What is 4D Printing?

The term "4-dimensional" printing or more exact statement 3+1 dimensional printing is assigned to 3D printing of parts using stimuli-responsive materials. These materials may exhibit different responses to a range of stimuli such as temperature, pH, humidity, salt concentration, electric field or magnetic field. The responses may be mechanical, optical or of any other nature. 

One of the most studied stimuli-responsive materials is shape-memory materials. These materials have a single or a set of stable shapes and a range of transient shapes. Applying a force field together with the stimuli changes their permanent shape to a temporary or transient shape. The transient shape is conditionally stable and can change to the permanent shape if the force is removed and the stimuli alone are applied. 

One of the most practical stimuli is heat. A heat-responsive shape memory material changes to the transient shape when heated and a force is applied. It retains its shape when it is cooled down and recovers its permanent shape when it is heated again to a specific recovery temperature. 

Shape-memory metals and polymers are well-known and in use for a while. With the advent of 3D printing and based on its inherent ability to make complex objects, attention was attracted to shape-memory materials which can be printed using commercial 3D printers. While some shape-memory thermoplastic polymers are discovered or formulated, there is no shape-memory photoresin available on the market now. This is because photoresins are thermosetting materials and their chemical cross-links are not able to break and reform during the heat-force-cool cycles. Basically, when a chemical cross-link is formed, it is not possible to tear the bond down and form the bond again. Of course, there are some exceptions, and this is where the shape-memory photoresins come into existence. 

Shape-memory photoresins are used in vat polymerization 3D printers such as SLA or DLP ones to make parts. Parts made using these materials exhibit shape-memory characteristics provided that they are well-designed and optimized for this purpose. 

Parts printed using shape-memory photoresins vary in the following characteristics: deformation temperature, elasticity at deformation temperature, the extent of deformation at deformation temperature, the maximum tolerable rate of deformation, rate of shape fixation, recovery temperature, degree of recovery, recovery rate and the maximum number of tolerable heat-force-cool cycles. For example, a shape-memory photoresin may exhibit a deformation temperature of 50 Celsius, a degree of recovery of 100 per cent and a fate of recovery of 10 seconds at 80 Celsius, while another grade may show completely different values. 

The deformation-recovery behaviour of shape-memory photoresins depends on their detailed network structure, additives, catalysts and other components and the nature of the cross-links in the polymeric network. Therefore, it is possible to tailor the properties of the shape-memory photoresins to make them fit a wide range of applications. 

Heat-activated shape-memory photoresins can be used to print orthoses, deployable or origami structures, lightweight artificial muscles, temperature sensors, parts for soft robotic applications. Many other applications can be imagined for these smart materials. Mixed with the concept of metamaterial design, shape-memory photoresins will be able to open new perspectives to enhance the life quality of human beings.