Material “Shape-Shifts” in Response to External Forces

Johns Hopkins University, Baltimore, Maryland

A self-adapting material was developed that can change its stiffness in response to the applied force. This advancement can someday open the door for materials that can self-reinforce to prepare for increased force or stop further damage; for example, a bone implant or a bridge that can self-reinforce where a high force is applied without inspection and maintenance.

Creating similar synthetic materials has been challenging because such materials are difficult and expensive to create or require active maintenance when they are created. They also are limited in how much stress they can bear. Having materials with adaptable properties, like those of wood and bone, can provide safer structures, save money and resources, and reduce harmful environmental impact.

Natural materials can self-regulate by using resources in the surrounding environment; for example, bones use cell signals to control the addition or removal of minerals taken from blood around them. The researchers created a materials system that could add minerals in response to applied stress.

The team began by using materials that can convert mechanical forces into electrical charges as scaffolds, or support structures, that can create charges proportional to external force placed on them. The goal was that these charges could serve as signals for the materials to start mineralization from mineral ions in the environment.

The researchers immersed polymer films of these materials in a simulated body fluid mimicking ionic concentrations of human blood plasma. After the materials incubated in the simulated body fluid, minerals started forming on the surfaces. The team also discovered that they could control the types of minerals formed by controlling the fluid’s ion composition. They set up a beam anchored on one end to gradually increase stress from one end of the materials to the other and found that regions with more stress had more mineral buildup; the mineral height was proportional to the square root of stress applied.

The methods are simple, low-cost, and don’t require extra energy. The materials could one day be used as scaffolds to accelerate treatment of bone-related disease or fracture, smart resins for dental treatments, or other similar applications.

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