Restoring sensory feedback in prosthetic limbs is crucial for improving user acceptance, dexterity, and quality of life, potentially mitigating phantom limb pain. Peripheral nerve interfaces (PNIs) must offer high spatial resolution, minimal foreign body reaction, and remain within electrochemical safety limits for optimal performance. Long-term stability is paramount for successful chronic implantation. While some PNIs have shown long-term stability (years), many others have failed due to issues such as foreign body reactions, material degradation, or mechanical failures. Previous approaches, including longitudinal intrafascicular electrodes (LIFE), cuff electrodes (limited spatial selectivity), flat interface nerve electrodes (FINE), and microelectrode arrays (MEAs), have demonstrated both successes and limitations in chronic applications. The transverse intrafascicular multichannel electrode (TIME), with its polyimide-based thin-film design, showed promise in preclinical studies, but previous human trials revealed adhesion issues. This study aimed to evaluate the long-term (up to six months) performance and integrity of improved TIME implants in human subjects, addressing previous shortcomings and assessing the long-term stability of thin-film electrodes for permanent implantation.

The paper reviews several existing PNIs, highlighting their advantages and drawbacks. Longitudinal intrafascicular electrodes (LIFE) provided early success but lacked spatial selectivity. Cuff electrodes, while robust, offered poor spatial selectivity. Flat interface nerve electrodes (FINE) improved selectivity but required careful consideration of applied force. Microelectrode arrays (MEAs), initially designed for intracortical use, showed subchronic success but suffered from high failure rates due to mechanical and biological factors in chronic studies. The authors highlight the need for long-term stability and improved spatial selectivity, leading to the development and testing of the TIME electrode.

This study involved implanting twelve latest-generation TIME electrodes into the median and ulnar nerves of three transradial amputees for up to six months. Impedance measurements were conducted weekly to monitor electrode performance. After explantation, the electrodes underwent thorough analysis, including light microscopy, scanning electron microscopy (SEM), white light interferometry, and micro-computed tomography (µ-CT). Finite element analysis (FEA) was used to simulate the effects of thermal stress on different ground contact designs. The study also involved a subjective assessment of sensory feedback reported by the participants. Electrochemical impedance spectroscopy (EIS) was used to characterize the electrodes before and after implantation and hydration. A detailed analysis was also conducted on the connector to assess its integrity.

The redesigned TIME electrodes with split ground contacts demonstrated significantly improved mechanical stability compared to previous designs. Impedance values generally remained within the safe range (<150 kΩ) for a significant portion of the implantation period. While a decrease in the number of electrically functional contacts was observed, this correlated with connector failures rather than inherent electrode degradation. The modified connector design used in the third patient showed a significant improvement in channel retention. Post-explantation analysis revealed no signs of corrosion or significant morphological changes in the thin-film metallization, except for damage attributed to the explantation process itself. Even those electrodes classified as having significant damage during optical analysis maintained impedance values within the functional range. The sensory feedback provided by the electrodes was generally consistent and reliable throughout the study period, showing good patient response. In vitro tests showed no morphological change to the electrode even after 6.5 billion pulses which corresponds to over 6 years of daily stimulation. However, issues were discovered with the connector, causing channel failure and the need for design modifications. The new design showed great improvements in long term stability.

The results indicate that the improved design of the TIME electrodes, particularly the split ground contact and inclusion of adhesion layers, has dramatically increased their long-term stability in humans. The observed impedance changes and loss of electrical functionality were primarily attributed to connector failures, highlighting the importance of robust connector design for long-term implantable devices. The absence of corrosion and significant morphological changes to the thin-film electrodes after six months underscores the biocompatibility and long-term stability of the materials used. The finding that even mechanically damaged electrodes were electrically functional highlights redundancy of the split ground contact design. These findings support the feasibility of using thin-film electrodes in permanent nerve interfaces for restoring sensory feedback in prosthetic limbs. Furthermore, the study emphasizes the need for careful consideration of connector design and explantation procedures to minimize damage to the electrode and ensure long-term functionality.

This study demonstrates the long-term stability and functionality of improved polyimide-based thin-film transverse intrafascicular multichannel electrodes (TIME) for restoring sensory feedback in prosthetic hands. The redesigned ground contact and connector significantly enhance the longevity and reliability of these implants. Further research should focus on improving connector design and developing minimally invasive explantation techniques to better preserve implant integrity for post-implant analysis.

The relatively small sample size (three patients) limits the generalizability of the findings. The explantation process, necessary for detailed analysis, may have introduced mechanical damage to the electrodes, potentially underestimating their in vivo lifespan. The observed connector failures highlight a critical area for future improvement.