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Mechanically Adaptive Organic Transistors for Implantable Electronics Jonathan Reeder, Martin Kaltenbrunner, Taylor Ware, David Arreaga-Salas, Adrian Avendano-Bolivar, Tomoyuki Yokota, Yusuke Inoue, Masaki Sekino, Walter Voit, Tsuyoshi Sekitani, and Takao Someya* Recent progress in flexible electronics has begun to decouple high-performance electronic properties from rigid mechanical behavior, and has enabled new classes of flexible, stretchable, and textile electronics.[1–6] Soft electronic devices are able to facilitate methods for addressing healthcare problems that require chronic device implantation and have been demonstrated for use in stimulation and recording of the central[7–9] and peripheral[10,11] nervous system, and mapping cardiac electrophysiology.[12] Intimate conformability is essential for obtaining stable interfaces on complex surfaces such as skin, tissue, or organs. Significant reduction of substrate thickness has enabled ultraflexible and conformal electronics for characterizing thermal properties of human skin,[13] and organic electronics such as tactile sensing skins,[14] skins for recording electrocardiograms[15] and implantable electrodes for recording neural activity.[9] This paves the way for future biomedical devices where the bulk mechanical properties of the devices can be designed independently of an flexible, ultrathin electronic layer. Requirements for acute and chronic bioelectronics include the need to fabricate consistent, reliable devices with stable electrical properties, which are robust and resilient at mild to severe room temperature deformation during surgical handling, which can be implanted with precision near small biological areas of interest, and which maintain electrical properties during interaction with tissue. The device durability, ease of handling, and the immune response of bioelectronics after implantation are in part determined by the mechanical and geometric properties of the substrate, which makes the
J. Reeder, Dr. M. Kaltenbrunner, Dr. T. Yokota, Dr. Y. Inoue, Prof. M. Sekino, Prof. T. Sekitani, Prof. T. Someya Electrical and Electronic Engineering and Information Systems The University of Tokyo 7–3–1 Hongo, Bunkyo-ku, Tokyo 113–8656, Japan E-mail: [email protected] J. Reeder, Dr. T. Ware, D. Arreaga-Salas, A. Avendano-Bolivar, Prof. W. Voit Department of Materials Science and Engineering The University of Texas at Dallas 800 W. Campbell Rd., Richardson, Texas 75080–3021, USA Dr. M. Kaltenbrunner, Dr. T. Yokota, Dr. Y. Inoue Prof. M. Sekino, Prof. T. Sekitani, Prof. T. Someya Exploratory Research for Advanced Technology (ERATO) Japan Science and Technology Agency (JST) 2–11–16, Yayoi, Bunkyo-ku, Tokyo 113–0032, Japan
DOI: 10.1002/adma.201400420
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substrate design critical. Indeed, electronics on silicon and glassy polymers typically respond poorly to implantation in soft biological tissue (10–500 kPa), due to their high modulus (>100 GPa and 1–10 GPa, respectively), and it has been shown that soft electronic substrates may improve the chronic viability of biomedical devices.[16] As a result, extensive research has been performed on low modulus substrates such as poly(dimethylsiloxane) (PDMS) and elastomeric polyurethanes (100 kPa–3 MPa) as soft substrates for bioelectronics,[17,18] though the deformability of the substrate makes high-yield fabrication difficult and precludes these devices from penetrating soft tissue. For these reasons, it is desirable to use an electronic substrate which can change mechanical properties in vivo to improve the modulus and geometry match with body tissue after rigid insertion. Stimuli-responsive materials, especially shape-memory polymers (SMPs),[19] have attracted much attention and have enabled electronics which soften when exposed to physiological conditions.[20,21] For example, organic thin-film transistors (OTFTs) manufactured on 10 µm-thick plastic film were laminated to a programmable SMP layer to electrically functionalize the surface of medical catheters.[22] Additionally, lithographically patterned OTFTs have been manufactured directly on SMP substrates and were demonstrated to have stable device properties during multiple shape-memory cycles.[23] However, in order to fabricate OTFTs on SMPs for implantable electronics, it is critical to simultaneously achieve both high mobility and low voltage operation of OTFTs on shape changing substrates which adapt at physiologically relevant temperatures, which will enable the detection of small body signals and limit undesirable physiological responses. Additionally, in order to examine the feasibility of OTFTs on mechanically active substrates for implantable electronics, such as deployment triggered by body temperature, reliability and stability of the devices inside the body has to be established. By exploiting the features of shape-memory polymer substrates, we demonstrate shape changing and softening organic electronics which softly conform or deploy into 3D shapes after exposure to a stimulus. Fabrication of low-voltage-driven organic thin-film transistors directly on stimuli-responsive polymers allows for electronics fabrication on a rigid substrate which subsequently softens and changes shape after heating above the glass-transition temperature of the substrate. We also demonstrate a novel fabrication technique for creating 3D electronics with soft and deployable form factors. Using this technique, we demonstrate devices which can be inserted through a small slit and deployed to a 3D shape in order to grasp a cylinder
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25 µm-thick film of a monomer solution and polymerizing under 365 nm UV radiation. The resulting surface of the SMP was evaluated by atomic force microscopy (AFM) and revealed the surface roughness to be 0.3 nm (root mean square) (Figure S1, Supporting Information). A 5 nm-thick layer of Al was used to protect the SMP substrate from degradation during later fabrication processes. Figure 1d illustrates OTFTs which are fabricated on a 1.4 µm PET film and encapsulated by 1 µm of Parylene. 100 µm of SMP monomer solution is subsequently cast on top to finish the device. For OTFT with direct fabrication, the aluminum oxide dielectric (5 nm) was formed through plasma-ashing of the gate electrode. For OTFT with SMP coating, the aluminum oxide dielectric was potentiostatically anodized (19 nm). In vivo conforming and deployable OTFT tests were measured using OTFTs with SMP coating. Bending and dry conforming tests were Figure 1. OTFTs on SMP substrates. a) Illustration of an OTFT on an SMP substrate conforming to warm surface. b) OTFTs are able to deploy from their planar fabrication state to measured for OTFTs with direct fabrication. The optical microscopy image in Figure 2a helices with r = 2.25 mm Scale bar is 10 mm. c) Cross-section of an OTFT on fabricated on an shows a completed OTFT with direct fabriSMP substrate. Aluminum serves as the gate contact, and aluminum oxide and an n tetradecyl phosphonic acid SAM form the dielectric. The organic semiconductor DNTT ensures air-stable cation, whose channel width and length are operation, and gold contacts complete the device. d) Cross-section of an OTFT fabricated on 1000 µm and 100 µm, respectively. Following an ultrathin polymer foil and coated with SMP. fabrication on an SMP substrate, OTFTs were evaluated in their planar state with a VDS of −2.0 V and a VGS swept from 0 to −2.0 V. Figure 2b with radius of 2.25 mm when exposed to a small temperature change. When implanted in physiological tissue, initially depicts the transfer curve and gate leakage current (left axis), as planar devices soften and conform to the biological environwell as the square root of IDS (right axis) which is the basis for ment and withstand 24 hours of fluid exposure and deformaderiving mobility and the threshold voltage. Figure 2c depicts tions associated with implantation in living tissue while mainthe output curves of one of these devices, which exhibits a drain taining electrical properties without significant degradation. current of more than 6 µA when VGS and VDS are −2.0 V. The The room temperature bending stability is evaluated to deteraverage mobility of ten devices tested in the planar state was mine bending limits and methods of degradation of various 1.51 ± 0.12 cm2 V−1 s−1, with a threshold voltage of −1.02 ± 0.35 V, bending configurations with respect to bending direction and and an average on/off current ratio of 104. stress type. Air stability and high mobility are achieved by using Figure 3 shows OTFTs fabricated on 25 µm-thick SMP subthe high-performance organic semiconductor dinaphtho[2,3strates conforming to 3D surfaces after heating above the Tg. b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT).[24] Low-voltage operaFigure 3a shows the planar device resting on a cylindrical rod with a radius of 5 mm. After warming, the subsequent drop tion of 3 V is achieved by using an ultrathin hybrid dielectric in substrate modulus causes the device to succumb to gravity layer comprised of aluminum oxide and a self-assembled and conform to the supporting structure. Subsequent smaller monolayer (SAM).[25,26] Robust electrical performance during conformal radii down to r = 2 mm are examined. Conforming shape changing is enabled by a low-stress glass transition of the tests were performed in a dry environment, so a temperature of SMP substrate, which is essential for maintaining the integrity 70 °C was used to simulate the glass transition and facilitate the of thin dielectric layers during deformations. modulus drop of the SMP. The shear modulus of the substrate Mechanically adaptive OTFTs are fabricated by two methods. as measured by dynamic mechanical analysis (DMA) is approxThe first type of OTFT is manufactured directly on SMP subimately 4.0 MPa at 70 °C, 80 MPa when at 37 °C in fluid in the strates (Figure 1c), which is hereafter referred to as OTFT with body and at 600 MPa dry at 25 °C.[27] As shown in Figure 3b, direct fabrication. The second type is manufactured on 1.4 µm [ 14 ] poly (ethylene terephthalate) (PET) foils, the electric characteristics of OTFT are slightly changed due to encapsulated by heating and deformation: the mobility decreases only by 0.1% 1 µm of Parylene, and subsequently coated with a layer of and 1.1%, respectively, when heated and conformed to radii SMP (Figure 1d) using a transfer-by-polymerization process,[21] of 3 mm and 2 mm. The Vth shifts by −0.06 V and −0.14 V, which is referred to as OTFT with SMP coating. This transferby-polymerization process utilizes the thin PET film as the resulting in 10% and 21% drops in the on current, respectively. device encapsulant, as well as enables the fabrication of 3D We demonstrate that a planar OTFT which responds to electronic structures as will be shown later. The SMP substrates a temperature change and deploys into a helix after being for OTFT with direct fabrication are formed by spin coating a inserted through a 150 µm-thick opening in a thermal barrier
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(Movie S1, Supporting Information), as shown in Figure 4. This structure is created by reforming the SMP substrate during the SMP polymerization step which allows the globally stable shape to be set to an arbitrary 3D geometry, in this case a helix with a radius of 2.25 mm. The SMP is cast on the ultrathin OTFT (2 µm), which adheres to and remains compliant to the mechanical behavior of the SMP. After 3D polymerization, the substrate and active device are heated and deformed flat which primes the structure to be deployed back into its initial 3D shape. Once the device is positioned while flat, heat is applied (70 °C) which causes the SMP film to recover to its initial helical shape (Figure 4a). Device performance was evaluated after 3D fabrication, after heating and flattening, and after thermal shape recovery. Figure 4b shows the transfer curve shifts slightly negative after heating and flattening, and once again after thermal shape recovery. The active device is subjected to 0.8% compressive strain as it recovers to a radius of 2.25 mm. The Vth shifts by −0.22 V and −0.41 V when deformed flat and subsequently recovered to a helix, respectively. The mobility decreases by 0.7% and 0.8%, and the on current dropped by 24% and 36%, respectively.
Conforming behavior of devices in vivo is also examined. An OTFT fabricated on a PET film and encapsulated with 1 µm of Parylene and 100 µm of SMP was implanted under the skin of a rat for 24 hours to evaluate the effects of body fluids and constant deformation on device performance. Figure 5a demonstrates that the devices are conformed in vivo by body temperature without any external thermal sources. The tissue surrounding the devices moved freely due to respiration and voluntary behavior. OTFT characteristics were evaluated before implantation and after explantation and exhibit a +0.7 V shift of the Vth, 0.6% drop in on current, and an approximate 32% drop in mobility after removal from the tissue (Figure 5b). There was a slight increase in off current and gate current after explanting. The explanted device also exhibits a small hysteresis with higher current on the back-sweep than front-sweep. In addition to stability inside the body, it is also important to characterize the effect of stresses at room temperature. A device must be able to withstand multidirectional stresses from both compressive and tensile bending at room temperature before implantation so that surgeons can handle devices without inducing bending-related degradation. Four bending
Figure 3. Conforming behavior of an OTFT. a) An initially planar OTFT, which after heating conforms to radii of r = 5 mm (top right), r = 3 mm (bottom right), and r = 2 mm (bottom left). Transistors fabricated on 25 µm-thick SMP substrates were heated to 70 °C (Tg + 10 °C). b) Limited degradation is shown in devices after heating and conforming to radii of 3 mm and 2 mm. The scale bars are 10 mm.
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Figure 4. A deploying OTFT. a) A planar OTFT deploys into a helix and wraps around a rod (r = 2.25 mm) after it passes through a 150 µm-thick opening in a thermal barrier. A clear divider separates ambient air (background) from warm air (surrounding the rod). b) Comparing the transfer curves shows slight shifting of the Vth after both thermal deformation steps. The scale bars are 10 mm.
configurations were employed to test how tensile and compressive stresses, as well as the direction of the applied stress, may change the OTFTs in these conditions. Bending tests indicated that OTFTs in all four configurations were functional, as shown in Figure S2 in the Supporting Information, at bending radii less than 1 mm, with some withstanding bending to radii of 100 µm, which is robust enough for surgeons to prepare devices before implantation. Figure S3 in the Supporting Information provides a visual investigation of modes of degradation and failure for devices subjected to extreme bending (r = 100 µm, 11% strain present in active layer). Movie S2 in the Supporting Information shows a bending test from r = 10 mm to r = 100 µm and Figure S4 in the Supporting Information shows the bending setup for each configuration. This work describes OTFTs on SMP substrates which can mechanically adapt from planar geometries into 3D shapes when heated above the SMP glass-transistion temperature
or when exposed to physiological conditions. High mobility is achieved by using DNTT as the semiconductor and results in an average mobility of 1.51 ± 0.12 cm2 V−1 s−1 (Figure 2), which is partly enabled by the ultrasmooth surface of the SMP substrate (0.3 nm root mean square, Figure S3, Supporting Information).[28] The use of a thin plasma-formed or anodic aluminum oxide dielectric and an n-tetradecyl phosphonic acid SAM ensures low-voltage operation and decreases the interfacial trap density.[25,26] Low-voltage operation is essential for realizing implantable electronics, and was a limiting factor of the only mechanically responsive OTFT devices described in the literature to date.[23] It is important to note the stability of the dielectric after the SMP layer is heated above the Tg, indicating there is not significant stress generated in the SMP during progression through the glass transition, such as thermal stress. A low-stress transition is imperative to achieving stable electronic performance during shape changing due to the thinness
Figure 5. In vivo conforming behavior of an OTFT. a) An OTFT coated with SMP conforms to the body tissue of a living rat after being implanted for 24 hours. b) Comparison of the transfer curves before implantation and after explantation show good electrical properties after exposure to body fluids and deformation associated with implantation in living tissue for 24 hours, exhibiting only a small positive shifting of the Vth after explanation. The slight increase in off and gate current are the result of a partially conductive substrate caused by water uptake in the SMP. The scale bars for (a) are 5 mm for the top two images and 2 mm for the bottom, large image.
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in mechanical stiffness does not damage or destroy the electrical properties of the OTFTs. The small increase in off current and gate current exhibited is due to water uptake in the SMP layer, which results in a slightly conductive substrate. After explantation, the device retained its conformed geometry due to cooling and shape fixing of the substrate. It is important to note that the applied temperature for the conforming experiment in Figure 3 (70 °C) is much higher than the typical body temperature. However, when devices with an SMP layer are exposed to physiological conditions, a small amount of water uptake (3%) facilitates plasticization of the polymer network causing the Tg to drop to near body temperature.[27] The Tg of the SMP was carefully designed for rigid implantation in biological tissue and subsequent softening and conforming. Considering this effect gives surgeons adequate time for implantation before softening. Two processing methods were used for coupling OTFTs and SMPs, namely, OTFTs with direct fabrication on SMPs, and OTFTs with SMP coating. OTFTs with direct fabrication were patterned directly on the SMP substrate, while OTFTs with SMP coating were patterned on a 1.4 µm PET film and encapsulated with Parylene and then coated with an SMP layer using a transfer-by-polymerization process. For both processing methods, the SMP layer can be partially polymerized, deformed into a complex 3D shape, and then fully cured, allowing for the fabrication of 3D electronics. However, partially polymerized films continue to polymerize at a slow rate even after removal from UV exposure due to the thermally induced reaction of unreacted functional groups at room temperature. For this reason, OTFTs for deployable devices were fabricated on a separate 1.4 µm PET layer before coating with SMP, as it minimized the time in between the partial polymerization of the SMP and reshaping of the device into a helix. In vivo stability studies were performed using OTFTs coated with SMP so that the active device layer was encapsulated by the 1.4 µm PET substrate and 1 µm Parylene on opposite sides, which had been demonstrated as a suitable encapsulation scheme for acute in vitro tests.[14] In conclusion, we demonstrate OTFTs that are planar during fabrication but can adapt to ultimately interface with an arbitrary 3D or dynamic surface by exploiting the mechanical properties of shape-memory polymers. As these devices are exposed to fluids in the body and move through the substrate Tg, their electrical properties are resilient. After the initial shape changes in the device due to softening after insertion, the device is able to withstand the natural motion of the surrounding soft tissue. Additionally, we present OTFTs that can not only conform to complex shapes in physiological environments, but also deploy into a 3D shape to actively grip a target site. These acutely physiologically stable OTFTs have the potential to address challenging biomedical problems by enabling new means of creating intimate interfaces between body tissue and highperformance, robust bioelectronics.
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of the hybrid aluminum oxide and SAM dielectric. This excellent dimensional stability of the substrate through the glass transition can be attributed to the highly crosslinked polymer network of the SMP.[29] Devices soften and conform to cylinders after heating above the Tg of the SMP substrate. In Figure 3, conforming to a 3D surface is triggered by heating the device above the substrate Tg. Without causing any significant electrical degradation, the device was conformed to radii as small as 2 mm, which is similar in dimension to common peripheral nerve cuffs.[30,31] Devices were heated to 70 °C to enable conforming, as this temperature is above the glass-transition region when the SMP substrate is dry. When exposed to physiological conditions the glass-transition temperature of the SMP shifts towards body temperature, enabling physiological conforming. Relatively small changes in mobility and Vth after heating to 70 °C indicates that the temperature increase is not detrimental to the device, and is corroborated by recent work by Kuribara et al. which report that similar DNTT-based transistors show small changes in device properties below 90 °C.[32] There was also no change in gate leakage current after conforming, indicating that the aluminum oxide dielectric remains pinhole-free even after heating above the Tg of the SMP and conforming to radii as small as 2 mm. This stable dielectric behavior for the conforming is encouraging for creating future devices which can be deformed above the substrate Tg without significant changes in dielectric properties. Devices are able to be deployed to actively grip 3D surfaces after insertion through a thin slit to simulate a minimally invasive surgery. In Figure 4, an OTFT was deployed to a programmed shape that is 15× larger than the insertion profile of the device when heated above the Tg of the substrate. Convective heating causes shape recovery of the polymerized shape of the SMP as the device passes through a small opening, actively gripping the cylinder on the warm side of the thermal barrier. This makes these devices very appealing for minimally invasive surgeries. By reshaping the substrate during polymerization, the globally stable state of the SMP can be set to any arbitrary 3D shape. When the substrate is heated and deformed flat and then quenched, these strains can be stored using the shape-memory effect to create a device which deploys into the polymerized, 3D shape. The similar amount of shifting of the Vth after both instances of heating indicates that thermal degradation plays a more important role than the 0.8% compressive bending strain in the device degradation. This is supported by the results shown in Figure S2 in the Supporting Information, which shows negligible degradation of OTFTs bent at room temperature to impose compressive stress at similar radii. As can be seen with the conforming experiment, there was no change in gate leakage current, indicating a pinhole-free dielectric layer after deploying. Acute in vivo stability is also demonstrated. In Figure 5, adaptive OTFTs were conformed in vivo using only physiological conditions to trigger softening of the substrate. An initially rigid, planar OTFT with SMP coating was implanted near the abdomen of a living rat, and over the first few hours after surgery, fluid uptake plasticized the SMP and triggered softening from above 1 GPa to approximately 80 MPa. This large change
Experimental Section SMP Substrate Synthesis: Tricyclodecane dimethanol diacrylate (TCMDA), 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO),
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and 2,2-dimethoxy-2-phenyl acetophenone (DMPA) were purchased from Sigma–Aldrich. Tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate (TMICN) was purchased from Wako Chemicals. All the chemicals were used as received without further purification. The thiol–ene/acrylate SMP composition consisted of stoichiometric quantities of TATAO and TMICN and 31 mol% TCMDA. 0.1 wt% DMPA of total monomer concentration was dissolved into the solution. The vial was covered in aluminum foil to prevent incident light from contacting the monomer solution and kept at room temperature. The appropriate amount of TMICN was added to the covered vial. Without exposing the solution to light, the vial was mixed thoroughly by vortexing and sonication until there were no visible air bubbles in the solution. To make the SMP substrate, monomer solution was spun at 1500 rpm for 30 s to form a 25 µm-thick film. When OTFTs were fabricated on ultrathin polymer foils, 100 µm of SMP were cast on top of the device using glass slides and spacers as a mold. Polymerization was performed using a crosslinking chamber with five overhead 365 nm UV bulbs. Polymerization consisted of UV exposure for a total of 20 min, followed by post-curing the SMP substrates at 120 °C for 12 h, or at 75 °C for 45 min for SMP cast on OTFTs fabricated on ultrathin polymer foils. More information about the SMP substrate can be found in the Supporting Information and additional discussion of the materials and methods for preparation can be found in the literature by Ware et al.[27] OTFT Fabrication on SMP Substrates: SMP substrates were prepared for device fabrication by evaporating 5 nm of aluminum over the surface of the film. This layer served as a protective shield which prevented degradation of the substrate during subsequent processes, such as oxygen plasma treatment. A 30 nm-thick aluminum gate electrode was then patterned via shadow mask using thermal evaporation, and exposed to oxygen plasma (5 min at 300 W) to form an approximately 5 nm-thick aluminum oxide dielectric layer. A self-assembled monolayer (SAM) of an n-tetradecyl phosphonic acid was deposited on the aluminum oxide surface by submersing the gate electrodes in a 5 mM solution of the SAM molecules in isopropanyl alcohol for 12 h. DNTT, 30 nm thick, was then thermally evaporated on top of the SAM layer at a pressure of 1 × 10−6 mbar and a rate of 0.1 Å/s. Subsequently, 60 nm of Au was thermally evaporated to form the source and drain to complete the device. OTFT Fabrication on Ultrathin Polymer Foils: OTFTs on ultrathin polymer foils have an identical structure to devices fabricated on SMP substrates, with the exception of thicker anodically formed aluminum oxide of 19 nm. After source/drain fabrication, 1 µm of Parylene diX-SR was used to encapsulate the device followed by 100 µm of SMP. Additional information on the fabrication of OTFTs on ultrathin polymer foils can be found in the literature by Kaltenbrunner et al.[14] Conformability Testing: For conforming outside the body, 25 µm-thick SMP substrates were fabricated and OTFTs direct patterned on the SMP surface. Using convective heating, the environment surrounding the device and supporting cylinders with various radii were heated to 70 °C. For in vivo stability tests, 100 µm of SMP was cast on top of Parylene diX-SR encapsulated OTFTs and polymerized and post cured for 45 min at 75 °C. The protocol for the animal experiment was approved by the Institutional Animal Care and Use Committee of the Graduate School of Engineering, the University of Tokyo. Ten-week-old rats were initially anaesthetized with 5% isoflurane and maintained under anesthesia with 2% isoflurane. The body temperature was monitored and kept constant at 37.5 °C with a heating pad. Incisions were made on the skin above the abdominal external oblique muscle. The devices were implanted under the skin, and the skin was sutured. The animals recovered from anesthesia after the implantation, and were kept in cages. The tissues surrounding the devices moved freely due to respiration and voluntary behavior. The device was explanted after 24 hours, rinsed, and then placed in a low humidity environment over night before OTFT characterization. Water desorption of the SMP was necessary to remove conductive paths between the non-encapsulated parts of the source, drain and gate used to contact the device for measuring, and to enable a high-vacuum process to expose fresh metal contacts for OTFT analysis. Deployable OTFT Fabrication and Testing: OTFTs were fabricated on 1.4 µm PET films and encapsulated with 1 µm of Parylene diX-SR.
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100 µm of SMP were then cast on top and partially polymerized for 6 s. The device was then reshaped into a helix with r = 2.25 mm and fully polymerized and postcured for 45 min at 75 °C. Heating to 70 °C, deforming flat, and then quenching to room temperature temporarily held the device in a planar geometry due to the shape-memory effect. After insertion through a 150 µm opening, the device was exposed to convective heating (T = 70 °C) to trigger shape recovery. OTFT Characterization: All the measurements were carried out in air using a Keithley 4200 parameter analyzer.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was partly supported by National Science Foundation Graduate Research Fellowship under Grant Nos. 2011113646 and 2013155309, the National Science Foundation East Asia and Pacific Summer Institute. J.R. acknowledges the JSPS Summer Program. Received: January 26, 2014 Revised: February 19, 2014 Published online: April 15, 2014
[1] T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi, T. Sakurai, Proc. Natl. Acad. Sci. USA 2004, 101, 9966. [2] K. Cherenack, L. van Pieterson, J. Appl. Phys. 2012, 112, 091301. [3] T. Someya, Y. Kato, T. Sekitani, S. Iba, Y. Noguchi, Y. Murase, H. Kawaguchi, T. Sakurai, Proc. Natl. Acad. Sci. USA 2005, 102, 12321. [4] S. J. Benight, C. Wang, J. B. Tok, Z. Bao, Prog. Polym. Phys. 2013, 38, 1961. [5] S. Wagner, S. Bauer, MRS Bull. 2012, 37, 207. [6] J. A. Rogers, T. Someya, Y. Huang, Science 2010, 327, 1603. [7] J. Viventi, D.-H. Kim, L. Vigeland, E. S. Frechette, J. A. Blanco, Y.-S. Kim, A. E. Avrin, V. R. Tiruvadi, S.-W. Hwang, A. C. Vanleer, D. F. Wulsin, K. Davis, C. E. Gelber, L. Palmer, J. Van der Spiegel, J. Wu, Y. Huang, D. Contreras, J. Xiao, J. A. Rogers, B. Litt, Nat. Neurosci. 2011, 14, 1599. [8] R. J. Vetter, J. C. Williams, J. F. Hetke, E. A. Nunamaker, D. R. Kipke, IEEE Trans. Biomed. Eng. 2004, 51, 896. [9] D. Khodagholy, T. Doublet, P. Quilichini, M. Gurfinkel, P. Leleux, A. Ghestem, E. Ismailova, T. Hervé, S. Sanaur, C. Bernard, G. Malliaras, Nat. Commun. 2013, 4, 1575. [10] T. Stieglitz, M. Schuettler, J.-U. Meyer, Biomed. Microdevices 2000, 2, 283. [11] D. J. Tyler, D. M. Durand, IEEE Trans. Neural Syst. Rehabil. Eng. 2002, 10, 294. [12] J. Viventi, D. H. Kim, J. D. Moss, Y. S. Kim, J. A. Blanco, N. Annetta, A. Hicks, J. Xiao, Y. Huang, D. J. Callans, J. A. Rogers, B. Litt, Sci. Transl. Med. 2010, 2, 24ra22. [13] R. C. Webb, A. P. Bonifas, A. Behnaz, Y. Zhang, K. J. Yu, H. Cheng, M. Shi, Z. Bian, Z. Liu, Y.-S. Kim, W.-H. Yeo, J. S. Park, J. Song, Y. Li, Y. Huang, A. M. Gorbach, J. A. Rogers, Nat. Mater. 2013, 12, 938. [14] M. Kaltenbrunner, T. Sekitani, J. Reeder, T. Yokota, K. Kuribara, T. Tokuhara, M. Drack, R. Schwödiauer, I. Graz, S. Bauer-Gogonea, S. Bauer, T. Someya, Nature 2013, 499, 458. [15] H. Fuketa, K. Yoshioka, Y. Shinozuka, K. Ishida, T. Yokota, N. Matsuhisa, Y. Inoue, M. Sekino, T. Sekitani, M. Takamiya,
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[17] [18]
[19] [20] [21]
[22] [23]
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[24] U. Zschieschang, T. Yamamoto, K. Takimiya, H. Kuwabara, M. Ikeda, T. Sekitani, T. Someya, H. Klauk, presented at Device Res. Conf. South Bend, IN, USA, June 2010. [25] A. Jedaa, M. Burkhardt, U. Zschieschang, H. Klauk, D. Habich, G. Schmid, M. Halik, Org. Electron. 2009, 10, 1442. [26] P. Cosseddu, S. Lai, M. Barbaro, A. Bonfiglio, Appl. Phys. Lett. 2012, 100, 093305. [27] T. Ware, D. Simon, K. Hearon, C. Liu, S. Shah, J. Reeder, N. Khodaparast, M. P. Kilgard, D. J. Maitland, R. L. Rennaker, W. Voit, Macromol. Mater. Eng. 2012, 297, 1193. [28] W. H. Lee, C. Wang, J. Vac. Sci. Technol. B Microelectron. Nanometer Struct.–Process., Meas., Phenom. 2009, 27, 1116. [29] D. Simon, T. Ware, R. Marcotte, B. R. Lund, D. W. Smith Jr., M. Di Prima, R. L. Rennaker, W. Voit, Biomed. Microdevices 2013, 15, 925. [30] D. Ceballos, A. Valero, E. Valderrama, T. Stieglitz, X. Navarro, J. Neurosci. Methods 2000, 98, 105. [31] G. G. Naples, J. T. Mortimer, A. Scheiner, J. D. Sweeney, IEEE Trans. Biomed. Eng. 1988, 35, 905. [32] K. Kuribara, H. Wang, N. Uchiyama, K. Fukuda, T. Yokota, U. Zschieschang, C. Jaye, D. Fischer, H. Klauk, T. Yamamoto, T. Someya, Nat. Commun. 2012, 3, 723.
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[16]
T. Someya, T. Sakurai, presented at IEEE Int. Solid-State Circuits Conf. Digest of Technical Papers (ISSCC), San Fransisco, CA, USA February 2013. J. Subbaroyan, D. C. Martin, D. R. Kipke, J. Neural. Eng 2005, 2, 103. D.-Y. Khang, H. Jiang, Y. Huang, J. A. Rogers, Science 2006, 311, 208. S. P. Lacour, S. Benmerah, E. Tarte, J. FitzGerald, J. Serra, S. McMahon, J. Fawcett, O. Graudejus, Z. Yu, B. MorrisonIII, Med. Biol. Eng. Comput. 2010, 48, 945. D. P. Nair, N. B. Cramer, T. F. Scott, C. N. Bowman, R. Shandas, Polymer 2010, 51, 4383. J. Harris, A. Hess, S. Rowan, C. Weder, C. Zorman, D. Tyler, J. Capadona, J. Neural. Eng 2011, 8, 046010. T. Ware, D. Simon, D. E. Arreaga-Salas, J. Reeder, R. Rennaker, E. W. Keefer, W. Voit, Adv. Funct. Mater. 2012, 22, 3470. T. Sekitani, U. Zschieschang, H. Klauk, T. Someya, Nat. Mater. 2010, 9, 1015. A. Avendano-Bolivar, T. Ware, D. Arreaga-Salas, D. Simon, W. Voit, Adv. Mater. 2013, 25, 3095.
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Mechanically Adaptive Organic Transistors for Implantable Electronics Jonathan Reeder, Martin Kaltenbrunner, Taylor Ware, David Arreaga-Salas, Adrian Avendano-Bolivar, Tomoyuki Yokota, Yusuke Inoue, Masaki Sekino, Walter Voit, Tsuyoshi Sekitani, and Takao Someya* Recent progress in flexible electronics has begun to decouple high-performance electronic properties from rigid mechanical behavior, and has enabled new classes of flexible, stretchable, and textile electronics.[1–6] Soft electronic devices are able to facilitate methods for addressing healthcare problems that require chronic device implantation and have been demonstrated for use in stimulation and recording of the central[7–9] and peripheral[10,11] nervous system, and mapping cardiac electrophysiology.[12] Intimate conformability is essential for obtaining stable interfaces on complex surfaces such as skin, tissue, or organs. Significant reduction of substrate thickness has enabled ultraflexible and conformal electronics for characterizing thermal properties of human skin,[13] and organic electronics such as tactile sensing skins,[14] skins for recording electrocardiograms[15] and implantable electrodes for recording neural activity.[9] This paves the way for future biomedical devices where the bulk mechanical properties of the devices can be designed independently of an flexible, ultrathin electronic layer. Requirements for acute and chronic bioelectronics include the need to fabricate consistent, reliable devices with stable electrical properties, which are robust and resilient at mild to severe room temperature deformation during surgical handling, which can be implanted with precision near small biological areas of interest, and which maintain electrical properties during interaction with tissue. The device durability, ease of handling, and the immune response of bioelectronics after implantation are in part determined by the mechanical and geometric properties of the substrate, which makes the
J. Reeder, Dr. M. Kaltenbrunner, Dr. T. Yokota, Dr. Y. Inoue, Prof. M. Sekino, Prof. T. Sekitani, Prof. T. Someya Electrical and Electronic Engineering and Information Systems The University of Tokyo 7–3–1 Hongo, Bunkyo-ku, Tokyo 113–8656, Japan E-mail: [email protected] J. Reeder, Dr. T. Ware, D. Arreaga-Salas, A. Avendano-Bolivar, Prof. W. Voit Department of Materials Science and Engineering The University of Texas at Dallas 800 W. Campbell Rd., Richardson, Texas 75080–3021, USA Dr. M. Kaltenbrunner, Dr. T. Yokota, Dr. Y. Inoue Prof. M. Sekino, Prof. T. Sekitani, Prof. T. Someya Exploratory Research for Advanced Technology (ERATO) Japan Science and Technology Agency (JST) 2–11–16, Yayoi, Bunkyo-ku, Tokyo 113–0032, Japan
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substrate design critical. Indeed, electronics on silicon and glassy polymers typically respond poorly to implantation in soft biological tissue (10–500 kPa), due to their high modulus (>100 GPa and 1–10 GPa, respectively), and it has been shown that soft electronic substrates may improve the chronic viability of biomedical devices.[16] As a result, extensive research has been performed on low modulus substrates such as poly(dimethylsiloxane) (PDMS) and elastomeric polyurethanes (100 kPa–3 MPa) as soft substrates for bioelectronics,[17,18] though the deformability of the substrate makes high-yield fabrication difficult and precludes these devices from penetrating soft tissue. For these reasons, it is desirable to use an electronic substrate which can change mechanical properties in vivo to improve the modulus and geometry match with body tissue after rigid insertion. Stimuli-responsive materials, especially shape-memory polymers (SMPs),[19] have attracted much attention and have enabled electronics which soften when exposed to physiological conditions.[20,21] For example, organic thin-film transistors (OTFTs) manufactured on 10 µm-thick plastic film were laminated to a programmable SMP layer to electrically functionalize the surface of medical catheters.[22] Additionally, lithographically patterned OTFTs have been manufactured directly on SMP substrates and were demonstrated to have stable device properties during multiple shape-memory cycles.[23] However, in order to fabricate OTFTs on SMPs for implantable electronics, it is critical to simultaneously achieve both high mobility and low voltage operation of OTFTs on shape changing substrates which adapt at physiologically relevant temperatures, which will enable the detection of small body signals and limit undesirable physiological responses. Additionally, in order to examine the feasibility of OTFTs on mechanically active substrates for implantable electronics, such as deployment triggered by body temperature, reliability and stability of the devices inside the body has to be established. By exploiting the features of shape-memory polymer substrates, we demonstrate shape changing and softening organic electronics which softly conform or deploy into 3D shapes after exposure to a stimulus. Fabrication of low-voltage-driven organic thin-film transistors directly on stimuli-responsive polymers allows for electronics fabrication on a rigid substrate which subsequently softens and changes shape after heating above the glass-transition temperature of the substrate. We also demonstrate a novel fabrication technique for creating 3D electronics with soft and deployable form factors. Using this technique, we demonstrate devices which can be inserted through a small slit and deployed to a 3D shape in order to grasp a cylinder
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25 µm-thick film of a monomer solution and polymerizing under 365 nm UV radiation. The resulting surface of the SMP was evaluated by atomic force microscopy (AFM) and revealed the surface roughness to be 0.3 nm (root mean square) (Figure S1, Supporting Information). A 5 nm-thick layer of Al was used to protect the SMP substrate from degradation during later fabrication processes. Figure 1d illustrates OTFTs which are fabricated on a 1.4 µm PET film and encapsulated by 1 µm of Parylene. 100 µm of SMP monomer solution is subsequently cast on top to finish the device. For OTFT with direct fabrication, the aluminum oxide dielectric (5 nm) was formed through plasma-ashing of the gate electrode. For OTFT with SMP coating, the aluminum oxide dielectric was potentiostatically anodized (19 nm). In vivo conforming and deployable OTFT tests were measured using OTFTs with SMP coating. Bending and dry conforming tests were Figure 1. OTFTs on SMP substrates. a) Illustration of an OTFT on an SMP substrate conforming to warm surface. b) OTFTs are able to deploy from their planar fabrication state to measured for OTFTs with direct fabrication. The optical microscopy image in Figure 2a helices with r = 2.25 mm Scale bar is 10 mm. c) Cross-section of an OTFT on fabricated on an shows a completed OTFT with direct fabriSMP substrate. Aluminum serves as the gate contact, and aluminum oxide and an n tetradecyl phosphonic acid SAM form the dielectric. The organic semiconductor DNTT ensures air-stable cation, whose channel width and length are operation, and gold contacts complete the device. d) Cross-section of an OTFT fabricated on 1000 µm and 100 µm, respectively. Following an ultrathin polymer foil and coated with SMP. fabrication on an SMP substrate, OTFTs were evaluated in their planar state with a VDS of −2.0 V and a VGS swept from 0 to −2.0 V. Figure 2b with radius of 2.25 mm when exposed to a small temperature change. When implanted in physiological tissue, initially depicts the transfer curve and gate leakage current (left axis), as planar devices soften and conform to the biological environwell as the square root of IDS (right axis) which is the basis for ment and withstand 24 hours of fluid exposure and deformaderiving mobility and the threshold voltage. Figure 2c depicts tions associated with implantation in living tissue while mainthe output curves of one of these devices, which exhibits a drain taining electrical properties without significant degradation. current of more than 6 µA when VGS and VDS are −2.0 V. The The room temperature bending stability is evaluated to deteraverage mobility of ten devices tested in the planar state was mine bending limits and methods of degradation of various 1.51 ± 0.12 cm2 V−1 s−1, with a threshold voltage of −1.02 ± 0.35 V, bending configurations with respect to bending direction and and an average on/off current ratio of 104. stress type. Air stability and high mobility are achieved by using Figure 3 shows OTFTs fabricated on 25 µm-thick SMP subthe high-performance organic semiconductor dinaphtho[2,3strates conforming to 3D surfaces after heating above the Tg. b:2′,3′-f ]thieno[3,2-b]thiophene (DNTT).[24] Low-voltage operaFigure 3a shows the planar device resting on a cylindrical rod with a radius of 5 mm. After warming, the subsequent drop tion of 3 V is achieved by using an ultrathin hybrid dielectric in substrate modulus causes the device to succumb to gravity layer comprised of aluminum oxide and a self-assembled and conform to the supporting structure. Subsequent smaller monolayer (SAM).[25,26] Robust electrical performance during conformal radii down to r = 2 mm are examined. Conforming shape changing is enabled by a low-stress glass transition of the tests were performed in a dry environment, so a temperature of SMP substrate, which is essential for maintaining the integrity 70 °C was used to simulate the glass transition and facilitate the of thin dielectric layers during deformations. modulus drop of the SMP. The shear modulus of the substrate Mechanically adaptive OTFTs are fabricated by two methods. as measured by dynamic mechanical analysis (DMA) is approxThe first type of OTFT is manufactured directly on SMP subimately 4.0 MPa at 70 °C, 80 MPa when at 37 °C in fluid in the strates (Figure 1c), which is hereafter referred to as OTFT with body and at 600 MPa dry at 25 °C.[27] As shown in Figure 3b, direct fabrication. The second type is manufactured on 1.4 µm [ 14 ] poly (ethylene terephthalate) (PET) foils, the electric characteristics of OTFT are slightly changed due to encapsulated by heating and deformation: the mobility decreases only by 0.1% 1 µm of Parylene, and subsequently coated with a layer of and 1.1%, respectively, when heated and conformed to radii SMP (Figure 1d) using a transfer-by-polymerization process,[21] of 3 mm and 2 mm. The Vth shifts by −0.06 V and −0.14 V, which is referred to as OTFT with SMP coating. This transferby-polymerization process utilizes the thin PET film as the resulting in 10% and 21% drops in the on current, respectively. device encapsulant, as well as enables the fabrication of 3D We demonstrate that a planar OTFT which responds to electronic structures as will be shown later. The SMP substrates a temperature change and deploys into a helix after being for OTFT with direct fabrication are formed by spin coating a inserted through a 150 µm-thick opening in a thermal barrier
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(Movie S1, Supporting Information), as shown in Figure 4. This structure is created by reforming the SMP substrate during the SMP polymerization step which allows the globally stable shape to be set to an arbitrary 3D geometry, in this case a helix with a radius of 2.25 mm. The SMP is cast on the ultrathin OTFT (2 µm), which adheres to and remains compliant to the mechanical behavior of the SMP. After 3D polymerization, the substrate and active device are heated and deformed flat which primes the structure to be deployed back into its initial 3D shape. Once the device is positioned while flat, heat is applied (70 °C) which causes the SMP film to recover to its initial helical shape (Figure 4a). Device performance was evaluated after 3D fabrication, after heating and flattening, and after thermal shape recovery. Figure 4b shows the transfer curve shifts slightly negative after heating and flattening, and once again after thermal shape recovery. The active device is subjected to 0.8% compressive strain as it recovers to a radius of 2.25 mm. The Vth shifts by −0.22 V and −0.41 V when deformed flat and subsequently recovered to a helix, respectively. The mobility decreases by 0.7% and 0.8%, and the on current dropped by 24% and 36%, respectively.
Conforming behavior of devices in vivo is also examined. An OTFT fabricated on a PET film and encapsulated with 1 µm of Parylene and 100 µm of SMP was implanted under the skin of a rat for 24 hours to evaluate the effects of body fluids and constant deformation on device performance. Figure 5a demonstrates that the devices are conformed in vivo by body temperature without any external thermal sources. The tissue surrounding the devices moved freely due to respiration and voluntary behavior. OTFT characteristics were evaluated before implantation and after explantation and exhibit a +0.7 V shift of the Vth, 0.6% drop in on current, and an approximate 32% drop in mobility after removal from the tissue (Figure 5b). There was a slight increase in off current and gate current after explanting. The explanted device also exhibits a small hysteresis with higher current on the back-sweep than front-sweep. In addition to stability inside the body, it is also important to characterize the effect of stresses at room temperature. A device must be able to withstand multidirectional stresses from both compressive and tensile bending at room temperature before implantation so that surgeons can handle devices without inducing bending-related degradation. Four bending
Figure 3. Conforming behavior of an OTFT. a) An initially planar OTFT, which after heating conforms to radii of r = 5 mm (top right), r = 3 mm (bottom right), and r = 2 mm (bottom left). Transistors fabricated on 25 µm-thick SMP substrates were heated to 70 °C (Tg + 10 °C). b) Limited degradation is shown in devices after heating and conforming to radii of 3 mm and 2 mm. The scale bars are 10 mm.
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Figure 4. A deploying OTFT. a) A planar OTFT deploys into a helix and wraps around a rod (r = 2.25 mm) after it passes through a 150 µm-thick opening in a thermal barrier. A clear divider separates ambient air (background) from warm air (surrounding the rod). b) Comparing the transfer curves shows slight shifting of the Vth after both thermal deformation steps. The scale bars are 10 mm.
configurations were employed to test how tensile and compressive stresses, as well as the direction of the applied stress, may change the OTFTs in these conditions. Bending tests indicated that OTFTs in all four configurations were functional, as shown in Figure S2 in the Supporting Information, at bending radii less than 1 mm, with some withstanding bending to radii of 100 µm, which is robust enough for surgeons to prepare devices before implantation. Figure S3 in the Supporting Information provides a visual investigation of modes of degradation and failure for devices subjected to extreme bending (r = 100 µm, 11% strain present in active layer). Movie S2 in the Supporting Information shows a bending test from r = 10 mm to r = 100 µm and Figure S4 in the Supporting Information shows the bending setup for each configuration. This work describes OTFTs on SMP substrates which can mechanically adapt from planar geometries into 3D shapes when heated above the SMP glass-transistion temperature
or when exposed to physiological conditions. High mobility is achieved by using DNTT as the semiconductor and results in an average mobility of 1.51 ± 0.12 cm2 V−1 s−1 (Figure 2), which is partly enabled by the ultrasmooth surface of the SMP substrate (0.3 nm root mean square, Figure S3, Supporting Information).[28] The use of a thin plasma-formed or anodic aluminum oxide dielectric and an n-tetradecyl phosphonic acid SAM ensures low-voltage operation and decreases the interfacial trap density.[25,26] Low-voltage operation is essential for realizing implantable electronics, and was a limiting factor of the only mechanically responsive OTFT devices described in the literature to date.[23] It is important to note the stability of the dielectric after the SMP layer is heated above the Tg, indicating there is not significant stress generated in the SMP during progression through the glass transition, such as thermal stress. A low-stress transition is imperative to achieving stable electronic performance during shape changing due to the thinness
Figure 5. In vivo conforming behavior of an OTFT. a) An OTFT coated with SMP conforms to the body tissue of a living rat after being implanted for 24 hours. b) Comparison of the transfer curves before implantation and after explantation show good electrical properties after exposure to body fluids and deformation associated with implantation in living tissue for 24 hours, exhibiting only a small positive shifting of the Vth after explanation. The slight increase in off and gate current are the result of a partially conductive substrate caused by water uptake in the SMP. The scale bars for (a) are 5 mm for the top two images and 2 mm for the bottom, large image.
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in mechanical stiffness does not damage or destroy the electrical properties of the OTFTs. The small increase in off current and gate current exhibited is due to water uptake in the SMP layer, which results in a slightly conductive substrate. After explantation, the device retained its conformed geometry due to cooling and shape fixing of the substrate. It is important to note that the applied temperature for the conforming experiment in Figure 3 (70 °C) is much higher than the typical body temperature. However, when devices with an SMP layer are exposed to physiological conditions, a small amount of water uptake (3%) facilitates plasticization of the polymer network causing the Tg to drop to near body temperature.[27] The Tg of the SMP was carefully designed for rigid implantation in biological tissue and subsequent softening and conforming. Considering this effect gives surgeons adequate time for implantation before softening. Two processing methods were used for coupling OTFTs and SMPs, namely, OTFTs with direct fabrication on SMPs, and OTFTs with SMP coating. OTFTs with direct fabrication were patterned directly on the SMP substrate, while OTFTs with SMP coating were patterned on a 1.4 µm PET film and encapsulated with Parylene and then coated with an SMP layer using a transfer-by-polymerization process. For both processing methods, the SMP layer can be partially polymerized, deformed into a complex 3D shape, and then fully cured, allowing for the fabrication of 3D electronics. However, partially polymerized films continue to polymerize at a slow rate even after removal from UV exposure due to the thermally induced reaction of unreacted functional groups at room temperature. For this reason, OTFTs for deployable devices were fabricated on a separate 1.4 µm PET layer before coating with SMP, as it minimized the time in between the partial polymerization of the SMP and reshaping of the device into a helix. In vivo stability studies were performed using OTFTs coated with SMP so that the active device layer was encapsulated by the 1.4 µm PET substrate and 1 µm Parylene on opposite sides, which had been demonstrated as a suitable encapsulation scheme for acute in vitro tests.[14] In conclusion, we demonstrate OTFTs that are planar during fabrication but can adapt to ultimately interface with an arbitrary 3D or dynamic surface by exploiting the mechanical properties of shape-memory polymers. As these devices are exposed to fluids in the body and move through the substrate Tg, their electrical properties are resilient. After the initial shape changes in the device due to softening after insertion, the device is able to withstand the natural motion of the surrounding soft tissue. Additionally, we present OTFTs that can not only conform to complex shapes in physiological environments, but also deploy into a 3D shape to actively grip a target site. These acutely physiologically stable OTFTs have the potential to address challenging biomedical problems by enabling new means of creating intimate interfaces between body tissue and highperformance, robust bioelectronics.
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of the hybrid aluminum oxide and SAM dielectric. This excellent dimensional stability of the substrate through the glass transition can be attributed to the highly crosslinked polymer network of the SMP.[29] Devices soften and conform to cylinders after heating above the Tg of the SMP substrate. In Figure 3, conforming to a 3D surface is triggered by heating the device above the substrate Tg. Without causing any significant electrical degradation, the device was conformed to radii as small as 2 mm, which is similar in dimension to common peripheral nerve cuffs.[30,31] Devices were heated to 70 °C to enable conforming, as this temperature is above the glass-transition region when the SMP substrate is dry. When exposed to physiological conditions the glass-transition temperature of the SMP shifts towards body temperature, enabling physiological conforming. Relatively small changes in mobility and Vth after heating to 70 °C indicates that the temperature increase is not detrimental to the device, and is corroborated by recent work by Kuribara et al. which report that similar DNTT-based transistors show small changes in device properties below 90 °C.[32] There was also no change in gate leakage current after conforming, indicating that the aluminum oxide dielectric remains pinhole-free even after heating above the Tg of the SMP and conforming to radii as small as 2 mm. This stable dielectric behavior for the conforming is encouraging for creating future devices which can be deformed above the substrate Tg without significant changes in dielectric properties. Devices are able to be deployed to actively grip 3D surfaces after insertion through a thin slit to simulate a minimally invasive surgery. In Figure 4, an OTFT was deployed to a programmed shape that is 15× larger than the insertion profile of the device when heated above the Tg of the substrate. Convective heating causes shape recovery of the polymerized shape of the SMP as the device passes through a small opening, actively gripping the cylinder on the warm side of the thermal barrier. This makes these devices very appealing for minimally invasive surgeries. By reshaping the substrate during polymerization, the globally stable state of the SMP can be set to any arbitrary 3D shape. When the substrate is heated and deformed flat and then quenched, these strains can be stored using the shape-memory effect to create a device which deploys into the polymerized, 3D shape. The similar amount of shifting of the Vth after both instances of heating indicates that thermal degradation plays a more important role than the 0.8% compressive bending strain in the device degradation. This is supported by the results shown in Figure S2 in the Supporting Information, which shows negligible degradation of OTFTs bent at room temperature to impose compressive stress at similar radii. As can be seen with the conforming experiment, there was no change in gate leakage current, indicating a pinhole-free dielectric layer after deploying. Acute in vivo stability is also demonstrated. In Figure 5, adaptive OTFTs were conformed in vivo using only physiological conditions to trigger softening of the substrate. An initially rigid, planar OTFT with SMP coating was implanted near the abdomen of a living rat, and over the first few hours after surgery, fluid uptake plasticized the SMP and triggered softening from above 1 GPa to approximately 80 MPa. This large change
Experimental Section SMP Substrate Synthesis: Tricyclodecane dimethanol diacrylate (TCMDA), 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO),
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and 2,2-dimethoxy-2-phenyl acetophenone (DMPA) were purchased from Sigma–Aldrich. Tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate (TMICN) was purchased from Wako Chemicals. All the chemicals were used as received without further purification. The thiol–ene/acrylate SMP composition consisted of stoichiometric quantities of TATAO and TMICN and 31 mol% TCMDA. 0.1 wt% DMPA of total monomer concentration was dissolved into the solution. The vial was covered in aluminum foil to prevent incident light from contacting the monomer solution and kept at room temperature. The appropriate amount of TMICN was added to the covered vial. Without exposing the solution to light, the vial was mixed thoroughly by vortexing and sonication until there were no visible air bubbles in the solution. To make the SMP substrate, monomer solution was spun at 1500 rpm for 30 s to form a 25 µm-thick film. When OTFTs were fabricated on ultrathin polymer foils, 100 µm of SMP were cast on top of the device using glass slides and spacers as a mold. Polymerization was performed using a crosslinking chamber with five overhead 365 nm UV bulbs. Polymerization consisted of UV exposure for a total of 20 min, followed by post-curing the SMP substrates at 120 °C for 12 h, or at 75 °C for 45 min for SMP cast on OTFTs fabricated on ultrathin polymer foils. More information about the SMP substrate can be found in the Supporting Information and additional discussion of the materials and methods for preparation can be found in the literature by Ware et al.[27] OTFT Fabrication on SMP Substrates: SMP substrates were prepared for device fabrication by evaporating 5 nm of aluminum over the surface of the film. This layer served as a protective shield which prevented degradation of the substrate during subsequent processes, such as oxygen plasma treatment. A 30 nm-thick aluminum gate electrode was then patterned via shadow mask using thermal evaporation, and exposed to oxygen plasma (5 min at 300 W) to form an approximately 5 nm-thick aluminum oxide dielectric layer. A self-assembled monolayer (SAM) of an n-tetradecyl phosphonic acid was deposited on the aluminum oxide surface by submersing the gate electrodes in a 5 mM solution of the SAM molecules in isopropanyl alcohol for 12 h. DNTT, 30 nm thick, was then thermally evaporated on top of the SAM layer at a pressure of 1 × 10−6 mbar and a rate of 0.1 Å/s. Subsequently, 60 nm of Au was thermally evaporated to form the source and drain to complete the device. OTFT Fabrication on Ultrathin Polymer Foils: OTFTs on ultrathin polymer foils have an identical structure to devices fabricated on SMP substrates, with the exception of thicker anodically formed aluminum oxide of 19 nm. After source/drain fabrication, 1 µm of Parylene diX-SR was used to encapsulate the device followed by 100 µm of SMP. Additional information on the fabrication of OTFTs on ultrathin polymer foils can be found in the literature by Kaltenbrunner et al.[14] Conformability Testing: For conforming outside the body, 25 µm-thick SMP substrates were fabricated and OTFTs direct patterned on the SMP surface. Using convective heating, the environment surrounding the device and supporting cylinders with various radii were heated to 70 °C. For in vivo stability tests, 100 µm of SMP was cast on top of Parylene diX-SR encapsulated OTFTs and polymerized and post cured for 45 min at 75 °C. The protocol for the animal experiment was approved by the Institutional Animal Care and Use Committee of the Graduate School of Engineering, the University of Tokyo. Ten-week-old rats were initially anaesthetized with 5% isoflurane and maintained under anesthesia with 2% isoflurane. The body temperature was monitored and kept constant at 37.5 °C with a heating pad. Incisions were made on the skin above the abdominal external oblique muscle. The devices were implanted under the skin, and the skin was sutured. The animals recovered from anesthesia after the implantation, and were kept in cages. The tissues surrounding the devices moved freely due to respiration and voluntary behavior. The device was explanted after 24 hours, rinsed, and then placed in a low humidity environment over night before OTFT characterization. Water desorption of the SMP was necessary to remove conductive paths between the non-encapsulated parts of the source, drain and gate used to contact the device for measuring, and to enable a high-vacuum process to expose fresh metal contacts for OTFT analysis. Deployable OTFT Fabrication and Testing: OTFTs were fabricated on 1.4 µm PET films and encapsulated with 1 µm of Parylene diX-SR.
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100 µm of SMP were then cast on top and partially polymerized for 6 s. The device was then reshaped into a helix with r = 2.25 mm and fully polymerized and postcured for 45 min at 75 °C. Heating to 70 °C, deforming flat, and then quenching to room temperature temporarily held the device in a planar geometry due to the shape-memory effect. After insertion through a 150 µm opening, the device was exposed to convective heating (T = 70 °C) to trigger shape recovery. OTFT Characterization: All the measurements were carried out in air using a Keithley 4200 parameter analyzer.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was partly supported by National Science Foundation Graduate Research Fellowship under Grant Nos. 2011113646 and 2013155309, the National Science Foundation East Asia and Pacific Summer Institute. J.R. acknowledges the JSPS Summer Program. Received: January 26, 2014 Revised: February 19, 2014 Published online: April 15, 2014
[1] T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi, T. Sakurai, Proc. Natl. Acad. Sci. USA 2004, 101, 9966. [2] K. Cherenack, L. van Pieterson, J. Appl. Phys. 2012, 112, 091301. [3] T. Someya, Y. Kato, T. Sekitani, S. Iba, Y. Noguchi, Y. Murase, H. Kawaguchi, T. Sakurai, Proc. Natl. Acad. Sci. USA 2005, 102, 12321. [4] S. J. Benight, C. Wang, J. B. Tok, Z. Bao, Prog. Polym. Phys. 2013, 38, 1961. [5] S. Wagner, S. Bauer, MRS Bull. 2012, 37, 207. [6] J. A. Rogers, T. Someya, Y. Huang, Science 2010, 327, 1603. [7] J. Viventi, D.-H. Kim, L. Vigeland, E. S. Frechette, J. A. Blanco, Y.-S. Kim, A. E. Avrin, V. R. Tiruvadi, S.-W. Hwang, A. C. Vanleer, D. F. Wulsin, K. Davis, C. E. Gelber, L. Palmer, J. Van der Spiegel, J. Wu, Y. Huang, D. Contreras, J. Xiao, J. A. Rogers, B. Litt, Nat. Neurosci. 2011, 14, 1599. [8] R. J. Vetter, J. C. Williams, J. F. Hetke, E. A. Nunamaker, D. R. Kipke, IEEE Trans. Biomed. Eng. 2004, 51, 896. [9] D. Khodagholy, T. Doublet, P. Quilichini, M. Gurfinkel, P. Leleux, A. Ghestem, E. Ismailova, T. Hervé, S. Sanaur, C. Bernard, G. Malliaras, Nat. Commun. 2013, 4, 1575. [10] T. Stieglitz, M. Schuettler, J.-U. Meyer, Biomed. Microdevices 2000, 2, 283. [11] D. J. Tyler, D. M. Durand, IEEE Trans. Neural Syst. Rehabil. Eng. 2002, 10, 294. [12] J. Viventi, D. H. Kim, J. D. Moss, Y. S. Kim, J. A. Blanco, N. Annetta, A. Hicks, J. Xiao, Y. Huang, D. J. Callans, J. A. Rogers, B. Litt, Sci. Transl. Med. 2010, 2, 24ra22. [13] R. C. Webb, A. P. Bonifas, A. Behnaz, Y. Zhang, K. J. Yu, H. Cheng, M. Shi, Z. Bian, Z. Liu, Y.-S. Kim, W.-H. Yeo, J. S. Park, J. Song, Y. Li, Y. Huang, A. M. Gorbach, J. A. Rogers, Nat. Mater. 2013, 12, 938. [14] M. Kaltenbrunner, T. Sekitani, J. Reeder, T. Yokota, K. Kuribara, T. Tokuhara, M. Drack, R. Schwödiauer, I. Graz, S. Bauer-Gogonea, S. Bauer, T. Someya, Nature 2013, 499, 458. [15] H. Fuketa, K. Yoshioka, Y. Shinozuka, K. Ishida, T. Yokota, N. Matsuhisa, Y. Inoue, M. Sekino, T. Sekitani, M. Takamiya,
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 4967–4973
www.advmat.de www.MaterialsViews.com
[17] [18]
[19] [20] [21]
[22] [23]
Adv. Mater. 2014, 26, 4967–4973
[24] U. Zschieschang, T. Yamamoto, K. Takimiya, H. Kuwabara, M. Ikeda, T. Sekitani, T. Someya, H. Klauk, presented at Device Res. Conf. South Bend, IN, USA, June 2010. [25] A. Jedaa, M. Burkhardt, U. Zschieschang, H. Klauk, D. Habich, G. Schmid, M. Halik, Org. Electron. 2009, 10, 1442. [26] P. Cosseddu, S. Lai, M. Barbaro, A. Bonfiglio, Appl. Phys. Lett. 2012, 100, 093305. [27] T. Ware, D. Simon, K. Hearon, C. Liu, S. Shah, J. Reeder, N. Khodaparast, M. P. Kilgard, D. J. Maitland, R. L. Rennaker, W. Voit, Macromol. Mater. Eng. 2012, 297, 1193. [28] W. H. Lee, C. Wang, J. Vac. Sci. Technol. B Microelectron. Nanometer Struct.–Process., Meas., Phenom. 2009, 27, 1116. [29] D. Simon, T. Ware, R. Marcotte, B. R. Lund, D. W. Smith Jr., M. Di Prima, R. L. Rennaker, W. Voit, Biomed. Microdevices 2013, 15, 925. [30] D. Ceballos, A. Valero, E. Valderrama, T. Stieglitz, X. Navarro, J. Neurosci. Methods 2000, 98, 105. [31] G. G. Naples, J. T. Mortimer, A. Scheiner, J. D. Sweeney, IEEE Trans. Biomed. Eng. 1988, 35, 905. [32] K. Kuribara, H. Wang, N. Uchiyama, K. Fukuda, T. Yokota, U. Zschieschang, C. Jaye, D. Fischer, H. Klauk, T. Yamamoto, T. Someya, Nat. Commun. 2012, 3, 723.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
[16]
T. Someya, T. Sakurai, presented at IEEE Int. Solid-State Circuits Conf. Digest of Technical Papers (ISSCC), San Fransisco, CA, USA February 2013. J. Subbaroyan, D. C. Martin, D. R. Kipke, J. Neural. Eng 2005, 2, 103. D.-Y. Khang, H. Jiang, Y. Huang, J. A. Rogers, Science 2006, 311, 208. S. P. Lacour, S. Benmerah, E. Tarte, J. FitzGerald, J. Serra, S. McMahon, J. Fawcett, O. Graudejus, Z. Yu, B. MorrisonIII, Med. Biol. Eng. Comput. 2010, 48, 945. D. P. Nair, N. B. Cramer, T. F. Scott, C. N. Bowman, R. Shandas, Polymer 2010, 51, 4383. J. Harris, A. Hess, S. Rowan, C. Weder, C. Zorman, D. Tyler, J. Capadona, J. Neural. Eng 2011, 8, 046010. T. Ware, D. Simon, D. E. Arreaga-Salas, J. Reeder, R. Rennaker, E. W. Keefer, W. Voit, Adv. Funct. Mater. 2012, 22, 3470. T. Sekitani, U. Zschieschang, H. Klauk, T. Someya, Nat. Mater. 2010, 9, 1015. A. Avendano-Bolivar, T. Ware, D. Arreaga-Salas, D. Simon, W. Voit, Adv. Mater. 2013, 25, 3095.
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