4D printing
[1][2][3] It is therefore a type of programmable matter, wherein after the fabrication process, the printed product reacts with parameters within the environment (humidity, temperature, voltage, etc.)
As opposed to fused-deposition modeling, where the extruded material hardens immediately to form layers, 4D printing is fundamentally based in stereolithography, where in most cases ultraviolet light is used to cure the layered materials after the printing process has completed.
They developed a bilayer film using cellulose stearoyl esters with different substitution degrees on either side.
[8] Understanding anisotropic swelling and mapping the alignment of printed fibrils allowed A. Sydney Gladman et al. to mimic the nastic behavior of plants.
Branches, stems, bracts, and flowers respond to environmental stimuli such as humidity, light, and touch by varying the internal turgor of their cell walls and tissue composition.
[11] Taking precedent from this, the team developed a composite hydrogel architecture with local anisotropic swelling behavior that mimics the structure of a typical cell wall.
Cellulose fibrils combine during the printing process into microfibrils with a high aspect ratio (~100) and an elastic modulus on the scale of 100 GPa.
The nanoclay is a rheological aid that improves liquid flow, and the glucose prevents oxygen inhibition when the material is cured with ultraviolet light.
Gladman et al. found that as θ approaches 0°, the curvature approximates the classical Timoshenko equation and performs similarly to a bimetallic strip.
Understanding this, then, the team could carefully control the effects of anisotropy and break lines of symmetry to create helicoids, ruffled profiles, and more.
[14] Yiqi Mao et al. used this to create a series of digital SMP hinges that have differing prescribed thermo-mechanical and shape memory behaviors, which are grafted onto rigid, non-active materials.
Thus, the team was able to develop a self-folding sample that could fold without interfering with itself, and even interlock to create a more robust structure.
[15] Qi Ge et al. designed digital SMPs based on constituents with varying rubbery moduli and glass-transition temperatures with extremely high-failure strains of up to 300% larger than existing printable materials.
The thick joints were made of SMPs for robustness, while the tips of the microgrippers could be designed separately to accommodate a safe contact for the object of transport.
The material is then exposed to a specific wavelength of light, as the photoinitiator is consumed it polymerizes the remaining mixture, inducing photo initiated stress relaxation.
The portion of material exposed to the light can be controlled with stencils to create specific bending patterns.
[16] Dr. Lijie Grace Zhang's research team at the George Washington University [17] created a new type of 4D-printable, photo-curable liquid resin.
A laser 3D-printed sample of this resin was subjected to temperature fluctuations from -18 °C to 37 °C and exhibited full recovery of its original shape.
Printed scaffolds of this material proved to be successful foundations for human bone marrow mesenchymal stem cell (hMSCs) growth.
This research article is one of the first that explore the use of plant oil polymers as liquid resins for stereolithography production in biomedical applications.
Research team of Leonid Ionov (University of Bayreuth) has developed novel approach to print shape-morphing biocompatible/biodegradable hydrogels with living cells.
The approach allows fabrication of hollow self-folding tubes with unprecedented control over their diameters and architectures at high resolution.
The versatility of the approach is demonstrated by employing two different bio polymers (alginate and hyaluronic acid) and mouse bone marrow stromal cells.
Harnessing the printing and post-printing parameters allows attaining average internal tube diameters as low as 20 μm, which is not yet achievable by other existing bio printing approaches and is comparable to the diameters of the smallest blood vessels.
Consequently, the presented 4D bioprinting strategy allows the fabrication of dynamically reconfigurable architectures with tunable functionality and responsiveness, governed by the selection of suitable materials and cells.
Polyaniline and polypyrrole (PPy) are, in particular, good conducting materials and can be doped with tetrafluoroborate to contract and expand under an electric stimulus.
Chan et al. fabricated a multiple-temperature sensing device with various switches triggered at different temperatures.
The incorporation of the metallic coating was demonstrated to have no adverse impact on the shape memory performance of the switches.
The combination of carbon nanotubes and magnetically responsive particles has been bioprinted for use in promoting cell growth and adhesion, while still maintaining a strong conductivity.
Tibbits also mentions the advantage of 4D-printing for shipping applications - it will allow products to be packaged flat to later have their designed shape activated on site by a simple stimulus.
