the pores. Nonetheless, the derived expres-
sion for piezoionic coefficient provides useful
predictions regarding the effects of perme-
ability, viscosity, ion size, concentration, and
conductivity. For example, increased ion con-
centration leads to larger sense voltage (Fig. 2E).
A faster transient piezoionic response is ob-
served in the pAAm-based hydrogel when
polymer content is reduced (Fig. 2I). The
higher fluid permeability (fig. S5) again leads
to a faster poroelastic time constant in the 15%
film than in the 30% film, as expected. A faster
response is also observed as the gel thickness
decreases. The current response time decreases
from ~5 s at 1-mm thickness to 30 ms at
0.2 mm (Fig. 2J; close-up view of current re-
sponse in fig. S10), owing to the local pressure
spreading over a larger area as the hydrogel
thickness increases, and hence the pressure
gradient decreasing, as supported by simula-tion results in figs. S11 and S12, as well as
movies S11 to S13. The transient response
time varies greatly, from 30 ms at 0.2-mm
thickness to >10 s at 2-mm thickness (fig. S9),
similar to the response range (3 ms to 2 s) for
rapidly- to ultraslowly-adapting human mech-
anoreceptors ( 28 ). Cutaneous mechanorecep-
tors attain different frequency responses by
using specialized encapsulations around nerve
endings to spatiotemporally modulate theSCIENCEscience.org 29 APRIL 2022¥VOL 376 ISSUE 6592 505
Fig. 3. Properties and operation of a 16-element piezoionic mechano-
receptor array.(A) Mechanoreceptor unit offset voltage as a function of the
AA content, showing an increase in“resting”potential magnitude as backbone
charge increases (data are mean ± SD,N= 3). (B) Indentation induced peak
piezoionic voltages as a function of the AA content, showing that increased
AA content results in increased magnitude of the generated voltage (data are
mean ± SD,N= 3). (C) Peak generated voltage as a function of applied pressure
(data are mean ± SD,N= 3). Fit lines represent smoothing spline fits for
(A) to (C). (D) Photograph of a 16-element piezoionic mechanoreceptor array on
a wrist, demonstrating conformability. (E) Photograph and (F) corresponding
normalized voltage bar plot of the piezoionic mechanoreceptor array detecting a
single touch (movie S1). (G) Photograph and (H) corresponding normalized
voltage bar plot of the piezoionic mechanoreceptor array detecting multiple
touches. (movie S2).RESEARCH | REPORTS