Stereolithography — a method of 3D printing — uses an ultraviolet laser controlled by a computer-aided design system to trace patterns across the surface of a photoactive polymer solution. The light causes the polymers to link together, forming solid 3D structures from the solution. The tracing process is repeated until an entire object is built from the bottom up. Stereolithographic printing usually uses photoactive polymers that link together with covalent bonds, which are strong, but irreversible.

3D-printed, intricate, temporary microstructures can be degraded on demand using a biocompatible chemical trigger. (Wong Lab/Brown University)

In this work, researchers created structures with potentially reversible ionic bonds using light-based 3D printing. Precursor solutions were made with sodium alginate, a compound derived from seaweed that is known to be capable of ionic cross-linking.

The 3D-printed biomaterials can degrade on demand, which can be useful in making intricately patterned microfluidic devices, or in making cell cultures that can change dynamically during experiments. The attachments between polymers come apart when the ions are removed, which is achieved by adding a chelating agent that grabs the ions. This enables patterning of transient structures that dissolve away on demand.

By using different combinations of ionic salts — magnesium, barium, and calcium — structures could be created with varying stiffness that could then be dissolved away at varying rates. The researchers demonstrated that alginate could be used as a template for making lab-on-a-chip devices with complex microfluidic channels. The shape of the channel is printed using alginate, then a permanent structure is printed around it using a second biomaterial. The alginate is dissolved away, and the hollow channel remains with no cutting or complex assembly required.

The degradable alginate structures are useful for making dynamic environments for experiments with live cells. A series of experiments was performed with alginate barriers surrounded by human mammary cells, observing how the cells migrate when the barrier is dissolved away. These kinds of experiments can be useful in investigating wound-healing processes or the migration of cells in cancer.

The experiments showed that neither the alginate barrier nor the chelating agent used to dissolve it away had any appreciable toxicity to the cells. That suggests that degradable alginate barriers are a promising option for such experiments.

The biocompatibility of the alginate is promising for additional future applications, including making scaffolds for artificial tissue and organs. The technique could be used to template vasculature such as blood vessels using alginate, and then dissolve it away.

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