Rapid 4D printing with shape memory polymers

Shape memory polymers or shape-changing materials are smart materials that have attracted considerable attention in materials science and biomedical engineering in recent years to build smart structures and devices. Digital light processing is a method based on vat photopolymerization with much faster technology to print a full layer in a single step to create smart materials.

Fahad Alam and a team of electrical, computer and nuclear engineering scientists from King Abdullah University of Science and Technology, Saudi Arabia, have developed a simple and rapid method to 3D print intelligent structures based on memory polymers. shape with digital printing by light. 3D printer and custom resin.

They combined a liquid crystal (a material that can change shape with temperature) with resin, to introduce shape memory properties for directly 3D printing heat-sensitive structures, while avoiding the complexity of preparing the resin. The team printed the structures with different geometries and measured the shape memory response. Shape memory polymers can be easily prepared for use as smart tools, toys, and metamaterials.

The article is published in the journal NPG Materials Asia.

Shape memory polymers

Shape memory polymers belong to a class of dual-shape smart polymers that can undergo mechanical deformation and return to their original shape in response to environmental parameters. The recovery of shape memory polymers depends on the application of external stimuli such as heat, light, electricity, humidity, and pH changes.

These materials are shape-shifting constructs that have attracted considerable interest in recent years due to their versatility and industrial viability. The research team demonstrated 4D printing of shape memory polymers via digital light processing; a 3D printing method based on vat photopolymerization. The results highlighted the suitability of complex 3D printed structures for a variety of applications.

Creating the shape memory effect

The research team investigated the shape memory effect of 3D printed samples by studying the shape induction and recovery process. The method made it possible to print complex 3D designs easily and in high resolution. These constructs are useful in various applications such as flexible smart patches, variable-sized mechanical tools, and deformable toys. In this work, Alam and colleagues developed a shape memory polymer based on a liquid crystal mixed with a photocurable resin, to develop a semi-crystalline polymer and described its mechanism of action, based on previous studies .

The team observed the internal morphology of 3D printed cross sections with or without liquid crystals using scanning electron microscopy. They then observed the responses of the shape memory polymers in relation to their ability to recover after being subjected to a load. The present work showed the influence of digital 3D light processing to create shape memory polymers with 4D effects. The scientists quantified the shape memory response to show the ratio of recovery angle as a function of time.

Adjustable mechanical properties

Researchers explored the promising applications of 3D printed smart memory polymers. To do this, Alam and his colleagues determined the mechanical properties of the materials by performing tensile tests on a dog bone specimen, to show how the mechanical properties of printed materials can be tuned by regulating the shape of the structures in trellis.

They confirmed the mechanical tunability of smart materials by performing finite element simulations and compared the experimental results with tensile tests from finite element analysis. The mechanical performances of 2D networks observed experimentally and predicted via simulation are consistent. Based on flexibility and stretchability, Alam and his team tested the samples for deformation testing and for joint motion sensing applications.

To facilitate joint movement via polymer integration, the scientists applied a conductive nano-silver coating as the electrode, which required further optimization of the printing parameters. The scientists measured changes in electrical resistance by stretching and compressing the structure to facilitate patient movement.

The results of measuring the resistance of the prepared mesh electrode patch showed its potential for use as a smart patch for detecting joint movements; this can be applied to a human knee, elbow joint, artificial limb, or real limbs to detect movement. Such electrode patches can be customized according to patient size through simple and rapid manufacturing processes.

Outlook

In this way, Fahad Alam and his team presented a method to 3D print smart materials by first using shape memory polymers for easy and rapid manufacturing through digital light processing. Scientists customized 3D printed objects to create structures that changed over time, this is called 4D printing. They achieved this by combining liquid crystals with a resin and printing them using a commercial desktop printer. Researchers have used this method to make a variety of complex objects, including lattice patches, bendable toys, smart packaging, and mechanical keys.

Scientists subjected these objects to heat, to temporarily change their shape and for later shape recovery applications. The team used tensile testing to show the adaptable nature of shape memory polymers to suit specific applications in biomedical engineering. Such 3D printed lattice patches are well suited for strain sensing in joint motion applications. The researchers recorded changes in the electrical resistance of the 3D-printed smart patch to detect the movement of joints in patients’ artificial limbs and arms.

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