Biomedical engineers at Duke University have demonstrated that a class of interwoven composite materials called semi-interpenetrating polymer networks (sIPNs) can be produced by living cells. The approach could make these versatile materials more biologically compatible for biomedical applications such as time-delayed drug delivery systems.

The concept of sIPNs has been around for more than 100 years and has been used in automotive parts, medical devices, molding compounds, and engineering plastics. The general idea is for one or more polymers to assemble around another polymer scaffold in such a way that they become interlocked. Even though the polymers are not chemically bonded, they cannot be pulled apart and form a new material with properties greater than the simple sum of its parts.

Traditional methods for manufacturing sIPNs typically involve producing the constituent parts called monomers and mixing them together in the right chemical conditions to control their assembly into large networks in a process called polymerization.

“When it works, it’s a fantastic platform that can incorporate different functionalities into the self-assembled layer for biomedical or environmental applications,” said Lingchong You, professor of biomedical engineering at Duke. “But the process is often not as biocompatible as you might want. So we thought why not use living cells to synthesize the second layer to make it as biocompatible as possible?”

In the new paper, Zhuojun Dai, a former postdoc in the You lab, who is now an associate professor at the Shenzhen Institute of Synthetic Biology, uses a platform that the lab has been developing for several years called “swarmbots” to do just that.

The swarmbots are living cells that are programmed to produce biological molecules within their walls and then explode once their population reaches a certain density. In this case, they’re programmed to produce monomers called elastin-like polypeptides (ELPs) fused to functional features called SpyTag and SpyCatcher. These two molecular structures form a lock-and-key system, allowing the ELPs to self-assemble into a polymer chain when mixed. As they grow, these polymers entangle themselves with the polymeric microcapsules containing the cells to form sIPNs.

Each monomer can contain multiple SpyTags or SpyCatchers and can also be fused to proteins that generate a readout or have specific functions. It’s sort of like making a chain-link fence out of many tiny charm bracelets that have room for clasps and charms.