Researchers have succeeded in developing a novel way of making tough, biocompatible materials called “hydrogels” in sophisticated and intricately patterned shapes—3D printing. The newly created process could offer new ways of delivering drugs or cells into the body, scaffolds for regenerating load-bearing tissues or tough but flexible actuators for robots.

At top, the structure of the hydrogel material is shown at different scales, down to the molecular level. At bottom, material 3-D printed using this method is so flexible that it can be squeezed flat, and then spring back to its full original shape. At top, the structure of the hydrogel material is shown at different scales, down to the molecular level. At bottom, material 3-D printed using this method is so flexible that it can be squeezed flat, and then spring back to its full original shape.

A paper in the journal Advanced Materials describes the process in detail, co-authored by MIT Associate Professor of Mechanical Engineering Xuanhe Zhao and colleagues at MIT, Duke University, and Columbia University.

Zhao says the new process could make it possible to 3D print “extremely tough and robust” hydrogel implants, which could be infused with cells and drugs and then placed in the body.

Hydrogels are defined by water molecules encased in rubbery polymer networks that provide shape and structure. Their material characteristics are similar to natural tissues such as cartilage, which is used by our bodies for shock absorption. These new 3D printed hydrogel structures could be used for replacement of knee cartilage in meniscus surgeries, for example.

Synthetic hydrogels are generally weak and tend to break easily, but a number of tough and flexible ones have been developed in the last 10 years. However, most previous methods of creating tough and flexible hydrogels involved creating “harsh chemical environments” that would kill encapsulated living cells, according to Zhao.

The new materials can synthesize together with living cells (think stem cells) which could then allow high viability of the cells, states Zhao, who holds a joint appointment in MIT’s Department of Civil and Environmental Engineering.

Previous efforts did not yield an ability to print complex 3D structures with tough hydrogels. The new biocompatible hydrogel can be fabricated into 3D structures such as a hollow cube, hemisphere, pyramid, twisted bundle, multilayer mesh, or physiologically relevant shapes such as a human nose or ear.

Zhao continues to explain that the new method also uses a commercially available 3D printing mechanism. “The innovation is really about the material—a new ink for 3-D printing of biocompatible tough hydrogel,” which as is turns out is a composite of two different biopolymers. “Each [material] individually is very weak and brittle, but once you put them together, it becomes very tough and strong. It’s like steel-reinforced concrete.”

The polymers act differently, but combined they create a material with the properties of both. One type of polymer provides elasticity, while the other allows it to “dissipate energy under deformation without breaking.” A biocompatible “nanoclay” rounds out the characteristics of the new material by helping it fine-tune the viscosity of the material, which allows for high levels of flow control as the material passes through the 3D-printing nozzle.

The flexible characteristics of this material are remarkable—a printed shape such as a pyramid “can be compressed by 99 percent, and then spring back to its original shape.” Sungmin Hong, a lead author of the paper, writes that it can also be stretched to five times its original size. This type of resilience reproduces key features of natural bodily tissues that have evolved to withstand a variety of forces, collisions and other physical events.

Not only could this material be used to custom-print shapes for the replacement of cartilaginous tissues in ears, noses, or load-bearing joints—the results of lab tests suggest that this material is even tougher than natural cartilage.

The downside of this material is its limited ability in terms of resolution. Currently limited to 500 micrometers in size, “We are enhancing the resolution to be able to print more accurate structures for applications,” says Zhao.

Source: MIT