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or polymer filaments that are inserted longitudinally into individual nerve
fascicles, so as to lay in-between and parallel to the nerve fibers
(
Lawrence, Dhillon, Jensen, Yoshida, & Horch, 2004; Yoshida & Horch,
1993; Yoshida & Stein, 1999
). The intrafascicular location makes them able
to record and/or stimulate only small groups of axons providing high selec-
tivity. An evolution of these electrodes is the thin-film LIFE (tfLIFE) based
on a thin micropatterned polyimide substrate. This highly flexible substrate
filament is folded in half, and each side can host a number of active sites
within a small surface, thus allowing selective multiunit nerve recording
and stimulation (
Navarro et al., 2007
). Although initially developed for
use with FES, LIFE electrodes are being used also for bidirectional interfac-
ing peripheral nerves after lesions and amputations. After 3 months, implan-
tation in the rat sciatic nerve tfLIFE did not induce noticeable abnormalities
in nerve function and morphology, and only a mild inflammatory reaction
around the electrode was observed (
Lago, Udina, Ramachandran, &
Navarro, 2007
). Several cases have been reported of LIFEs implanted in
human amputees, providing evidence of their effective use for controlling
advanced hand prostheses, and for delivering sensory feedback to the subject
(
Dhillon, Kr¨ger, Sandhu, & Horch, 2005; Rossini et al., 2010
).
Transverse intrafascicular multichannel electrodes
(TIMEs) (
Fig. 2.1
D) are
designed to be implanted transversally into the nerve in order to access dif-
ferent groups of nerve fibers. In comparison with LIFE that has high selec-
tivity to interface a small population of nerve fibers within one fascicle, the
TIME is able to record or stimulate different subsets of axons in various fas-
cicles over the nerve cross-section and obtain a reasonable spatial selectivity
(
Boretius et al., 2010
). Studies in rats have shown that after implantation in
the sciatic nerve, it was possible to selectively activate different muscles
innervated by distinct branches of the nerve or even within the same fascicle,
while LIFE was only able to stimulate one of each fascicle (
Badia, Boretius,
Andreu, et al., 2011
). In addition, no significant axonal damage was found
2 months after implantation in the sciatic nerve, suggesting the TIME as a
good candidate for chronic implantation even in small peripheral nerves
(
Badia, Boretius, Pascual-Font, et al., 2011
).
The
self-opening intrafascicular neural interface
(SELINE) is an evolution of
tfLIFE and TIME electrodes. The electrode is composed of a looped pol-
yimide thin film body and lateral wings with active sites on them
(
Cutrone et al., 2011
). After insertion of the structure in the nerve, it is
gently pulled away allowing the wings to open transversely. Therefore,
the wings move within fascicles, offering a second dimension for contacting