Section: Application Domains

Invasive stimulation (implanted FES)

Invasive FES means that the selectivity issue has to be dealt with, both from theoretical and technological points of view. To take advantage of spatial and topological nerve organization, invasive stimulation must be able to focus the current in specific nerve areas to elicit subgroups of muscles, while avoiding undesired functional effects (i.e., undesired fiber recruitment). Although multipolar electrodes are available, it is still challenging to find the optimal electrode configuration to reach the given 3D current spreading (i.e., selective stimulus). Indeed, this is not intuitive and modeling is mandatory. On the other hand, implantable stimulators must provide for both dynamical electrode configuration and a complex stimulation profile.

Selectively activating part of the nerve requires an active contact configuration (anode, cathode, high impedance), distribution of the current over the selected contacts, and accurate control of the overall total injected current, both from amplitude and time dimensions. To meet these needs, the neurostimulator has been designed based on a 2-stage device [50]. The first stage is the output stage based on a dedicated analog ASIC (application-specific integrated circuit) that is able to drive 12 channels of stimulation in absolute synchronization, with a programmable and controlled current distribution over selected contacts. The latest ASIC version we designed is CORAIL (circuit fabrication by November 2016): this analog/digital integrated circuit ensures current distribution but also such features as the storage of multiple electrode configurations and the possibility to internally combine poles. The second stage consists of a digital architecture embedded in an FPGA containing a dedicated processor for programming complex stimulation profiles, a monitoring module ensuring the respect of safety constraints stemming from both target tissue protection and electrode integrity preservation (in terms of quantity of injected charge limits), and a protocol stack for remote programming and online control of stimulation parameters. This complex digital system was formally developed using HILECOP §6.1.1.