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614 XIE ET AL.

Motion capture, especially, can be commonly sensitive strain sensors (10,18,25). For example, a
found in surveillance, military, entertainment, recently reported carbon nanotube–silicone rubber
sports, and medical applications (14,15). Conven- based strain sensor can be stretched to maximum
tional human motion capture is primarily based on strain of 500% with a good reversible response (26).
optical systems, inertial sensors, magnetic systems, Herein, we describe the invention of a simpli-
or mechanical systems. Optical systems, which are fied two-step manufacturing process to create
intensively studied and widely used, typically come ultra-stretchable materials with tunable conductivity
in two categories: systems with markers and sys- that are particularly applicable for wearable elec-
tems without markers. Marker systems require very tronics and associated technologies. At the heart of
complex equipment, a special environment, and are the fabrication of this novel iono-elastomer is the
financially and spatiotemporally expensive. Mark- nanoscale hierarchical self-assembly of function-
erless systems, while more convenient and more alized, commercially available polymers in a protic
broadly applicable, have many drawbacks, such as ionic liquid, followed by chemical crosslinking. The
requiring further digital processing using complex invention uses this novel iono-elastomer to cre-
algorithms, sensitivity to the environment of use, and ate a transparent, lightweight, customizable, and
generally not being as accurate as marker systems. skin-mountable strain sensor patch. The potential
A review of these and other prevalent methods pro- for commercialization, including market size and
vides an overview of the advantages and drawbacks competitive landscape, and potential benefits to soci-
of the current methods (16). Improvements that can ety of this invention are presented and discussed.
reduce cost, shrink the size and/or volume of the
device, and minimize the influence on performers Description of Ultra-Stretchable Conductive
while maintaining accuracy are highly desired. As Iono-Elastomer Invention
body motion can often involve relatively large strains The raw materials were downselected to create a
(≥55%) (17,18), a possible solution is the creation of highly stretchable, conductive material that could
new wearable, flexible, and highly extensible strain spontaneously self-assemble at the nanoscale to form
sensors. a hierarchically-microstructured iono-elastomer. A
The design criteria for high-performance wearable, commercial triblock copolymer (Pluronic F127)
flexible, and stretchable strain sensors includes high (27), which is a macromolecule with linear and/or
sensitivity (i.e., large gauge factor (GF) for measur- radial arrangements of two or more different blocks
ing small human motions), high flexibility and high of varying monomer compositions, was selected for
extensibility (capable of accommodating elongational the mechanical building block (28). Block copoly-
strains of ≥55%), good stability (capable of measuring mers can impart mechanical strength to the system
repetitive deformations with low hysteresis), and fast via self-assembly in suitable self-assembly media, as
response speed (fast signal acquisition). Moreover, shown in Figure 1 (a) (29). Conductivity is provided
it is desirable that these devices have a low material by ethylammonium nitrate (EAN) (30), which is
and fabrication cost and be technically simple, light- a room temperature protic ionic liquid. An ionic
weight, and small, as well as being biocompatible for liquid is chosen for its remarkable physio-chemical
skin-mountable applications and comfortable to wear properties: high ion conductivity (up to 100 mS/cm),
(19,20). Although conventional strain sensors have wide electrochemical windows (up to 5.8 V), and high
advantages in low fabrication cost, they typically electrochemical and thermal stability (31). Further-
have poor stretchability and sensitivity (maximum more, it has negligible vapor pressure, which implies
strain of 5% and GF ~ 2). Recent advances in creating that it does not evaporate at any service temperature
advanced strain sensors have focused on nanoma- (32,33). Importantly, EAN can also act as an effective
terials, e.g., graphene (18,21,22), carbon nanotubes self-assembly media for the block copolymer (34). In
(17,19,23), nanoparticles (24), and nanowires (8). addition, both block copolymers and ionic liquids are
Among them, carbon nanomaterial-based sensors two representative classes of “designer compounds,”
have shown outstanding performance as highly meaning that specific combinations selected from the