Authors Jeong Wook Seo, Su Ryon Shin, Yeon Joo Park, and Jojae Bae share the findings of their recent study in ‘Hydrogel Production Platform with Dynamic Movement Using Photo-Crosslinkable/Temperature Reversible Chitosan Polymer and Stereolithography 4D Printing Technology,’ filling in some major gaps regarding fabrication of networks in tissue engineering.
While acknowledging that scientists have made enormous progress with 3D printing, and especially in the area of bioprinting, there are still obvious challenges to overcome. Much focus has been placed on creating structures that mimic the design of human tissue; however, in this study, we realize that typical polymers used must also be able to move like the complex microstructures they are meant to replace in bioprinting.
By integrating 4D printing into tissue engineering, structures being created are expected to morph to the desired environment—thus mimicking movement via biocompatible materials that are stimulated by moisture, temperature, or other types of introduced environments.
Shape memory polymers and their ability to deform are used in a variety of projects today, as researchers examine ways to improve mechanical properties, performance and recyclability, and use them with other devices like actuators. Many different users have also experimented with the material aspects of 4D printing, integrating it into fashion and haute couture, automobile interiors, and much more.
“Morphological deformation is caused by changes in the physical properties (e.g. swelling) or mechanical strength of the active components of the structure, such as swelling ratio, expansion, and contraction, etc.,” explained the researchers. “The shape memory property (SMP), which reverts to the original structure after transformation, also enables reversible shape transformation.”
SLA 3D printing was used in this study, but with a polymer-based resin meant to ‘satisfy several conditions’ necessary for success.
A Schematic describing the SLA-3D printing process for structure fabrication using resin precursors. The SLA-laser, which is beamed under the resin tank, causes the resin to build up under the build plate during photo-crosslinking. B Spectrum of light absorbance for 4D resin solution. The purple region is the expected laser output wavelength region of Form 2. C 3D model designed for 4D hydrogel construct. The 4D printing model consists of a fixed part and a bending part. All units are in mm.
“Since lamination is based on photopolymerization by a laser, SLA printing requires a polymer that can be photo-crosslinked. In the case of 4D printing, changes in the shape, properties, and functions of the printed structure should be induced according to the stimulus. Finally, the organ’s movement should have SMP, because it is a transformation that reverts to its original form by reversible change. Once these three requirements are realized, 4D properties can be assigned to existing 3D structures.”
Hydroxybutyl methacrylated chitosan (HBC-MA) was found to be suitable for this study, and in fact, the researchers developed it previously, defining the materials as “a photocrosslinkable temperature-reversible chitosan polymer, which reacts sensitively to temperature changes.”
Control and 4D resin formulation used for 4D SLA printing
The researchers went on to test samples for:
- Swelling properties – dynamic swelling was associated with temperature, stating that “within the control structure group, there was no statistically significant difference in the swelling ratio according to change in temperature. However, all the groups in the control structure samples showed statistically significant differences from the cooling (10 C) group of the 4D structure (###p\ 0.001). These results confirm that the volume difference due to swelling and deswelling according to the change in temperature resulted in shape deformation, enabling a 4D characteristic.”
- Mechanical properties – showing that temperature sensitivity did affect mechanical strength of the 4D structure: “Thermal-crosslinking of HBC-MA due to the rise in temperature would not only form additional pores, but also would reduce the moisture content, resulting in a denser network. On the other hand, the decrease in temperature caused swelling in the HBC-MA polymers and lowered the mechanical strength due to the loose network configuration and decreased density caused by the expanded volume as a result of the increased water content.”
- Bending ratio – the potential for 4D printing capability—confirmed with a calculation method expressing ‘the bending curvature of 4D printing as a relative ratio.’
A Schematic describing the entire process of reversible dynamic movement of 4D printing. The 4D structure produced by the 4D printing modeling design showed bending movements upon cooling and unbending movements upon heating. B Pore structure of the printed 4D structure. Small pores formed by HBC-MA are observed between the pore-free walls made of photopolymerized PEG-MA. C Pore structure after bending. Cracks from bending during swelling were observed. D Pore structure after unbending. At 37 C, thermal-crosslinked pores with deswelling were observed. E Equilibrium swelling characteristics of control vs 4D printing structure. The 4D structure showed different swelling ratios with temperature. There was no statistically significant difference with temperature in the control group. *p\ 0.05, **p \0.01 between the indicated group; ###p\ 0.001 versus 10 C group of 4D structure. Data are shown in mean ± SD, n = 3
“The 4D structure with HBC-MA showed tunable physicochemical properties (e.g., morphology, swelling, and mechanics) according to temperature, and was capable of dynamic movement through volume expansion and contraction properties,” explained the researchers.
“In conclusion, HBC-MA can be regarded as a potential material for tissue engineering and medical applications with 4D printing properties.”
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[Source / Images: ‘Hydrogel Production Platform with Dynamic Movement Using Photo-Crosslinkable/Temperature Reversible Chitosan Polymer and Stereolithography 4D Printing Technology’]