Conformable neural interfaces : from engineering stretchability at the micro-scale to translational applications

Recently, flexible and soft bioelectronic interfaces have been proposed as a solution to improve existing neural interfaces that currently present mechanical mismatch with the soft tissue. These are devices fabricated with thin polymeric or elastomeric backbones to enable an optimal contact to the target tissue as well as reduced mechanical mismatch. For this the electrical interconnects that carry the electrical information from the tissue or deliver the stimulation charges need to be elastic to accommodate the stresses and remain conductive. Several strategies for creating stretchable tracks have been proposed in the past but are not compatible with high-density low-profile electrode arrays for chronic implantation in vivo.A method of patterning thin material stacks of flexible polymers and platinum layers embedded in silicone membranes was proposed in the Laboratory for Soft Bioelectronic Interfaces at EPFL. By creating specific microscopic cuts in the materials permits the out-of-plane deformation of the film and enables stretchability above 50% strain while remaining conductive. However, the previous fabrication method was not optimal, was not hermetically encapsulated and not reliable. In this thesis, novel soft electrode arrays were designed and fabricated using an improved manufacturing process named core-shell micropatterning. This enables fully encapsulated electrical tracks that can be micropatterned with superior electromechanical performances and notable increase in hermetic barrier lifetimes. Additionally, this method was applied to other material systems enabling new functions such as completely transparent devices to enable concurrent optical stimulation and microscopy through the implanted electrodes.These fabricated devices were applied to several device concepts in multiple animal models. First, microelectrode arrays for electrocorticography (ECoG) recordings were designed to be implanted over the auditory cortex of rat and non-human primate (NHP) models to capture evoked potentials from acoustic stimulation. This enabled the precise tonotopic mapping and the spatial representation of the auditory cortex. Next, auditory brainstem implants (ABI), targeting the cochlear nucleus (CN) after the auditory nerve were designed. This implant needs to conform to the curved surface of the CN and stimulate the auditory pathway. In non-human primates, the placement of this device is challenging and new tools were developed for the insertion of the device. Finally, the combination of soft ABI and ECoG was implanted in chronic settings in a NHP to evoke auditory percepts from brainstem stimulation that could be monitored using the cortical evoked potentials. This will enable the fine-tuning of stimulation protocols for hearing restoration and verify the stability of the implants over time.Overall, this work represents a framework to design and fabricate soft electrode arrays that can be applied for recording and stimulation in small to large animal models in a reliable way. The careful design of the system around the implant such as fine-tuning of microfabrication parameters, the characterization in vitro, device geometry, connection scheme, experimental setup and tracking of the device properties in vivo enables reliable long-term implantations in new experimental paradigms.