A sound option

When the concept of 3D printing was first introduced in the works of sci-fi writer Murray Leinster in 1935, it seemed a far reach from ever becoming reality. But almost 80 years later, 3D printers are used worldwide in a range of industries, including medicine. Despite how established the field has become, the concept that 3D printing could one day be done using sound may at first seem as equally farfetched as Leinster’s ideas once were – but that’s exactly what researchers at Concordia University have done. They named the new methodology direct sound printing (DSP). “DSP works based on the concept of chemical interaction between cavitation bubbles and the printing medium,” explains Mohsen Habibi, former research assistant at Concordia University and a key member of the team who developed DSP.

Cavitation describes a process whereby pressure variations in a liquid can in a short period of time cause countless small cavities to form and then implode. “These tiny bubbles are famous for their destructive force in many engineering fields,” Habibi adds. He uses the example of propellers on watercrafts, where the continuous stream of bubbles created by the action of the propeller collapse and erode the metal over time. Another example can be seen in nature. When under attack, a mantis shrimp can strike its club with an acceleration force proportional to the firing of a 22-caliber bullet. The strike moves the water so quickly that it creates a low-pressure area and forms a bubble. The bubble then collapses in a burst of high-energy light and sound that is strong enough to almost match the power of the strike itself.

The team at Concordia saw potential in the phenomenon of cavitation and hypothesised that it could be used to drive chemical reactions in certain conditions. “In DSP, we could tame this force in nature to use it for creation, instead of destruction,” says Habibi. He goes on to explain the limitations of light and heat mean they weren’t viable for the specific targeting of energy required for 3D printing. “Therefore, we looked for an alternative route to deposit energy in the remote locations while still being capable of inducing chemical reactions,” he continues. “Sound and specifically ultrasound seemed an interesting alternative since they can pass through objects and also create chemical reactions through the generation of tiny bubbles in the medium, via sonochemistry.”

Printing inside the body

One of the potential capabilities of DSP that is being considered by the team of researchers is printing implants or devices directly into the human body. “Direct sound printing enables non-invasive 3D printing inside the body without an open surgery,” says Habibi. “With this concept, a prosthesis can be created inside the body without the need to hospitalise the patient and perform open surgery, which means a shorter recovery time and less medical expenses. The field of DSP is still at its early stages but has received huge attraction from industries and we hope that soon we can have meaningful progress on that front.”

Of course, that’s not to say DSP doesn’t come with its own set of issues. In particular, when it comes to the regulatory considerations and hurdles associated with using DSP for medical purposes, especially when dealing with implants or other internal structures. “One of the medical applications of high-intensity focused ultrasound is tumour ablation, which the FDA has approved as a method for treating prostate cancer,” explains Shervin Foroughi, a PhD candidate at Concordia University and part of the research team with Habibi. “Since DSP employs high-intensity ultrasound energy, some of the already existing regulatory considerations of the HIFU ablation can be applied while implementing DSP for inbody printing. On the other hand, the DSP process can be controlled and adjusted with respect to the desired construct to be fabricated at the target. In fact, the application of the DSP for in-body printing is at its early stage, and we believe that regulatory considerations corresponding to the DSP in-body printing should be developed in the future.”

“We looked for an alternative route to deposit energy in the remote locations while still being capable of inducing chemical reactions. Sound and specifically ultrasound seemed an interesting alternative since they can pass through objects and also create chemical reactions through the generation of tiny bubbles in the medium, via sonochemistry.”
Mohsen Habibi

Medical device manufacturing

While we may not be using DSP to print directly into the body anytime soon, we may start to see the process being used to create medical devices in the next few years, which is where the research team sees value in the technology right now. Specifically, Habibi and the team point to thermoset polymers. These are often the common choice for handheld medical devices due to their durability and noncorrosive qualities, but also as a biocompatible material for use inside the body or for in vitro organ-on-a-chip devices, as is the case with polydimethylsiloxane (PDMS). As Habibi and his colleagues note in their published Nature Communications paper, thermoset polymers require arduous post-processing to remove toxic photopolymerisation by-products and unreacted compounds by solvents used in light-based methods; for thermal-based methods, the addition of additives to alter the viscosity of the printing material – explained as its ability to resist the forces upon it as it cures using extreme heat – as well as the use of a supportive bath to achieve the same end, can create geometric inaccuracies in the final product.

“An effective on-demand curing of heat-curing polymers is yet to be introduced due to the difficulty of applying very short heating and cooling rates at small, localised regions,” explains Habibi. “Sonochemistry can be a solution to print such materials due to its highly localised temperature with fast heating and cooling rates.” To delve more into the science at work: acoustic cavitation creates chemically active regions in the printing resin or resin mixture medium, in which the resin undergoes fast phase transformation from liquid to solid under sonochemical reactions. This means with DSP, heat-curing polymers with a free radical polymerisation process such as heatcuring thermosets, which could not be printed using light or heat-based processes, can now be printed directly.

Habibi and his colleagues’ paper did highlight a few issues, including difficulty with multimaterial printing using the mechanism harnessed in their research, as the build chamber needed to be refilled each time a new material was used for printing. Another difficulty was printing overhanging structures, as the printed spot needed to be supported. The researchers also noted that an alternative mechanism based on the same DSP concept along with the use of support materials can help resolve these problems, however. This is something that Habibi says they have already begun to develop.

The future of DSP

Clearly, there are adaptations and refinements required to realise the lofty ambitions of Habibi and the team, but given the promise DSP holds for 3D printing, he expects the method to develop rapidly as fellow researchers contribute and collaborate to the science. “DSP introduced a completely different paradigm in 3D printing,” he says. “We expect to see more applications of DSP in engineering and medical fields, which will lead to the commercialisation of products for different industries. In addition, we expect that more and more researchers will be attracted to this new field and start to participate in the collaborating efforts to develop this area, just like in every other discipline in science.”

As for when we can expect to see DSP being used to print prosthetics directly into the body, we may be waiting a while. “We presented the concept of using DSP to print directly into the body in 2022 and we are working on the real-use case,” says Habibi. “We expect to see the results soon and to publish them in peer-reviewed journals. It is difficult to predict the future at such a level of cutting-edge technologies, but we are determined to make it happen.”

In terms of what’s next for the team of researchers, we can expect to see them to continue to refine the direct sound printing process and prepare it for use in clinical practice, as well as outside of the medical industry. As Habibi explains: “Besides the medical aspect of DSP, we are working on the mechanical and acoustic aspects of this technology to mature the method as much as possible. We plan to publish out results in 2023 in two papers. We also hope that these developments make the technology ready for commercial use soon.”