Ultrasound waves harden 3D-printed tools for biomedicine

Scientists at Duke University and Harvard Medical School have devised a biocompatible ink that solidifies into different 3D shapes and structures by absorbing ultrasound waves. As the ink responds to sound waves instead of light, it can be used in minimally invasive medicine to print precise structures directly within the body without the need for open surgery. Such structures also include scaffolds for tissue regeneration and micro-scale drug delivery systems for sustained therapy.

Although 3D printing tools are becoming common for producing prototypes of medical devices and engineering tissues used in wound healing, their production usually involves a difficult process needing a robust printing platform. This has prompted research into photo-sensitive inks that respond directly to targeted beams of light and quickly harden into a desired structure.

As ultrasound has been used clinically for many years to visualize internal organs and deep tissues in the body, the team hypothesized it could also provide energy to permit 3D printing of structures when combined with precisely designed ultrasound-responsive sonicated ink (called sono-ink). As reported in Science [Kuang et al. Science (2023) DOI: 10.1126/science.adi1563], a new printing method called deep-penetrating acoustic volumetric printing (DVAP) was therefore developed to offer greater penetration to reach tissues, bones and organs with high spatial precision.

When the sono-ink, a combination of hydrogels, microparticles and molecules, is delivered to the target area, a specialized ultrasound printing probe then sends focused ultrasound waves into the ink, hardening parts of it into detailed structures. As sono-ink is viscous, it can be injected into a targeted area quite easily. On completion, any remaining ink non-solidified that can be removed with a syringe.

The components of the sono-ink allow for adjusting the formula for many uses, including scaffolding to help heal a broken bone or adding bone mineral particles to make up for bone loss. Such flexibility also means it is possible to engineer the hardened formula to be more durable or more degradable, and the colors of their final print can also be changed.

As Y. Shrike Zhang, who designed the sono-ink, told Materials Today, “Our approach uses focused ultrasound (FUS) for DAVP and represents a significant advancement in the field of additive manufacturing by allowing, for the first time, 3D printing in scenarios where traditional light-based methods are ineffective.”

The team hope to further refine the technique, and to optimize the sono-inks and the ultrasound printing technology for greater precision, versatility, and biocompatibility. They also plan to investigate further applications in clinical and healthcare settings, and to design prototypes for particular medical uses.

“Our approach uses focused ultrasound (FUS) for DAVP and represents a significant advancement in the field of additive manufacturing by allowing, for the first time, 3D printing in scenarios where traditional light-based methods are ineffective.”Y. Shrike Zhang