Hydrogel manipulated into 3D forms for use in soft robotics

Scientists have found a way to direct the growth of hydrogel to mimic plant or animal tissue structure and shapes, an advance that could be utilised in soft robotics.

soft robotics
Self-assembled hydrogel bowl

The findings, published in Proceedings of the National Academy of Sciences, suggest new applications in areas where hydrogel is commonly used, such as such as tissue engineering or compliant robots. The team from Nanyang Technological University, Singapore (NTU Singapore) and Carnegie Mellon University (CMU) has filed a patent on the development.

In nature, plant or animal tissues are formed as new biomass is added to existing structures. Their shape is the result of different parts of those tissues growing at different rates.

Mimicking this behaviour of biological tissues in nature, the research team comprising CMU scientists Changjin Huang, David Quinn, K. Jimmy Hsia and NTU President-designate Prof Subra Suresh, showed that through manipulation of oxygen concentration, it is possible to pattern and control the growth rate of hydrogels to create the desired complex 3D shapes.

The team found that higher oxygen concentrations slow down the cross-linking of chemicals in the hydrogel, inhibiting growth in that specific area.

Mechanical constraints such as soft wire, or glass substrate which chemically binds with the gel, can also be used to manipulate the self-assembly and formation of hydrogels into complex structures.

Such complex organ structures are essential for performing specialised body functions. For example, humans' small intestines are covered with microscopic folds known as villi, which increase the gut's surface area for more efficient absorption of food nutrients.

The new technique is said to differ from previous methods which create 3D structures by adding/printing or subtracting layers of materials. This technique, however, relies on continuous polymerisation of monomers inside the porous hydrogel, similar to the process of enlargement and proliferation of living cells in organic tissues.

Most living systems adopt a continuous growth model, so the new technique which mimics this approach will potentially be a useful tool for researchers to study growth phenomena in living systems.

"Greater control of the growth and self-assembly of hydrogels into complex structures offers a range of possibilities in medical and robotics fields. One field that stands to benefit is tissue engineering, where the goal is to replace damaged biological tissues, such as in knee repairs or in creating artificial livers," said Prof Subra Suresh.

Growth-controlled and structure-controlled hydrogels are also useful in the study and development of flexible electronics and soft robotics, providing increased flexibility compared to conventional robots, and mimicking how living organisms move and react to their surroundings.