Researchers at the University of North Texas have developed a real-time in-situ ultrasound monitoring system capable of tracking soft hydrogel 3D printing processes with subwavelength resolution. The work, published in Communications Engineering, provides new methods for observing critical transient phenomena during the additive manufacturing of gelatin-alginate hydrogels, including layer bonding, surface deformation, and gravitational spreading.
The research team, led by Yuqi Jin and Arup Neogi, integrated a 10 MHz immersion ultrasound transducer into a BIO X bioprinter platform. Their system allows for the continuous collection of acoustic signals as material is deposited, enabling quality control not only during printing but also during post-processing such as ionic crosslinking.
Ultrasound as a tool for bioprint quality control
While optical techniques like OCT and X-ray imaging have been explored for AM monitoring, they are often limited by material opacity or electromagnetic interference. In contrast, ultrasound offers greater penetration depth and is compatible with high water-content hydrogels. The team’s setup used a monostatic configuration with a transducer embedded in a custom aluminum substrate to track the reflected waveforms of each printed layer in real-time.
Numerical simulations using COMSOL Multiphysics were used to model signal behavior during printing. The ultrasound reflections enabled the researchers to detect minute changes in layer thickness, elasticity, and mechanical disturbances caused by nozzle scratching or inconsistent extrusion.
Monitoring geometry, surface quality, and material behavior
In tests using fully infilled hydrogel blocks, the researchers observed clear acoustic indicators of geometry build-up, followed by layer height degradation caused by cumulative weight and nozzle contact. Fourier analysis of reflected signals revealed both elastic and plastic deformations, with frequency dispersion correlating to surface roughness.
The ultrasound system was also tested on grid-infused scaffold prints, revealing gravitational sagging and structural shrinkage due to unsupported geometry. These phenomena, difficult to detect via conventional ex-situ methods, were visualized through phase spectra and dynamic bulk modulus (DBM) profiles.
The team further extended monitoring to the post-crosslinking stage. By immersing samples in CaCl₂ solution, they observed real-time stiffening of the printed structures alongside geometric distortion. Monitoring enabled the identification of optimal crosslinking durations to enhance mechanical properties without compromising structural integrity.
Toward smart bioprinting systems
The in-situ ultrasound system demonstrated in this study provides a pathway toward real-time defect detection, parameter tuning, and automated process optimization for soft material 3D printing. Although currently limited to localized measurements, the authors propose expanding the system using 2D phased arrays for full-plane coverage and integration with closed-loop feedback systems.
By providing insight into subtle mechanical and geometrical changes during bioprinting, the system could aid in producing consistent, high-quality hydrogel constructs for tissue engineering, drug delivery, and regenerative medicine applications.
This research builds on recent developments in hydrogel-based additive manufacturing. In 2021, scientists developed a high-strength seaweed-based hydrogel for 3D printing applications, highlighting alginate’s mechanical potential. More recently, 3D printed hydrogels have been explored for radiation shielding in space environments, underscoring their functional versatility. Meanwhile, a study on xolography introduced a volumetric approach for fabricating soft tissue constructs, pushing the boundaries of bioprinting resolution. These advances underscore the need for robust, real-time monitoring systems, such as the ultrasound-based approach demonstrated in the current study, to ensure quality control and reproducibility across complex hydrogel printing processes.
Read the full article in Communications Engineering
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Featured image shows in-situ monitoring experimental results from the printing of grided block. Image via Communications Engineering / University of North Texas.
