On the viability of implantable electrodes for the natural control of artificial limbs: review and discussion

Abstract

The control of robotic prostheses based on pattern recognition algorithms is a widely studied subject that has shown promising results in acute experiments. The long-term implementation of this technology, however, has not yet been achieved due to practical issues that can be mainly attributed to the use of surface electrodes and their highly environmental dependency. This paper describes several implantable electrodes and discusses them as a solution for the natural control of artificial limbs. In this context "natural" is defined as producing control over limb movement analogous to that of an intact physiological system. This includes coordinated and simultaneous movements of different degrees of freedom. It also implies that the input signals must come from nerves or muscles that were originally meant to produce the intended movement and that feedback is perceived as originating in the missing limb without requiring burdensome levels of concentration. After scrutinizing different electrode designs and their clinical implementation, we concluded that the epimysial and cuff electrodes are currently promising candidates to achieving a long-term stable and natural control of robotic prosthetics, provided that communication from the electrodes to the outside of the body is guaranteed.

Figure 1

Implantable electrodes (IEs) for prosthetic control. This classification uses 4 categories taking account of the target and the location within the target: 1) extra-muscular, 2) intra-muscular, 3) extra-neural, 4) intra-neural. The invasiveness is proportional to selectivity and inverse to the simplicity of the implantation. IMES stands for Implantable Myoelectric Sensors.

Figure 2

A bipolar epimysial electrode. Illustration of a bipolar epimysial electrode with two PtIr discs mounted and covered by silicon sheets in which round windows allow the exposure of the PtIr.

Figure 3

Intra-muscular electrodes. Illustrations of (a) a coiled wire electrode where the exposed tip is the sensing part, and (b) an implantable myoelectric sensor where the sensing part is the electrode caps.

Figure 4

Cuff electrode. Cuff electrode with circumferential (A, B, D and E) and discrete (C) contacts with possible differential configurations in bipolar (1), tripolar (2) [46] and short-circuit tripolar (3) [47]. In the latter configuration, the outer electrode pair is short-circuited in order to yield a screening effect [47].

Figure 5

Spiral cuff electrode. Illustration of a self-sizing spiral cuff electrode with continuous or ring contacts.

Figure 6

Intrafascicular electrodes. Illustrations of (a) a polymer-based intrafascicular electrode (polyLIFE) based in the design from [67] and a multi-contact longitudinal intrafascicular electrode.

Figure 7

Micro-electrode array. Illustration of a micro-electrode array where the tip of each needle is the sensing area. Sketch based in [73].

Figure 8

Sieve electrode. Illustration of a sieve electrode where the sensing area is around the holes, even if 100% of the holes do not have a contact.