A passive, low-cost resonant sensor was developed with the potential application of wireless monitoring of hydrolytic enzyme activity in closed systems. The resonators are rapidly prototyped from polyimide substrates (25à µm thickness) which are coated with a thin layer of copper (35à µm thickness). The patterns of the resonators, which are Archimedean spirals, are drawn on these substrates using an indelible marker with an XY plotter. These substrates are etched with a solution containing hydrogen peroxide and hydrochloric acid in order to remove the undesired copper.

The initial resonant frequency of these resonators can be controlled by the Archimedean coil length and pitch size of the spiral. The frequency response window is tuned for the 1-100 MHz range for better penetration through soil, water, and tissue. The resonant frequency can be measured up to 5cm stand-off distance by a 3D-printed coplanar, two-loop coil reader antenna. This reader is attached to a vector network analyzer for monitoring the magnitude of S21 scattering parameter. The central hypothesis is that the Archimedean spiral sensors respond to any change in relative permittivity of the medium in contact with the resonator. This response is represented as a clear shift in the resonant frequency of the resonator. For instance, changing the medium from air to water results in approximately 50MHz redshift in the resonant frequency.

In order to measure hydrolytic enzyme activity, the resonant sensors are coated by an enzyme substrate (e.g. hydrogel). The degradation of the enzyme substrate causes a change in the relative permittivity which results in a shift in the resonant frequency (up to 7MHz redshift). By fitting a transport-reaction model, which simulates the radial digestion profile, on the experimental data the activity (turnover rate, or kcat value) of the enzyme is calculated. This approach is used for testing purified Subtilisin A and unpurified bacterial protease samples at different concentrations ranging from 30mg/ml to 200mg/ml with kcat values of 0.003-0.002 and 0.009-0.004 gsubstrate/genzyme per second, respectively. The sensor response rate can be tuned by changing the substrate composition (i.e. changing the gelatin and glycerol plasticizer weight percentage in the hydrogel). Finally, the applicability of these resonant sensors in a real-life problem is demonstrated by wirelessly measuring the proteolytic activity of farm soil with a measured kcat of 0.00152 gsubstrate/(gsoilà ·s) using 3D-printed plastic cases.