Soft and stretchable nanocomposites can match the mechanical properties of neural tissue, thereby minimizing foreign body reactions to provide optimal stimulation and recording specificity. Soft materials for neural interfaces should simultaneously fulfill a wide range of requirements, including low Young's modulus (<<1 MPa), stretchability (>= 30%), high conductivity (>> 1000 S cm(-1)), biocompatibility, and chronic stability (>> 1 year). Current nanocomposites do not fulfill the above requirements, in particular not the combination of softness and high conductivity. Here, this challenge is addressed by developing a scalable and robust synthesis route based on polymeric reducing agents for smooth, high-aspect ratio gold nanowires (AuNWs) of controllable dimensions with excellent biocompatibility. AuNW-silicone composites show outstanding performance with nerve-like softness (250 kPa), high conductivity (16 000 S cm(-1)), and reversible stretchability. Soft multielectrode cuffs based on the composite achieve selective functional stimulation, recordings of sensory stimuli in rat sciatic nerves, and show an accelerated lifetime stability of >3 years. The scalable synthesis method provides a chemically stable alternative to the widely used AgNWs, thereby enabling new applications within electronics, biomedical devices, and electrochemistry.

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.

Vår kropp innehåller ett stort nätverk av nerver som förmedlar kommunikationen mellan hjärnan och kroppens organ. Detta system kallas det perifera nerv-systemet och består av mjuka och töjbara nervknippen som gradvis förmedlar en mer specifik funktionalitet då nerverna förgrenar sig till organen ute i kroppen som de ansluter till. Kommunikationen inom nerver är baserad på snabba fluktuationer i membranpotential längs med nervcellerna, så kallade aktionspotentialer. Med elektroniska nervgränssnitt kan dessa aktionspotentialer läsas ut eller induceras artificiellt. Detta ger möjligheter till att diagnosticera störningar i nervsystemet och med hjälp av nydanande elektrisk terapi behandla dem. Under det senaste årtiondet har därför intresset för perifera nervgränssnitt ökat inom biomedicinsk teknik. Nervgränssnittens funktion bygger på att elektroder implanteras i nära anslutning till de nerver som de ämnar läsa ut signaler ifrån eller stimulera. Ett av de vanligaste nervgränssnitten är så kallade cuff-elektroder, som viras runt nerven och stimulerar den genom att skicka elektriska pulser genom elektroder lokaliserade på insidan gentemot nerven. En utmaning är att kroppen regerar på alla främmande objekt, där styrkan i reaktionen beror på objektets biokompatibilitet. På senare tid har flera studier påvisat att skillnader i mekaniska egenskaper mellan vävnad och implantat kan ha stor påverkan på kroppens svar mot främmande objekt. Mjuka implantat som matchar nervers mekaniska egenskaper kan därför minska kroppssvaret och detta har skapat ett intresse för att utveckla tunna och mjuka nervgränssnitt. För att matcha en nervs mekaniska egenskaper räcker det dock inte att gränssnittet är tunt och flexibelt, det behöver också vara töjbart.

En vanlig strategi för att åstadkomma töjbara elektriska ledare är att skapa kompositmaterial bestående av ledande partiklar och elastomergummi. Ledningsförmågan i sådana kompositer kan bevaras även vid omfattande töjning då partiklarna kan röra sig i kompositen och på så sätt bevara det ledande nätverket. Töj-bar elektronik baserade på guldnanotrådar och silikongummi är en lovande kandidat för framtidens töjbara nervgränssnitt då materialen är biokompatibla, hög-presterande under töjning, och mönstringsbara med flera olika metoder. Denna avhandling utforskar sådana töjbara elektroniska material och komponenter för skapandet av elektriska gränssnitt för det perifera nervsystemet. I artikel I och II så utvecklas cuff-elektroder baserade på guldnanotrådar och silikongummi. Elektroderna är fullt funktionella vid 50% töjning och dess funktion verifieras in vivo. Artiklarna påvisar fördelarna med konformerbara elektroder genom att i artikel I åstadkomma nervstimulering med extremt låga spänningar av ischiasnerven i råttor och i artikel II uppnå selektiv stimulering av ischiasnerven i grisar med en flerelektrod-cuff. I artikel III studeras egenskaperna hos kompositer av guldnanotrådar och ett flertal olika elastomergummin. Studien visar på hur olika egenskaper av elastomeren avgör den elektriska prestandan för kompositer vid töjning. En alternativ teknik för att stimulera nerver utvecklas i artikel IV i form av ett töjbart jonleveranssystem som fungerar upp till 100% töjning.

The mass implementation of renewable energies is limited by the absence of efficient and affordable technology to store electrical energy. Thus, the development of new materials is needed to improve the performance of actual devices such as batteries or supercapacitors. Herein, the facile consecutive chemically oxidative polymerization of poly(1-amino-5-chloroanthraquinone) (PACA) and poly(3,4-ethylenedioxythiophene (PEDOT) resulting in a water dispersible material PACA-PEDOT is shown. The water-based slurry made of PACA-PEDOT nanoparticles can be processed as film coated in ambient atmosphere, a critical feature for scaling up the electrode manufacturing. The novel redox polymer electrode is a nanocomposite that withstands rapid charging (16 A g−1) and delivers high power (5000 W kg−1). At lower current density its storage capacity is high (198 mAh g−1) and displays improved cycling stability (60% after 5000 cycles). Its great electrochemical performance results from the combination of the redox reversibility of the quinone groups in PACA that allows a high amount of charge storage via Faradaic reactions and the high electronic conductivity of PEDOT to access to the redox-active sites. These promising results demonstrate the potential of PACA-PEDOT to make easily organic electrodes from a water-coating process, without toxic metals, and operating in non-flammable aqueous electrolyte for large scale pseudocapacitors.