Real-time in-situ ultrasound monitoring of soft hydrogel 3D printing with subwavelength resolution.

3D bioprinting has excellent potential in tissue engineering, regenerative medicine, and drug delivery systems due to the ability to fabricate intricate structures that are challenging to make with conventional manufacturing methods. However, the complexity of parametric combinations and lack of product quality control have restricted soft hydrogel bioprinting from practical applications. Here we show an in-situ ultrasound monitoring system that reveals the alginate-gelatin hydrogel's additive manufacturing process. We use this technique to understand the parameters that influenced transient printing behaviors and material properties in approximately real-time. This unique monitoring process can facilitate the detection of minor errors/flaws during the printing. By analyzing the ultrasonic reflected signals in both time and frequency domains, transient printing information can be obtained from 3D printed soft hydrogels during the processes with a depth subwavelength resolution approaching 0.78 . This in-situ technique monitors the printing behaviors regarding the constructed film, interlayer bonding, transient effective elastic constant, layer-wise surface roughness (elastic or plastic), nozzle indentation/scratching, and gravitational spreading. The simulation-verified experimental methods monitored fully infilled printing and gridded pattern printing conditions. Furthermore, the proposed ultrasound system also experimentally monitored the post-crosslinking process of alginate-gelatin hydrogel in CaCl solution. The results can optimize crosslinking time by balancing the hydrogel's stiffness enhancement and geometrical distortion.