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614 XIE ET AL. IONO-ELASTOMERS FOR WEARABLE ELECTRONICS 615

Motion capture, especially, can be commonly sensitive strain sensors (10,18,25). For example, a related classes of block copolymers and ionic liquids breaks at 3000% elongation and has an ultimate ten-
found in surveillance, military, entertainment, recently reported carbon nanotube–silicone rubber can be used to tune the iono-elastomer’s physical sile strength of 200 MPa (36). Compared to a regular
sports, and medical applications (14,15). Conven- based strain sensor can be stretched to maximum and chemical properties. The variety and variability rubber band shown in Figure 4 (b), our iono-elas-
tional human motion capture is primarily based on strain of 500% with a good reversible response (26). of raw materials will not only cultivate diversity in tomer has about one order of magnitude higher
optical systems, inertial sensors, magnetic systems, Herein, we describe the invention of a simpli- our product and prototype invention but also lead extensibility. Remarkably, the conductivity of our
or mechanical systems. Optical systems, which are fied two-step manufacturing process to create to manifold commercialization streams. iono-elastomer increases with extension (36), which
intensively studied and widely used, typically come ultra-stretchable materials with tunable conductivity Utilizing the selected raw materials, we have suc- is a response opposite to that of most conductive
in two categories: systems with markers and sys- that are particularly applicable for wearable elec- cessfully demonstrated a simplified manufacturing materials, such as the calculation for the comparable
tems without markers. Marker systems require very tronics and associated technologies. At the heart of process to create stretchable conductive materials extension of a copper wire, as shown in Figure 5 (a).
complex equipment, a special environment, and are the fabrication of this novel iono-elastomer is the applicable for stretchable electronic technologies This is a unique and non-trivial material response
financially and spatiotemporally expensive. Mark- nanoscale hierarchical self-assembly of function- by self-assembly of concentrated solutions of the because, for instance, the electrical resistance of a
erless systems, while more convenient and more alized, commercially available polymers in a protic end-functionalized, commercially available, and inex- constant volume copper wire increases as it is (irre-
broadly applicable, have many drawbacks, such as ionic liquid, followed by chemical crosslinking. The pensive triblock copolymer Pluronic F127 in the versibly) extended into longer and thinner wire (as
requiring further digital processing using complex invention uses this novel iono-elastomer to cre- protic ionic liquid EAN followed by micelle corona depicted in Figure 5 (b)). The calculated, normal-
algorithms, sensitivity to the environment of use, and ate a transparent, lightweight, customizable, and crosslinking to generate elastomeric ion gels, termed ized electrical resistance as a function of elongation
generally not being as accurate as marker systems. skin-mountable strain sensor patch. The potential “iono-elastomers.”(35, 36) The chemical structures of strain is also plotted on Figure 5 (a), which shows
A review of these and other prevalent methods pro- for commercialization, including market size and Pluronic F127 and EAN are presented in Figure 2 (a) the opposite response of our iono-elastomer. This
vides an overview of the advantages and drawbacks competitive landscape, and potential benefits to soci- and (b), and a schematic of the synthesis and fabrica- novel mechano-electrical material property plays a
of the current methods (16). Improvements that can ety of this invention are presented and discussed. tion of the Pluronic F127 diacrylate iono-elastomer significant role in strain sensor device design because,
reduce cost, shrink the size and/or volume of the is shown in Figure 2 (c), (d), and (e). As shown in as resistance decreases under extension, the device
device, and minimize the influence on performers Description of Ultra-Stretchable Conductive Figure 2 (e), the resulting material is an optically clear, is anticipated to require less energy, thus increasing
while maintaining accuracy are highly desired. As Iono-Elastomer Invention free-standing elastomer, which is our “iono-elas- battery life. The origin of this novel electromechanical
body motion can often involve relatively large strains The raw materials were downselected to create a tomer.” This particular iono-elastomer exhibits an response is the complex microstructural rearrange-
(≥55%) (17,18), a possible solution is the creation of highly stretchable, conductive material that could unprecedented combination of high stretchability, ment of the hierarchically assembled micelles under
new wearable, flexible, and highly extensible strain spontaneously self-assemble at the nanoscale to form tunable ionic conductivity, and mechano-electrical uniaxial extension (36). To summarize, this stress-in-
sensors. a hierarchically-microstructured iono-elastomer. A response (36). duced microstructural rearrangement (depicted in
The design criteria for high-performance wearable, commercial triblock copolymer (Pluronic F127) Figure 3 demonstrates the stretchability of the Figure 6 (a)) consisting of the reversible formation of
flexible, and stretchable strain sensors includes high (27), which is a macromolecule with linear and/or iono-elastomer by stretching, twisting, and bending hexagonally close packed (HCP) layers of crosslinked
sensitivity (i.e., large gauge factor (GF) for measur- radial arrangements of two or more different blocks the material. To quantify the stretchability, we tested micelles produces ion channels between layers. This
ing small human motions), high flexibility and high of varying monomer compositions, was selected for the elongational properties of our iono-elastomer configuration reduces the tortuosity for ion transport
extensibility (capable of accommodating elongational the mechanical building block (28). Block copoly- using a Sentmanat Extensional Rheometer, as shown in the stretching direction (1) as compared to the ini-
strains of ≥55%), good stability (capable of measuring mers can impart mechanical strength to the system in Figure 4 (a) (36). The mechanical response shown tial configuration of randomly oriented face-centered
repetitive deformations with low hysteresis), and fast via self-assembly in suitable self-assembly media, as in Figure 4 (b) indicates that our iono-elastomer cubic (FCC) micelles; therefore, electrical resistance
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 Figure 1. (a) Schematic showing the hierarchically self-assembled microstructures formed from block copolymers in ionic liquid
Among them, carbon nanomaterial-based sensors two representative classes of “designer compounds,” and a list of the tunable parameters for reaching desired properties. (b) Left panel: Three ionic liquid categories—aprotic, protic and
have shown outstanding performance as highly meaning that specific combinations selected from the zwitterionic ionic liquids. Right panel: Desirable properties of ionic liquids have.