modalities have been investigated to transduce physical or analytical, biologically derived signals. Among these include electrochemical (amperometric, impedance, voltaic), [10,11] electrical, [12] optical, [8] piezoelectric, [13] radio-frequency (RF), [3,14] mechanical, [15] plasmonic, [16] and fluorescence/colometric [17] modalities. While such techniques have found numerous roles in various modern applications, many of these approaches possess limitations in sample preparation, power consumption, system size/weight, microelectronics requirements, and/or lack true wireless operation. These limitations complicate their implementation in many emerging applications, such as implantable sensing.Dielectric sensors are a sensor class that typically monitors analyte via permittivity shifts in the environment ("labelfree" biosensors). These can be built into RF formats via structuring in the form of an antenna/resonator. In such RF sensors, permittivity shifts in the environment modulates the capacitance of the construct. This in-turn changes the spectral response of the sensor (via a shift in the magnitude and/or resonant frequency). Such shifts can be readout remotely via inductive coupling with a reader/ readout coil, wherein sensor information will be manifested in the reflection coefficient of this readout coil. RF sensors possess many aspects that make them an ideal platform for sensing-they are biocompatible, possess native wireless operation, are typically nondegradative, and are structurally robust. These sensors have seen a measure of success as pressure sensors (wherein sensors are sealed from the liquid environment), [3,18] yet these sensors have seen limited success operating as analytical sensors in practical modern systems beyond proof-of-concept. [19][20][21] We recently demonstrated a compact, modern iteration of the traditional RF sensor, termed an interlayer-RF resonator. [22] In a typical RF sensor, a capacitor and an inductor are fabricated in separate regions and electrically connected to form a resonant circuit. [23] Instead, our constructs utilize a broad-side coupled (BC), split-ring/multi-turn resonator architecture, [24] wherein planar coils are interceded by a multifunctional interlayer (Figure 1a). They perform favorably in comparison to sensors based on split-ring resonator (SRR), [25] complementarycoupled SRR, [26] and patch antenna architectures. [27] This is due Wearable sensors promise to transform human understanding of body state. However, despite many wearable sensor modalities that exist, few demonstrate the raw capabilities required for many emerging healthcare applications-passivity (and microelectronics-free), wireless readout, long-term operation, and specificity. Hydrogel-interlayer radio-frequency resonators are demonstrated as a versatile platform for passive and wireless biosensing. Fabricated using a simple vinyl cutter, the base resonator is composed of unanchored, broad-side coupled coils interceded by multifunctional hydrogels-such resonators are tuned to be sensitive t...
