## 3 Results  
### IDE Sensor and Film Structure and Morphology  
Microscopic images of resulting IDE sensors, with and without spin-coated gelatin films before exposure to enzyme solutions, are displayed in Figure 1a. The gelatin films, depicted over the sensors on the left side of the image, were slightly frayed at the edges, but this did not directly affect sensor response as the edges were relatively distant from the IDEs. Figure 1b--d represent the same configuration, except after the sensor has been exposed to either PBS (b), trypsin (c), or all three enzymes (d) in PBS and has been dried. After surface profiling, it was evident that trypsin alone removes most the gelatin from the sensor surface, visualized in Figure 1c, while each other condition leaves a residue of film over the test sensor and more non-specific molecular adhesion to the control surface, such as the mixture-treated sensors depicted in Figure 1d. Table 1 describes the proportion ofremaining film thickness, mean values (\(n=2\)), after exposure to each solution condition, calculated using Equation (1), where \(t\) is the thickness at each time point:  
\[Film\;Thickness\;Remaining=\left(\frac{t_{pre-experiment}-t_{post-experiment}}{t_{pre- experiment}}\right) \tag{1}\]  
A value of 1 would indicate no film was lost, and each thickness was achieved after signal saturation and complete drying in ambient environment. SEM images of the drop-cast films in various conditions can be viewed in Figure 2, where (a,b) are side-view and (c,d) are top-down. In Figure 2a, the surface of the dry, cross-linked films appears rough with an edge thickness of \(\sim\)60 \(\upmu\)m. After exposure to trypsin for several hours, at which the absolute impedance had saturated, the sensor was rinsed with DIIH\({}_{2}\)O and completely dried. The resulting edge thickness was then measured at \(\sim\)9 \(\upmu\)m, reflecting the removal of film as a result of hydrolysis from trypsin. The ratio of remaining film to the original is slightly less than 0.15, which is relatively consistent with the ratio of film remaining with the spin-coated film from Table 1 (0.13). Furthermore, the film is much smoother, and can be observed again from the top-view in Figure 2c. \begin{table}
\begin{tabular}{c c} \hline
**Condition** & **Film Thickness Remaining (\%)** \\ \hline PBS (Phosphate Buffered Saline) & 78.6 \(\pm\) 0.05 \\ Amylase (1 mg/mL) & 24.3 \(\pm\) 0.08 \\ Lipase (1 mg/mL) & 52.4 \(\pm\) 0.30 \\ Trypsin (1 mg/mL) & 11.1 \(\pm\) 0.03 \\ Trypsin (0.5 mg/mL) & 21.7 \(\pm\) 0.10 \\ \hline \end{tabular}
\end{table} 
Table 1: Ratio of spin-coated film remaining after various enzyme conditions, analyzed with a contact profiler (\(n=2\)). Figure 1: Resulting interdigitated finger electrode (IDE) sensors, with (**Left**) and without (**Right**) gelatin films, (**a**) pre-exposure to solutions, and post-exposure to (**b**) buffer; (**c**) trypsin; or (**d**) all three enzymes in buffer. Scale bar = 1 mm. After the film was exposed to all three enzymes, however, the thickness reduced to only \(\sim\)0.68 of the original film after the impedance signal had saturated. This indicates saturation occurred before even half of the film had been degraded. Further, as depicted in Figure 2d, globules form in the film, some of which can exceed 15 um in diameter. This is likely due to the entrapment of non-specific enzymes in the gelatin matrix, possibly due to residue glutaraldehyde within the film, and possibly responsible for attenuating the peptide cleavage by trypsin [18; 19; 20]. Further, the immobilization of the non-specific enzymes in the gelatin may induce tighter bonds (i.e., aldimine with the amylase) to form, possibly explaining the decrease in thickness by them alone [21]. However, based on the results in Table 1, there is still less degradation with all three enzymes than what occurs with only amylase or lipase, suggesting enzymatic cross-interference in the solution before interactions occur at the gelatin surface.