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Wearable mechatronic devices for upper-limb amputees 219

This implied that it is possible to achieve the same results with vibrotactile
feedback, which is cheaper and easier to implement than other forms of
feedback. Markovic et al. (2018) evaluated the impact of these factors with
a longitudinal assessment in six amputee subjects, using a clinical setup
(socket, embedded control) and a range of tasks (box and blocks, block turn,
clothespin, and cups relocation). To provide sensory feedback the study used
a novel vibrotactile configuration. The study demonstrated, for the first
time, the relevance of an advanced, multi-variable feedback interface for
dexterous, multi-functional prosthesis control in a clinically relevant setting.
The aforementioned devices are made of rigid materials and compo-
nents. Alternatively, a promising and interesting technology is soft robotics
that exploits the use of soft and flexible materials for developing soft wearable
devices with better adaptability and affordance with the human body. In this
context, soft devices are created for generating vibrations and vibrotactile
feedback for amputees. Sonar and Paik (2016) designed a soft pneumatic
actuator capable of inducing vibrotactile feedback and sensing the force pro-
vided on the skin by using PZT sensors embedded in a matrix of silicone
reaching a bandwidth of 56Hz. Georgarakis et al. (2017) enhanced this actu-
ator by using different 2D shapes in the inflation area, obtaining a bandwidth
of 120Hz, but with an amplitude of vibration in the order of 10μm. More
recently, Huaroto et al. (2018) explored the 3D shapes of the inflation area to
reach much larger amplitude displacements in vibration mode with a signif-
icant bandwidth of about 70Hz and displacements of 2mm as required to
generate kinesthetic illusions (Marasco et al., 2018).

Electrotactile
Electrotactile stimulation communicates nontactile information via electri-
cal stimulation of the sense of touch. This information can be transmitted by
using electrodes over/under the skin (noninvasive and invasive electro stim-
ulation, respectively) (Szeto and Saunders, 1982). Electrotactile (or electro-
cutaneous) stimulation evokes sensations within the skin by stimulating
afferent nerve endings through a local electrical current (Antfolk et al.,
2013). Typical currents range within 1–20mA with pulse frequencies rang-
ing from 1Hz to 5kHz; biphasic pulses produce more comfortable sensa-
tions (Szeto and Saunders, 1982). Moreover, several studies compare this
technique with vibrotactile stimulation, which is naturally noninvasive
(Kaczmarek et al., 1991). For instance, Witteveen et al. (2012) investigated
a longitudinal and transversal orientation of an array of four feedback