Introduction:: Neurotransmitters play essential roles in regulating neural circuit dynamics both in the central nervous system as well as at the peripheral, including the gastrointestinal tract. Their real-time monitoring will offer critical information for understanding neural function and diagnosing disease. However, bioelectronic tools to monitor the dynamics of neurotransmitters in vivo, especially in the enteric nervous systems, are underdeveloped. This is mainly owing to the limited availability of biosensing tools that are capable of examining soft, complex and actively moving organs. Here we introduce a tissue-mimicking, stretchable, neurochemical biological interface termed NeuroString, which is prepared by laser patterning of a metal-complexed polyimide into an interconnected graphene/nanoparticle network embedded in an elastomer. NeuroString sensors allow chronic in vivo real-time, multichannel and multiplexed monoamine sensing in the brain of behaving mouse, as well as measuring serotonin dynamics in the gut without undesired stimulations and perturbing peristaltic movements. The described elastic and conformable biosensing interface has broad potential for studying the impact of neurotransmitters on gut microbes, brain–gut communication and may ultimately be extended to biomolecular sensing in other soft organs across the body.
Materials and Methods:: Here we introduce a soft and stretchable graphene-based biosensing neural interface, termed ‘NeuroString’, to seamlessly interface with CNS and GI tissue, and enable real-time simultaneous monitoring of monoamine dynamics in both tissues. Graphene was selected as the electrode material owing to its good biocompatibility, high super-capacitive response during fast-scan cyclic voltammetry (FSCV), known catalytic activity towards amine oxidation, as well as high mechanical compliance in bending, stretching and twisting resulting from its atomic-level thickness. However, graphene monolayer cracks at less than 5% strain. We address this issue by embedding laser-induced graphene nanofibre networks, with transition metal nanoparticles decorated on the surface, into a polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) elastomer matrix to achieve high levels of softness and stretchability while preserving the unique electrochemical properties of the nanomaterials. The desired electrochemical properties, combined with the rapid laser patterning and the ease of transfer process enabled by SEBS, result in a versatile materials platform with high stretchability and rapid fabrication of arbitrary patterns.
Results, Conclusions, and Discussions:: In summary, by developing a sensor based on the graphene–elastomer composite, we demonstrate that NeuroString functions as a soft bioelectronic interface to monitor the dynamics of monoamine neurotransmitters, including DA and 5-HT, in both the brain and the gut of living animals. With tissue-like mechanical properties, the NeuroString can interface acutely with the GI mucosa and is compatible with traditional medical inspection devices, such as endoscopy, for non-invasive biomolecular monitoring. NeuroString allows chronically stable and multiplexed neurochemical sensing in the mouse CNS. In the GI tract, the stretchability and softness of NeuroString offers high conformability with intestinal tissue without disturbing the peristaltic movement and without inducing undesired stimulation. The unique elastic features of NeuroString make it suitable for simultaneous monitoring of neurotransmitter signalling from both the central and the peripheral nervous systems, and potentially address current technical limitations in studying the dynamics of gut chemistry and its interplay with microbes. Although inheriting the limitations of the voltammetry method makes the sensitivity and selectivity of NeuroString in general worse than the latest genetically encoded fluorescence probes, such electrochemical methods are advantageous for translational use in humans. NeuroString can also simultaneously sense multiple biomolecules at multiple locations. Further development will be dedicated to improving the spatial resolution of the sensor using microfabrication/nanofabrication, to improve its selectivity and multiplexity by incorporating different molecular recognition probes and eventually to integrate it with wireless electronics and to validate its long-term implantation performance. Combining the excellent mechanical properties with the versatility of chemical sensing provided by the graphene surface chemistry, we predict that the NeuroString platform will be readily adaptable for studying the dynamics of various signalling biomolecules and electrophysiological signals throughout the body in primates.