Fig. 2: Characterizations of the soft photoacoustic patch.

a Simulated optical intensity distribution in tissue. The 3D distribution map comprises 51 horizontal planes stacked together with display transparency of 60%. The four slices at the bottom panel highlight the optical intensity distribution at different cross-sections. VCSEL: vertical-cavity surface-emitting laser. b Thermal imaging of the photoacoustic patch on the arm immediately after turning on the lasers and after one hour of continuous operation. The maximum temperature is below 36 °C, which is comfortable for long-term wear. c Testing of the VCSELs’ output stability as they continuously work for 1 h with a pulse duration of 200 ns at a pulse repetition frequency of 3 kHz. The insets correspond to the 100th and the 10 millionth pulses. The decrease in normalized light intensity after an hour is <4%. d Simulated photoacoustic detection sensitivity distribution in tissue. The 3D distribution map comprises 51 horizontal planes stacked together with display transparency of 60%. The four slices at the bottom panel highlight the detection sensitivity distribution at different cross-sections. e Photoacoustic impulse response of the patch, in both time and frequency domains, characterized by detecting signal of a hair excited by the VCSELs. f A photoacoustic image of a hair at a depth of 2 cm in a gelatin phantom. The blue and red curves are the Gaussian fit to the lateral and axial photoacoustic amplitude profiles (black dots), respectively. The lateral and axial resolutions are determined by the FWHM in different directions. g The lateral and axial resolutions at different depths.