Materials and Devices for Stretchable Electronic Nerve Interfaces

Abstract: Within our body, there is a large network of nerves that facilitates communication between the brain and the body’s organs. This network is called our peripheral nervous system, consisting of soft and stretchable nerve bundles that gradually increase in their functional specificity as they split and branch out the closer they get to their target organ. Communication within the nerve is based on action potentials, fast fluctuations in electric trans-membrane potential along the neurons within the nerve. These action potentials can be recorded and artificially triggered by interfacing electronically with peripheral nerves. In doing so, modern medicine is able to elucidate the mechanisms behind disorders related to the nervous system and even applies novel electronic therapies to treat them. Over the last decade, the field of biomedical engineering has therefore seen a surge of interest in electronic devices that interface with the peripheral nervous system, such as cuff electrodes. The device function is based on electrodes that are implanted in close proximity of the nerves they intend to record or stimulate. A cuff electrode, specifically, is wrapped around a peripheral nerve and applies stimulation pulses at electrodes located on the inside of the cuff to evoke action potentials within the nerve. Our body is not welcoming to foreign objects though. Any implant within our body triggers a foreign body reaction with an intensity dependent on the biocompatibility of the implant. Recent studies have found that one of the major factors governing the foreign body reaction is the mechanical mismatch of the implant to the interfacing tissue, with softer, more mechanically similar implants, exhibiting reduced foreign body response. This has prompted an ongoing push for thin and soft peripheral nerve interfaces. However, to truly match the mechanical properties of peripheral nerves, peripheral nerve interfaces need not only to be soft and flexible, they need to become as elastic and stretchable as the nerve themselves. A common strategy to achieve stretchable conductors is by incorporating highly conductive filler materials in an elastomeric matrix. The resulting composite remains conductive even when stretched due to the ability of the filler material to dislocate with the elastomeric matrix while retaining its interconnectivity and thus conductivity. Electronic composites based on gold nanowires and silicones are promising candidates for stretchable peripheral nerve interfaces, due to their material-based biocompatibility, good stretchability, and versatile patterning possibilities.Based on this, the thesis at hand investigated stretchable electronic composite materials and devices to interface with the peripheral nervous system. Publication I and II develop gold-nanowire/polydimethylsiloxane-based cuff electrodes, which are functional even at 50% strain, as peripheral nerve interfaces in vivo. These publications highlight the beneficial conformability of stretchable devices, with a stretchable bi-polar cuff for low-voltage stimulation of the rat sciatic nerve in publication I and a stretchable multi-electrode cuff for selective stimulation of the pig sciatic nerve in publication II. Publication III investigates stretchable gold-nanowire composites based on a variety of elastomers, therein, elucidating the influence of the varying elastomer properties on the electromechanical performance of gold-nanowire composites. Lastly, publication IV establishes a stretchable ion delivery device with potential use for the peripheral nervous system. The device is based on an ionically conductive membrane as the conductive filler, and the device can be reversibly stretched to 100% strain. Overall, this thesis presents stretchable materials and devices that advance the possibilities for peripheral nerve interfaces.