Advanced materials for 4D printing
The 4D printed materials can be designed on both micro and macro scale. The size, shape, modulus and connection pattern of these advanced materials play role in their transformation under the influence of external stimuli. Some of these materials are:
Hydro-reactive polymers/hydrogels: These materials change their shapes under the influence of water. One such material is produced at MIT where researchers made a composite polymers combining highly hydrophilic elements and non-active rigid elements. The 4D printed chain formed using this allowed 150 % swelling of certain parts where there was hydrophilic material while the rigid elements set structure and angle constraints of product. This chain spelt MIT when submerged in water.
Cellulose composite: Scientists have developed cellulose based materials that respond to humidity change in environment. They have come up with a bilayer film material using cellulose steraroyl esters with different substitution degrees on either side. Under influence of humidity, the hydrophobic side contracted while hydrophilic side swelled causing the material to roll up. The process is reversible as after reverting temperature and humidity changes, the sample unrolled again.
Thermo-reactive polymers/hydrogel: Poly(N-isopropylacrylamide), or pNIPAM, is one of the most commonly used thermo-responsive materials. A hydrogel of this material becomes hydrophilic and swollen in an aqueous solution of 32 °C while above this temperature, it starts to dehydrate and shrink resulting in shape transformation. Some hydrogels composed using this polymer, exhibit strong reversibility in which there is no shape deformation even after ten cycles of shape change. The material has been used by researchers from the University of Wollongong to create a smart valve that closes when touching hot water and opens when touching cold water.
Digital Shape-Memory Polymers: Shape-memory polymers (SMP) are materials that have the ability to recover their original shape from a deformed shape, which happened because of exposure to some external stimuli. Digital SMPs are printed using 3D printers to engineer the placement, geometry and mixing and curing ratios to offer different properties like glass transition or crystal-melt transition. One team of scientists from MIT self-assembly lab made a self-folding box using these kinds of polymers. Another team made microgrippers that could grab and release objects according to temperature input.
Thermal photo-reactive materials: These kinds of materials are temperature driven and bending strips are made of it which bends under the focussed beam of intense light. The material itself is made of chemical photo-reactive polymers. The portion that is exposed to light can also be controlled using a stencil to further explore the possible application of materials.
Present scenario in market for 4D printing
According to a report by business wire, the 4D printing market is expected to see a rise from US$ 28.103 million in 2017 to US$ 152.782 million by 2022. In practice, the self-assembling material is mixed with standard contents to produce 4D objects. There is also a software called Project Cyborg that facilitates simulation of 4D structures on the computer monitor.
4D printing has shown its potential in so many verticals but there are several hurdles that it has to overcome before it can be used as widespread commercial technology. Sometimes the process of changing shape is slow and sometimes the change affects the mechanical strength of the material. But with so many players like Stratasys limited, Autodesk Inc, Hewlett Packard Corporation and of course the self-assembly lab from MIT, the products are soon expected to be in the market at the consumer end as the idea of next 5D printing has already been presented.