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We report a new physics-based model for dual-gate amorphous-indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) which we developed and fine-tuned through experimental implementation and benchtop characterization.We fabricated and characterized a variety of test patterns, including a-IGZO TFTs with varying gate widths (100–1000 μm) and channel lengths (5–50 μm), transmission-line-measurement patterns and ground–signal–ground (GSG) radio frequency (RF) patterns. We modeled the contact resistance as a function of bias, channel area, and temperature, and captured all operating regimes, used physics-based modeling adjusted for empirical data to capture the TFT characteristics including ambipolar subthreshold currents, graded interbias-regime current changes, threshold and flat-band voltages, the interface trap density, the gate leakage currents, the noise, and the relevant small signal parameters. To design high-precision circuits for biosensing, we validated the dc, small signal, and noise characteristics of the model. We simulated and fabricated a two-stage common source amplifier circuit with a common drain output buffer and compared the measured and simulated gain and phase performance, finding an excellent fit over a frequency range spanning 10 kHz–10 MHz. 

Electrotactile stimulus is a form of sensory substitution in which an electrical signal is perceived as a mechanical sensation. The electrotactile effect could, in principle, recapitulate a range of tactile experience by selective activation of nerve endings. However, the method has been plagued by inconsistency, galvanic reactions, pain and desensitization, and unwanted stimulation of nontactile nerves. Here, we describe how a soft conductive block copolymer, a stretchable layout, and concentric electrodes, along with psychophysical thresholding, can circumvent these shortcomings. These purpose-designed materials, device layouts, and calibration techniques make it possible to generate accurate and reproducible sensations across a cohort of 10 human participants and to do so at ultralow currents (≥6 microamperes) without pain or desensitization. This material, form factor, and psychophysical approach could be useful for haptic devices and as a tool for activation of the peripheral nervous system. 

Electrophysiological stimulation has been widely adopted for clinical diagnostic and therapeutic treatments for modulation of neuronal activity. Safety is a primary concern in an interventional design leveraging the effects of electrical charge injection into tissue in the proximity of target neurons. While modalities of tissue damage during stimulation have been extensively investigated for specific electrode geometries and stimulation paradigms, a comprehensive model that can predict the electrochemical safety limits in vivo doesn’t yet exist. Here we develop a model that accounts for the electrode geometry, inter-electrode separation, material, and stimulation paradigm in predicting safe current injection limits. We performed a parametric investigation of the stimulation limits in both benchtop and in vivo setups for flexible microelectrode arrays with low impedance, high geometric surface area platinum nanorods and PEDOT:PSS, and higher impedance, planar platinum contacts. We benchmark our findings against standard clinical electrocorticography and depth electrodes. Using four, three and two contact electrochemical impedance measurements and comprehensive circuit models derived from these measurements, we developed a more accurate, clinically relevant and predictive model for the electrochemical interface potential. For each electrode configuration, we experimentally determined the geometric correction factors that dictate geometry-enforced current spreading effects. We also determined the electrolysis window from cyclic-voltammetry measurements which allowed us to calculate stimulation current safety limits from voltage transient measurements. From parametric benchtop electrochemical measurements and analyses for different electrode types, we created a predictive equation for the cathodal excitation measured at the electrode interface as a function of the electrode dimensions, geometric factor, material and stimulation paradigm. We validated the accuracy of our equation in vivo and compared the experimentally determined safety limits to clinically used stimulation protocols. Our new model overcomes the design limitations of Shannon’s equation and applies to macro- and micro-electrodes at different density or separation of contacts, captures the breakdown of charge-density based approaches at long stimulation pulse widths, and invokes appropriate power exponents to current, pulse width, and material/electrode-dependent impedance. 

A roadmap of the spine During spinal cord surgery, intraoperative neuromonitoring (IONM) is used to reduce the risk of damage. Electrodes on muscles or scalp record the response to large-amplitude electrical stimuli delivered to the spinal cord. However, this method does not allow precise spatiotemporal characterization of spinal cord neurophysiology. Now, Russman et al. developed a microelectrode array that can be placed on the spinal cord during surgery and record with high spatiotemporal definition and high sensitivity the electrophysiological response to low-current stimulation, providing precise maps of spinal cord electrophysiology. These maps can be used during surgery to improve IONM. 

Piezoelectric devices transduce mechanical energy to electrical energy by elastic deformation, which distorts local dipoles in crystalline materials. Amongst electromechanical sensors, piezoelectric devices are advantageous because of their scalability, light weight, low power consumption, and readily built-in amplification and ability for multiplexing, which are essential for wearables, medical devices, and robotics. This paper reviews recent progress in active piezoelectric devices. We classify these piezoelectric devices according to the material dimensionality and present physics-based device models to describe and quantify the piezoelectric response for one-dimensional nanowires, emerging two-dimensional materials, and three-dimensional thin films. Different transduction mechanisms and state-of-the-art devices for each type of material are reviewed. Perspectives on the future applications of active piezoelectric devices are discussed.