Sweat loss can help determine hydration status of individuals working in harsh conditions, which is especially relevant to those who wear thick personal protective equipment (PPE) such as firefighters. A wireless, passive, conformable sweat sensor sticker is described here that can be worn under and interrogated through thick clothing to simultaneously measure sweat loss volume and conductivity. The sticker consists of a laser-ablated, microfluidic channel and a resonant sensor transducer. The resonant sensor is wirelessly read with a handheld vector network analyzer coupled to two, co-planar, interrogation antennas that measure the transmission loss. A sweat proxy is used to fill the channels and it is determined that the sensor can orthogonally determine the sweat conductivity and volume filled in the channel via peak transmission loss magnitude and frequency respectively. A four-person study is then used to determine level of sensor variance caused by local tissue dielectric heterogeneity and sensor-reader orientation.

A passive, resonant sensor was developed that can be embedded in closed systems for wireless monitoring of hydrolytic enzyme activity. The resonators are rapidly prototyped from copper coated polyimide substrates that are masked using an indelible marker with an XY plotter and subsequently etched. The resonator's frequency response window is designed by the Archimedean coil length and pitch and is tuned for the 1-100 MHz range for better penetration through soil, water, and tissue. The resonant frequency is measured up to 5 cm stand-off distance by a coplanar, two-loop coil reader antenna attached to a vector network analyzer monitoring the S21 scattering parameter. The resonant frequency is modulated (up to 50 MHz redshift) by changing the relative permittivity of the medium in contact with the resonator (e.g., air to water). The resonant sensors are coated by an enzyme substrate, which, when degraded, causes a change in dielectric and a shift in resonant frequency (up to 7 MHz redshift). The activity (turnover rate, or k) of the enzyme is calculated by fitting the measured data via a custom transport and reaction model which simulates the radial digestion profile. This is used to test purified Subtilisin A and unpurified bacterial protease samples at concentrations of 30 mg/mL to 200 mg/mL with k ranges of 0.003-0.002 and 0.008-0.004 g/ g per second. The sensor response rate can be tuned by substrate composition (e.g., gelatin and glycerol plasticizer weight percentage). Finally, the utility of these sensors is demonstrated by wirelessly measuring the proteolytic activity of farm soil with a measured k of 0.00152 g/( g·s).

A passive resonant sensor with kirigami patterning is presented to wirelessly report material deformation in closed systems. The sensors are fabricated from copper‐coated polyimide by etching a conductive Archimedean spiral and then laser cutting kirigami patterns. The sensor response is defined as the resonant frequency in the transmission scattering parameter signal (S21), which is captured via a benchtop vector network analyzer. The sensors are tested over a 0–22 cm range of extension and show a significant shift in resonant frequency (e.g., 90 MHz shift for 10 cm stretch). Furthermore, the effect of resonator coil pitch on the extension sensor gain (MHz cm−1) and linear span of the sensor is studied. The repeatability of the sensor gain is confirmed by performing hysteresis cycles. The sensors is coated with polydimethylsiloxane films to protect from electrical shorting in aqueous environments. The coated resonators are placed in a pipe to report flow rates. The sensor with 1 mm coating is found to have the largest gain (0.17 MHz⋅s mL−1) and linear span (10–100 mL s−1). Thus, flexible resonant sensors with kirigami‐inspired patterns can be tuned via geometric and coating considerations to wirelessly report a large range of extension lengths for potential uses in health monitoring, motion tracking, deformation detection, and soft robotics.

This paper details a passive, inductor–capacitor (LC) resonant sensor embedded in a commercial dressing for low-cost, contact-free monitoring of a wound; this would enable tracking of the healing process while keeping the site closed and sterile. Spiral LC resonators were fabricated from flexible, copper-coated polyimide and interrogated using external reader antennas connected to a two-port vector network analyzer; the forward transmission scattering parameter (S21) magnitude was collected, and the resonant frequency (MHz) and the peak-to-peak amplitude of the resonant feature were identified. These increase during the healing process as the permittivity and conductivity of the tissue change. The sensor was first tested on gelatin-based tissue-mimicking phantoms that simulate layers of muscle, blood, fat, and skin at varying phases of wound healing. Finite element modeling was also used to verify the empirical results based on the expected variations in dielectric properties of the tissue. The performance of the resonant sensors for in vivo applications was investigated by conducting animal studies using canine patients that presented with a natural wound as well as a controlled cohort of rat models with surgically administered wounds. Finally, transfer functions are presented that relate the resonant frequency to wound size using an exponential model (R 2 = 0.58–0.96). The next steps in sensor design and fabrication as well as the reading platform to achieve the goal of a universal calibration curve are then discussed.

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