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Electronic AXONs: wireless microstimulators based on electronic rectification of epidermically applied currents

Periodic Reporting for period 2 - eAXON (Electronic AXONs: wireless microstimulators based on electronic rectification of epidermically applied currents)

Reporting period: 2018-11-01 to 2020-04-30

To build interfaces between the electronic domain and the human nervous system is one of the most demanding challenges of nowadays engineering. Fascinating developments have already been performed such as visual cortical implants for the blind and cochlear implants for the deaf. Yet implantation of most electrical stimulation systems requires complex surgeries which hamper their use in some clinical scenarios. In particular, previously developed systems based on central stimulation units are not adequate for applications in which a large number of sites must be individually stimulated over large and mobile body parts, thus hindering neuroprosthetic solutions for patients suffering paralysis due to spinal cord injury or other neurological disorders. A technological solution to address this challenge can consist in deploying a network of addressable single-channel wireless microstimulators implantable with simple procedures such as injection. Such solution was proposed and tried in the past. However, previous attempts did not achieve satisfactory success because the developed implants were stiff and too large. That is, those devices were too invasive for dense implantation. Further miniaturization was prevented because of the use of inductive coupling and batteries as energy sources. In the eAXON project we are exploring an innovative method for performing electrical stimulation in which the implanted microstimulators will operate as rectifiers of bursts of innocuous high frequency current supplied through skin electrodes shaped as garments. This approach has the potential to reduce the diameter of the implants to one-fifth the diameter of current microstimulators and, more significantly, to allow that most of the implants’ volume consists of materials whose density and flexibility match those of neighbouring living tissues for minimizing invasiveness. In fact, implants based on the proposed method will look like short pieces of flexible thread.
The main global objective of the eAXON project is to develop and to in vivo demonstrate electrical stimulation systems consisting of addressable wireless microelectronic implants (eAXONs) whose actuation principle will be based on electronic rectification of epidermically applied currents. These systems will exhibit an unprecedented level of minimal invasiveness which, in turn, will yield novel modes of application of electrical stimulation for therapeutics. A subjacent global objective of the proposal is to develop and demonstrate these systems specifically aiming at neuromuscular stimulation for developing neuroprosthetic systems of the type grouped under the term Functional Electrical Stimulation (FES).
Another global objective of the eAXON project is 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.
During its first 30 months, the eAXON project has progressed as it was anticipated in the project proposal. After consolidating a team of researchers composed by veteran and junior researchers in biomedical engineering and electronics, different technological milestones have been achieved and diverse numerical, in vitro and in vivo studies have been conducted which, so far, confirm the main hypotheses of the project and anticipate success for the global objectives. In particular it must be mentioned that the development of eAXON prototypes, that is, the injectable implants, is progressing adequately: a fully functional integrated circuit for demonstrating the eAXON principles will be ready by about month 36 and it has been stablished a set of packaging and interconnection technologies that will allow the implementation of robust and regulatory compliant eAXONs suitable for clinical use after the eAXON project. Also regarding technology it is worth noting that the development of the external systems required for operating the eAXONs is advancing as planned.
Regarding the numerical, in vitro and in vivo studies it is worth noting a few results which are only partially published: 1) it has been numerically and in vitro demonstrated that thread-like implants will be able to safely draw powers in the order of a few milliwatts by galvanic coupling, 2) it has been developed a novel sensing technology based on eAXONs and it has been numerically and in vitro demonstrated, 3) it has been in vivo demonstrated that distributed intramuscular stimulation, as the eAXONs will allow, avoids muscle fatigue and 4) it has been in vivo demonstrated the applicability of the eAXON technology for a very impactful clinical field totally different from neuroprosthetics (details are intentionally omitted due to confidentiality).
Presently, and as it was planned, the scholarly output of the eAXON project is rather modest. Few publications have been produced and few dissemination activities have been conducted. The main reason for that is the first half of the project was mainly devoted to team consolidation and to technology development. In addition, it must be noted that the manuscripts for some studies already concluded are under review or still under preparation or are withheld while patent applications are being considered and prepared.
Finally it is worth noting the alliance formed between the eAXON project and EXTEND project (identifier 779982 call H2020-ICT-2017-1). The members of the eAXON project also participate in the EXTEND project which, in essence, consists in exploring potential clinical uses of the eAXON technologies.
During the project, injectable wireless implants with a diameter below 1 mm able to perform intramuscular stimulation will be developed. In fact, prototypes according to such description have already been in vivo demonstrated in the project. The distinctive features of the implants that will be developed during the second half of the project will be: 1) these implants will be digitally addressable to enable selective stimulation at different nearby points, 2) these implants will be constructed using materials, encapsulation methods and fabrication procedures common in the medical devices industry for robustness and for regulatory compliance and 3) these implants will be able to record EMG signals.
In addition, through a series of experimental studies, the capabilities of the eAXON technology for developing neuroporsthetic systems will be demonstrated. In particular it will be demonstrated that it is possible to implant and to independently control a significant number of eAXONs (>20) in an animal limb for performing complex movement patterns. Such number of control channels for neuroprosthetics, which will be essentially given by the minimal invasiveness of the eAXONs and their implantation procedures, is unprecedented.
Picture of an intermediate integrated circuit prototype to be embedded within the eAXON implants
Artistic representation of the envisioned conformation of the eAXON implants