Ovidio Rodriguez‐Lopez

Target engagement determines therapeutic efficacy. In spinal cord stimulation (SCS), efficacy depends on the attributes of the electrodes used to deliver stimulation, particularly their position, interelectrode distance, size, and current orientation. We tested the effects of these electrode parameters using cervical stimulation to elicit forelimb muscle responses in the rat. We implanted devices over the C6 dorsal root entry zone (DREZ) in eight rats and measured responses in six forelimb muscles. We used custom designed electrode arrays to systematically vary position, distance, and size with linear arrays, and current orientation with circular arrays. Stimulation consisted of biphasic and pseudomonophasic pulse shapes with bipolar or distant return, and a high-definition montage with four returns surrounding a central contact to increase local current density. Efficacy was calculated from the motor thresholds of recruitment curves, and parameters were compared using linear mixed models. For position, stimulation over the DREZ was most effective, reducing thresholds by 25.9% ( p = 0.0002) relative to the midline; efficacy decreased as the electrode was positioned medial or lateral to the DREZ. For distance, placing the electrodes farther apart significantly improved efficacy ( p = 0.0022). For size, large electrodes increased SCS efficacy, reducing thresholds by 21.5% ( p = 0.0026) relative to small electrodes. After accounting for position and distance, current orientation did not affect SCS efficacy. Lastly, the high-definition montage decreased efficacy, increasing thresholds by 16.7% ( p = 0.003) relative to monopolar stimulation with a distant return. In conclusion, electrode position, distance, and size had the greatest effect on SCS efficacy, with the optimal parameters combining large electrodes at the DREZ position with a distant return. These results have the potential to lower the current needed to engage cervical spinal circuitry and to inform the design of SCS systems to improve dexterity in people with neurological injury or disease.

Neural implantable devices serve as electronic interfaces facilitating communication between the body and external electronic systems. These bioelectronic systems ideally possess stable electrical conductivity, flexibility, and stretchability to accommodate dynamic movements within the body. However, achieving both high electrical conductivity and mechanical compatibility remains a challenge. Effective electrical conductors tend to be rigid and stiff, leading to a substantial mechanical mismatch with bodily tissues. On the other hand, highly stretchable polymers, while mechanically compatible, often suffer from limited compatibility with lithography techniques and reduced electrical stability. Therefore, there exists a pressing need to develop electromechanically stable neural interfaces that enable precise communication with biological tissues. In this study, a polymer that is softening, flexible, conformal, and compatible with lithography to microfabricate perforated thin‐film architectures is utilized. These architectures offer stretchability and improved mechanical compatibility. Three distinct geometries are evaluated both mechanically and electrically under in vitro conditions that simulate physiological environments. Notably, the Peano structure demonstrates minimal changes in resistance, varying less than 1.5× even when subjected to ≈150% strain. Furthermore, devices exhibit a maximum mechanical elongation before fracture, reaching 220%. Finally, the application of multi‐electrode spinal cord leads employing titanium nitride for neural stimulation in rat models is demonstrated.

Neural interfaces have undergone significant advancements in recent decades, aiming for flexible, compact, and high‐density electrode arrays with a large number of channels. However, the scalability of these devices is often hindered by the number of wires needed to separately connect each electrode to external electronics. To address this limitation, thin‐film transistor (TFT)‐based electronic circuits offer a promising solution by enabling active addressing of stimulation electrodes and potentially increasing the channel count with fewer interconnections. This paper presents the integration of a circuit comprising two amorphous indium–gallium–zinc‐oxide (a‐IGZO) TFTs and a titanium nitride (TiN) electrode on a flexible polymer platform. This circuit precisely controls the electrode's On/Off states and the delivery of current for nerve stimulation. Characterization studies involving frequency and electrochemical analysis demonstrate the TFT‐based circuit's capability to operate at high frequencies, deliver biphasic stimulation pulses to the electrode, and store sufficient charge for effective neural stimulation. Moreover, the a‐IGZO TFTs exhibit remarkable stability during repeated gate voltage sweeps with minimal changes in electrical performance. This circuit has the potential to be extended to active‐matrix devices that enable electrode arrays with a high number of channels and enhanced spatial resolution, which is crucial for selective neural stimulation.

We fabricated all solution based In ₂ O ₃ TFTs with AlO _x –PVP and HfAlO _x –PVP hybrid dielectric layers and observed the reduction of hysteresis by incorporation of HfO _x into the pristine AlO _x –PVP hybrid dielectric with enhanced electrical performance.