Fig. 3

Protecting MRR ultrasonic detector for long-term in vivo imaging. a Scanning electron microscope (SEM) image of a MRR ultrasonic detector fabricated by sNIL. Scale bar: 10 μm; (b) Magnified view of the waveguide coupling region highlighted by the orange dashed box in a. Scale bar: 500 nm. c Schematic and (d) SEM image of a cross-section of the fabricated MRR. The schematic illustrates potential contaminants attached to the waveguide. Scale bar: 500 nm. e Optical resonance diminished in 2 h due to contamination when unprotected MRR is exposed to whole blood. f Schematic and (g) SEM image of a cross-section of the MRR protected by an additional PDMS layer. Scale bar: 500 nm. The schematic illustrates that contamination to the waveguide is prevented. h Optical resonance remained unaffected after the protected MRR is exposed to whole blood for 2 h. i Comparison of Q-factors of MRRs with and without the protection layer in whole blood. Error bars show the standard deviation of Q-factor when fitting with the experimentally measured resonance spectrum