Threadlike addressable neuromuscular microstimulators (eAXONs) have been developed and in vivo demonstrated. These injectable implants have a submillimetric diameter (0.97 mm diameter, 35 mm length) and consist of a microcircuit, which contains a single custom-developed integrated circuit, housed within a titanium capsule (0.7 mm diameter, 6.5 mm length), and two platinum–iridium coils that form two electrodes (3 mm length) located at opposite ends of a silicone body (Journal of Neural Engineering, 2022, 19(5): 056015 (doi: 10.1088/1741-2552/ac8dc4)). These devices are deployed percutaneously by injection and can form a network of microstimulators that can be controlled independently for producing complex movement patterns.
Several in vivo assays have been performed with the eAXON technology to demonstrate its potential for neuroprosthetics. For instance, in acute assays with anesthetized rabbits, it was demonstrated that antagonist movements of the foot can be produced by activating either eAXONs implanted in the tibialis anterior or in the gastrocnemius medialis muscles. And in acute assays with anesthetized sheep, in which eAXONs were implanted in the finger extensor muscle, in the finger flexor muscle, in the foot flexor muscle, and, in the foot extensor muscle, complex movement patterns were generated (e.g. the anesthetized sheep draw elliptical shapes on a whiteboard). Furthermore, a chronic study in rabbits has been carried out to demonstrate the long-term robustness of the eAXONs. (These results are pending publication.)
Because of its minimal invasiveness, a remarkable advantage offered by the eAXON technology is the capability to independently stimulate portions of a muscle. In addition to allowing finer muscle control, this allows activating the muscle fibers in a more physiological way for preventing muscle fatigue. This scheme is known as interleaved stimulation and its feasibility was demonstrated early in the project with conventional intramuscular electrodes (Journal of Neural Engineering, 2020, 17(4):046037 (doi: 10.1088/1741-2552/aba99e)) and later in the project with eAXONs implanted in sheep (pending publication).
In essence, the eAXON project not only has achieved the technological and scientific objectives originally planned but it has gone further by laying the foundations of a novel sensing technology (IEEE Transactions on Biomedical Circuits and Systems, 2020, 14(4):867-878 (doi: 10.1109/TBCAS.2020.3002326)) and of a brain stimulation and recording technology (patent application pending publication). In addition, co-developments within the EXTEND collaborative project (H2020-ICT-2017-1, grant agreement 779982) have demonstrated the eAXON technology in humans, a test scenario not considered in the eAXON project.
An additional global objective of the eAXON project was to illustrate that galvanic coupling through living tissues at high frequencies can be effectively and safely used for powering electronic implants in general; as an alternative to current energy transfer and harvest methods which require embedding bulky components within the implants. This objective was also achieved through a number of studies presented in journals and conferences that demonstrated that thread-like implants can safely draw powers in the order of a few milliwatts by galvanic coupling. Of particular relevance are the journal publications IEEE Access 2020, 8:37808-37820 (doi: 10.1109/ACCESS.2021.3096729) and IEEE Access, 2021, 9:100594-100605 (doi: 10.1109/ACCESS.2020.2975597). And, although these results were performed within the framework of the EXTEND project, it is also worth noting a study performed in humans published in IEEE Transactions on Biomedical Engineering, 2023, 70(2):659-670 (doi: 10.1109/TBME.2022.3200409).
