at 40 °C for 4 h. Next, the mixture was filtered with filter papers to remove hair debris. The keratin solution was subsequently transferred to a cellulose tubing of 10 kDa molecular weight cutoff and exhaustively dialyzed against deionized water to remove the remaining sodium sulfide. The dialyzed solution was then freeze-dried to obtain keratin powder and stored at −20 °C until use. For the keratin substrate, an enriched fraction of keratin intermediate filament proteins (KIFP) was utilized, following a separation protocol described previously.29 Briefly, keratin-associated proteins were first removed by incubating human hair in pH 9 Tris− HCl buffer containing 8 M urea, 200 mM dithiothreitol (DTT), and 25% ethanol at 50 °C for 72 h. The KIFP fraction was then separated by incubating human hair residue in pH 8.5 Tris−HCl buffer containing 5 M urea, 2.6 M thiourea, and 200 mM DTT for 24 h at 50 °C. After dialysis against deionized water, the KIFP solution (20 mg/ mL) was cast on a glass substrate and dried at room temperature for 2 days. The resultant keratin substrate thickness was approximately 5 μm. 2.3. TFT Fabrication and Electrical Characterization. The PCDTPT/keratin TFT device was fabricated on the heavily doped ptype Si (100) wafer. The wafer was cleaned with acetone and isopropyl alcohol in an ultrasonic bath for 10 min each and then dried with nitrogen. The cleaned substrates undergo plasma treatment for 30 min. Keratin solution was prepared by dissolving keratin powder in distilled water (keratin/distilled water = 1.5:8.5), and the solution was centrifuged at 10 000 rpm for 15 min. The keratin solution was spincoated on the wafer substrate at 2000 rpm for 30 s, followed by 43005
DOI: 10.1021/acsami.7b16330 ACS Appl. Mater. Interfaces 2017, 9, 43004−43012
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Topography image of keratin film on a p++-Si substrate. (b) Water contact angle of keratin film. (c) Capacitance vs frequency result of keratin layer under ambient and vacuum condition (p++-Si/keratin 646.7 nm/Al, frequency range: 20 Hz−1 MHz). The inset shows the crosssectional SEM image of spin-coated keratin thin films with 646.7 nm thickness. (d) Schematic of EDL formation in the keratin film. thermal annealing at 70 °C for 10 min. Semiconductor solution was prepared by dissolving 4 mg of PCDTPT in 1 mL of chlorobenzene. The semiconductor film was coated on keratin dielectric layer at 1500 rpm for 60 s under an argon atmosphere. The resultant films were then annealed at 200 °C for 10 min. Finally, the gold source and drain contacts (thickness, 100 nm) were evaporated through a shadow mask, thereby defining a channel length of 50−100 μm and width of 300− 500 μm. Reference PCDTPT devices have a 200 nm thick thermally grown SiO2 layer only. For capacitor device, Al electrode (100 nm) was evaporated on keratin film coated on the heavily doped p-type Si (100) wafer by thermal evaporation. The flexible PCDTPT/keratin TFT was fabricated on polyarylate (PAR) film. Ag bottom gate electrode (100 nm) was first evaporated on PAR substrate, and the same fabrication processes of PCDTPT and keratin are followed. Similar devices are also built on keratin substrates. The output and transfer characteristics of the transistors as well as current−voltage characteristics of the capacitor devices were measured under ambient and vacuum conditions (
