Voit, Walter E.
Permanent URI for this collectionhttps://hdl.handle.net/10735.1/4984
Walter Voit is an Associate Professor of Mecanical Engineering. He was a member of UTD's inaugural class of Eugene McDermott scholars in 2001 and finished his academic training with a PhD from Georgia Tech. He returned to UTD in 2010 as a member of the faculty. His research interests include:
- Shape memory polymers
- Polymer manufacturing
- Ionizing radiation
- Thermomechanical properties
- Biopolymer mechanics
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Item Chronic Softening Spinal Cord Stimulation Arrays(Institute of Physics Publishing) Garcia-Sandoval, Aldo; Pal, A.; Mishra, A. M.; Sherman, Sydney; Parikh, Ankit R.; Joshi-Imre, Alexandria; Arreaga-Salas, David; Gutierrez-Heredia, Gerardo; Duran-Martinez, Adriana C.; Nathan, J.; Hosseini, Seyed Mahmoud; Carmel, J. B.; Voit, Walter E.; 0000-0003-0135-0531 (Voit, WE); Garcia-Sandoval, Aldo; Sherman, Sydney; Parikh, Ankit R.; Joshi-Imre, Alexandria; Arreaga-Salas, David; Gutierrez-Heredia, Gerardo; Duran-Martinez, Adriana C.; Hosseini, Seyed Mahmoud; Voit, Walter E.Objective. We sought to develop a cervical spinal cord stimulator for the rat that is durable, stable, and does not perturb the underlying spinal cord. Approach. We created a softening spinal cord stimulation (SCS) array made from shape memory polymer (SMP)-based flexible electronics. We developed a new photolithographic process to pattern high surface area titanium nitride (TiN) electrodes onto gold (Au) interconnects. The thiol-ene acrylate polymers are stiff at room temperature and soften following implantation into the body. Durability was measured by the duration the devices produced effective stimulation and by accelerated aging in vitro. Stability was measured by the threshold to provoke an electromyogram (EMG) muscle response and by measuring impedance using electrochemical impedance spectroscopy (EIS). In addition, spinal cord modulation of motor cortex potentials was measured. The spinal column and implanted arrays were imaged with MRI ex vivo, and histology for astrogliosis and immune reaction was performed. Main results. For durability, the design of the arrays was modified over three generations to create an array that demonstrated activity up to 29 weeks. SCS arrays showed no significant degradation over a simulated 29 week period of accelerated aging. For stability, the threshold for provoking an EMG rose in the first few weeks and then remained stable out to 16 weeks; the impedance showed a similar rise early with stability thereafter. Spinal cord stimulation strongly enhanced motor cortex potentials throughout. Upon explantation, device performance returned to pre-implant levels, indicating that biotic rather than abiotic processes were the cause of changing metrics. MRI and histology showed that softening SCS produced less tissue deformation than Parylene-C arrays. There was no significant astrogliosis or immune reaction to either type of array. Significance. Softening SCS arrays meet the needs for research-grade devices in rats and could be developed into human devices in the future.Item Conformal Electrode Arrays to Enable in Vivo Recordings of the Enteric Nervous System(Society for Biomaterials) Ecker, Melanie; Guerrero, Edgar; Flores, Pedro Emanuel Roca; Voit, Walter E.; 0000-0003-0135-0531 (Voit, WE); Ecker, Melanie; Guerrero, Edgar; Flores, Pedro Emanuel Roca; Voit, Walter E.Statement of Purpose: The enteric nervous system (ENS) has the largest population of neurons in the peripheral nervous system, but it is not well understood and is less investigated than the central nervous system. Most of the information on the function and electrophysiology of the ENS was collected ex vivo, either on pathological samples or in cell cultures. Major problems when attaching neural electrodes to the gut, e.g. the small intestine, are that the tissue is of soft muscle, the geometry and its surface topology are complex, and it is constantly moving. Thus, conventional stiff electrodes and nerve cuffs cannot conform to the gut surface and are susceptible to motion artifacts since they will be moving relative to the bowel. Here, we demonstrate that the problems caused by conventional, stiff electrodes can be solved with the use of a thin-film electrode array fabricated on a self-softening polymeric substrate material. The hypothesis is that the polymer is capable of softening and changing its shape, so that the device can adapt to the shape and surface topology of its surroundings. It will wrap tightly around the gut, securing the electrodes in place to enable continuous periodic in vivo recordings of neurons from the myenteric and submucosal plexus within the small intestine. © 2019 Omnipress - All rights reserved.Item Effect of Annealing Atmosphere on IGZO Thin Film Transistors on a Deformable Softening Polymer Substrate(Institute of Physics Publishing) Gutierrez-Heredia, Gerado; Maeng, Jimin; Conde, J.; Rodriguez-Lopez, O.; Voit, Walter E.; 0000-0002-9198-1822 (Gutierrez-Heredia, G); 0000-0003-0135-0531 (Voit, WE); Gutierrez-Heredia, Gerado; Maeng, Jimin; Rodriguez-Lopez, O.; Voit, Walter E.The effect of annealing atmosphere on indium-gallium-zinc-oxide (IGZO) thin film transistors (TFTs) fabricated on a deformable softening polymer substrate is presented in this work. Different annealing conditions - ambient, oxygen, vacuum and forming gas - are employed in the fabrication of IGZO TFTs and the changes in electrical characteristics are examined. Fabricated devices exhibit shape memory properties due to thiol-ene/acrylate substrates allowing the softening of bioelectronics to demonstrate modulus changes in aqueous conditions at body temperature. Gold (Au) is used as the contact metal for the gate, drain and source for its good adherence and malleability required for this polymer. It is found that annealing treatments at 250 °C can improve the field effect mobility of the TFTs from 10⁻² up to 30 cm² V⁻¹ s⁻¹. These improvements are attributed to the reduction of oxygen concentration in the active film of the TFTs. The contact resistance is also reduced by the annealing treatments from approximately 1 MΩ to 20 kΩ, indicating improvement in physical contact at the IGZO-Au interface. In addition, the contributions of contact resistance and channel resistance to other electrical parameters are analyzed. This study will pave the way for the development and optimization of high-performance bioelectronic devices on smart polymers.Item Electrical Properties of Thiol-ene-Based Shape Memory Polymers Intended For Flexible Electronics(MDPI AG, 2019-05-17) Frewin, Christopher L.; Ecker, Melanie; Joshi-Imre, Alexandra; Kamgue, Jonathan; Waddell, Jeanneane; Danda, Vindhya Reddy; Stiller, Alison M.; Voit, Walter E.; Pancrazio, Joseph J.; 0000-0002-0603-6683 (Ecker, M); 0000-0002-4271-1623 (Joshi-Imre, A); 0000-0003-0135-0531 (Voit, WE); 0000-0001-8276-3690 (Pancrazio, JJ); Frewin, Christopher L.; Ecker, Melanie; Joshi-Imre, Alexandra; Kamgue, Jonathan; Waddell, Jeanneane; Danda, Vindhya Reddy; Stiller, Alison M.; Voit, Walter E.; Pancrazio, Joseph J.Thiol-ene/acrylate-based shape memory polymers (SMPs) with tunable mechanical and thermomechanical properties are promising substrate materials for flexible electronics applications. These UV-curable polymer compositions can easily be polymerized onto pre-fabricated electronic components and can be molded into desired geometries to provide a shape-changing behavior or a tunable softness. Alternatively, SMPs may be prepared as a flat substrate, and electronic circuitry may be built directly on top by thin film processing technologies. Whichever way the final structure is produced, the operation of electronic circuits will be influenced by the electrical and mechanical properties of the underlying (and sometimes also encapsulating) SMP substrate. Here, we present electronic properties, such as permittivity and resistivity of a typical SMP composition that has a low glass transition temperature (between 40 and 60 °C dependent on the curing process) in different thermomechanical states of polymer. We fabricated parallel plate capacitors from a previously reported SMP composition (fully softening (FS)-SMP) using two different curing processes, and then we determined the electrical properties of relative permittivity and resistivity below and above the glass transition temperature. Our data shows that the curing process influenced the electrical permittivity, but not the electrical resistivity. Corona-Kelvin metrology evaluated the quality of the surface of FS-SMP spun on the wafer. Overall, FS-SMP demonstrates resistivity appropriate for use as an insulating material. © 2019 by the authors.Item Environmental Dynamic Mechanical Analysis to Predict the Softening Behavior of Neural Implants(NLM (Medline)) Hosseini, Seyed Mahmoud; Voit, Walter E.; Ecker, Melanie; Hosseini, Seyed Mahmoud; Voit, Walter E.; Ecker, MelanieWhen using dynamically softening substrates for neural implants, it is important to have a reliable in vitro method to characterize the softening behavior of these materials. In the past, it has not been possible to satisfactorily measure the softening of thin films under conditions mimicking body environment without substantial effort. This publication presents a new and simple method that allows dynamic mechanical analysis (DMA) of polymers in solutions, such as phosphate buffered saline (PBS), at relevant temperatures. The use of environmental DMA allows measurement of the softening effects of polymers due to plasticization in various media and temperatures, which therefore allows a prediction of the materials behavior under in vivo conditions.Item Mechanical Simplification of Variable-Stiffness Actuators Using Dielectric Elastomer Transducers(MDPI AG, 2019-05-20) Allen, David P.; Bolívar, Edgar; Farmer, Sophie; Voit, Walter E.; Gregg, Robert D.; 0000-0002-9740-1278 (Allen, DP); 0000-0001-7697-4387 (Bolívar, E); 0000-0002-5898-0449 (Farmer, S); 0000-0003-0135-0531 (Voit, WE); 0000-0002-0729-2857 (Gregg, RD); Allen, David P.; Bolívar, Edgar; Farmer, Sophie; Voit, Walter E.; Gregg, Robert D.Legged and gait-assistance robots can walk more efficiently if their actuators are compliant. The adjustable compliance of variable-stiffness actuators (VSAs) can enhance this benefit. However, this functionality requires additional mechanical components making VSAs impractical for some uses due to increased weight, volume, and cost. VSAs would be more practical if they could modulate the stiffness of their springs without additional components, which usually include moving parts and an additional motor. Therefore, we designed a VSA that uses dielectric elastomer transducers (DETs) for springs. It does not need mechanical stiffness-adjusting components because DETs soften due to electrostatic forces. This paper presents details and performance of our design. Our DET VSA demonstrated independent modulation of its equilibrium position and stiffness. Our design approach could make it practical to obtain the benefits of variable-stiffness actuation with less weight, volume, and cost than normally accompanies them, once weaknesses of DET technology are addressed. © 2019 by the authors.Item A Mosquito Inspired Strategy to Implant Microprobes into the Brain(Nature Publishing Group, 2018-11-05) Shoffstall, Andrew J.; Srinivasan, Suraj; Willis, Mitchell; Stiller, Allison M.; Ecker, Melanie; Voit, Walter E.; Pancrazio, Joseph J.; Capadona, Jeffrey R.; 0000-0002-0603-6683 (Ecker, M); 0000-0003-0135-0531 (Voit, WE); 0000-0001-8276-3690 (Pancrazio, JJ); Stiller, Allison M.; Ecker, Melanie; Voit, Walter E.; Pancrazio, Joseph J.Mosquitos are among the deadliest insects on the planet due to their ability to transmit diseases like malaria through their bite. In order to bite, a mosquito must insert a set of micro-sized needles through the skin to reach vascular structures. The mosquito uses a combination of mechanisms including an insertion guide to enable it to bite and feed off of larger animals. Here, we report on a biomimetic strategy inspired by the mosquito insertion guide to enable the implantation of intracortical microelectrodes into the brain. Next generation microelectrode designs leveraging ultra-small dimensions and/or flexible materials offer the promise of increased performance, but present difficulties in reliable implantation. With the biomimetic guide in place, the rate of successful microprobe insertion increased from 37.5% to 100% due to the rise in the critical buckling force of the microprobes by 3.8-fold. The prototype guides presented here provide a reproducible method to augment the insertion of small, flexible devices into the brain. In the future, similar approaches may be considered and applied to the insertion of other difficult to implant medical devices.Item Recent Advances in the Science and Engineering of Organic Light-Emitting Diodes(SPIE) Kippelen, B.; Gaj, M. P.; Zhang, X.; Choi, S.; Fuentes-Hernandez, C.; Zhang, Y. D.; Barlow, S.; Marder, S. R.; Voit, Walter E.; Wei, Andrew; So, F.; Adachi, C.; Kim, J. J.; Voit, Walter E.; Wei, AndrewIn this talk, we will discuss recent advances in the science and engineering of organic light-emitting diodes (OLEDs). First, we will focus on materials in which light emission involves the process of thermally activated delayed fluorescence (TADF). In these materials, triplet excited states can convert into optically emissive singlet excited states by reverse intersystem crossing, allowing for nearly 100% internal quantum efficiency. This process can be used to design a new class of materials that are all organic, offering a lower cost alternative to conventional electrophosphorescent materials that contain heavy and expensive elements such as Pt and Ir. We will discuss molecular design strategies and present examples of materials that can be used as emitters or hosts in the emissive layer. In a second part of this talk, we will review recent progress in fabricating OLEDs on shape memory polymer substrates (SMPs). SMPs are mechanically active, smart materials that can exhibit a significant drop in modulus once an external stimulus such as temperature is applied. In their rubbery state upon heating, the SMP can be easily deformed by external stresses into a temporary geometric configuration that can be retained even after the stress is removed by cooling the SMP to below the glass transition temperature. Reheating the SMP causes strain relaxation within the polymer network and induces recovery of its original shape. We will discuss how these unique mechanical properties can also be extended to a new class of OLEDs. ©2016 SPIEItem Solution-Processed Oxide Thin Film Transistors on Shape Memory Polymer Enabled by Photochemical Self-Patterning(Cambridge University Press) Daunis, Trey B.; Barrera, Diego; Gutierrez-Heredia, Gerado; Rodriguez-Lopez, Ovidio; Wang, Jian; Voit, Walter E.; Hsu, Julia W. P.; 0000-0003-0135-0531 (Voit, WE); 0000-0002-7821-3001 (Hsu, JWP); Hsu, Julia W. P.; Daunis, Trey B.; Barrera, Diego; Gutierrez-Heredia, Gerado; Rodriguez-Lopez, Ovidio; Wang, Jian; Voit, Walter E.Solution-processed metal oxide electronics on flexible substrates can enable applications from military to health care. Due to limited thermal budgets and mismatched coefficients of thermal expansion between oxides and substrates, achieving good performance in solution-processed oxide films remains a challenge. Additionally, the use of traditional photolithographic processes is incompatible with low-cost, high-throughput roll-to-roll processing. Here, we demonstrate solution-deposited oxide thin film transistors (TFTs) on a shape memory polymer substrate, which offers unique control of final device shape and modulus. The key enabling step is the exposure of the precursor film to UV-ozone through a shadow mask to perform patterning and photochemical conversion simultaneously. These TFTs exhibit mobility up to 160 cm2/(V s), subthreshold swing as low as 110 mV/dec, and threshold voltage between -2 and 0 V, while maintaining compatibility with a flexible form factor at processing temperatures below 250 °C. ©2018 Materials Research Society.Item Top-Gate Organic Field-Effect Transistors Fabricated on Shape-Memory Polymer Substrates(SPIE-Int Soc Optical Engineering) Choi, Sangmoo; Fuentes-Hernandez, Canek; Wang, Cheng-Yin; Wei, Andrew; Voit, Walter E.; Zhang, Yadong; Barlow, Stephen; Marder, Seth R.; Kippelen, Bernard; 0000-0003-0135-0531 (Voit, WE); Wei, Andrew; Voit, Walter E.We demonstrate top-gate organic field-effect transistors (OFETs) with a bilayer gate dielectric and doped contacts fabricated on shape-memory polymer (SMP) substrates. SMPs exhibit large variations in Young's modulus dependent on temperature and have the ability to fix two or more geometric configurations when a proper stimulus is applied. These unique properties make SMPs desirable for three-dimensional shape applications of OFETs. The electrical properties of OFETs on SMP substrates are presented and compared to those of OFETs on traditional glass substrates.