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Wearable mechatronic devices for upper-limb amputees 221

matching and (2) modality matching (Kim et al., 2009). Kuiken et al. devel-
oped Targeted Reinnervation (TR) surgery that reroutes severed nerves
from the amputated limb to residual muscles and skin (Kuiken et al.,
2004, 2009; Dumanian et al., 2019). The results of these studies provided
a kind of somatotopic matching. So, in this context, some soft wearable
devices were developed to reproduce the modality matching in order to
complement the somatotopic matching (Huaroto et al., 2018, 2019). In
effect, the next generation of prosthesis robotics should satisfy these two
aforementioned conditions and enable direct connection to the nervous
human system. Hence, the next sections introduce the most important tech-
nologies developed up to now as modality-matched feedback devices.

Mechanotactile
One of the first devices developed to enable mechanotactile feedback was
proposed by Meek et al. (1989). Their device consisted of a “tactor” that
penetrates into the skin as a noninvasive stimulus using a force of about
9N, and a skin surface penetration of about 1cm. Many years later, other
devices were proposed, such as Kim’s tactor (see Fig. 3)(Kim et al.,
2009) and Casinis’ device, which was capable of providing a cutaneous stim-
ulation to a forearm in tangential direction by using two belts and two DC
motors with encoders to record the torsional angle (Casini et al., 2015). Most
rigid devices used for mechanotactile feedback are uncomfortable, have
visual discomfort, avoid the inherent suction between residual limb and
liner, and are likely to contaminate EMG signals (due to vibrations of
DC motors) (Kim et al., 2009). Currently, a new generation of robotics, soft
wearable robots, has opened the possibility of introducing soft and flexible,
smart and responsive materials in prosthetic applications with the possibility
of avoiding the loss of limb fixation suction due to holes in the liner to pass
touch/vibration to the residual limb, and the contamination of EMG signal
associated with rigid devices based on DC motors (Huaroto et al., 2018).
Some soft wearable devices have been recently developed that are capable
of exerting pressure and stretching on the skin (Agharese et al., 2018; Suarez
et al., 2018, and Huaroto et al., 2019).

Direct-neural
In the previous sections, we discussed noninvasive human stimulation;
however, nowadays several studies of direct-neural stimulation examined
how to connect a wearable robotic device directly to the human nervous
system (see Fig. 3).