A recent study, produced by a group of researchers from the Rutgers University Department of Mechanical and Aerospace Engineering and the New Jersey Institute of Technology’s Department of Mechanical and Industrial Engineering, has developed a three-dimensional printed technique for PNIPAAM; a temperature responsive hydrogel utilized for a number of applications within the science of engineering. In their results published by Scientific Reports, these researchers successfully used projection micro-stereolithography (PμSL) to produce a 3D printed PNIPAAM structure with sequential deformation.
Stimuli-Responsive Hydrogels
The extremely high levels of hydrophilicity and biocompatibility, combined with the soft physical properties of this material that are similar to living tissues, allow hydrogels to remain an ideal biomaterial for a wide variety of clinical purposes. In particular, stimulus-responsive hydrogels that can be sensitive to temperature, pH, chemical, light or electric field, have had a unique impact within the biomaterials industry, as this material allows users to have an enhanced control of the hydrogel’s properties during use.
Of the numerous hydrogels that have found success in the commercial market over the past several decades, one of the most widely used is poly(N-isopropylacrylamide) (PNIPAAM). With applications most often found in microfluidic devices, drug delivery vehicles, cell culture substrates and soft actuators, PNIPAAm can exhibit a unique hydrophilic behavior when present in an aqueous environment that results in a rapid water uptake and swelling of the material. When the temperature of the environment surrounding the hydrogel increases above its Lower Critical Solution Temperature, which typically measures between 32-35°C, the hydrophobic groups of the PNIPAAm become more efficient, resulting in the molecules of this material to transform into a compact globule structure.
Limitations in Manufacturing PNIPAAm
Current production techniques for PNIPAAm have remained limited to simple two-dimensional (2D) fabrication methods, such as molding and lithography, however, these processes have been shown to limit the full utilization of the unique material behavior of PNIPAAm. While some efforts have been made to create a three-dimensional (3D) shapes from 2D PNIPAAM sheets, it has remained a challenge to acquire a high resolution and high aspect ratio to the final PNIPAAm geometry. High-resolution 3D manufacturing techniques such as 3D laser chemical vapor deposition (3D-LCVD), electrochemical fabrication (EFAB) and micro-stereolithography (mSL) have been considered for the production of PNIPAAM, however, the extended fabrication time, high cost and reduced availability of materials for each of these methods limits their practical application for the production of PNIPAAM.
Projection Micro-Stereolithography (PμSL) for PNIPAAm production
PμSL is a fast, inexpensive and flexible method of lithography additive manufacturing that uses projection lithography, in which an entire layer of the material is polymerized by a single ultraviolet (UV) illumination for several seconds. Once the 3D model is created on computer-aided design (CAD) software, a series of digitally sliced cross-sectional images of the 3D model are generated to optically pattern the UV light. The UV light is then projected through a reduction lens and focused on the surface of the resin material of interest, in which it converts the liquid resin to a solid layer through photopolymerization. As in any other type of 3D printing method, the layers of the solid resin continue to form on top of each other in a preceding manner, until the final 3D object is completed.
To create the 3D printed PNIPAAM, the researchers dissolved NIPAAm as a monomer, N-N’-Methylene-bis(acrylamide) as a cross-linker and Phenylbis (2,4,6,-tri-methyl benzoyl) phosphine oxide as a photo-initiator (PI). By precisely controlling the growth and shrinkage of the hydrogel during the orienting process, the researchers found that at temperatures below 32°C, the hydrogel was capable of absorbing more water and swelling in size, a factor they quantified through the swelling ratio (SR). Similarly, when the temperature exceeded 32°C, the researchers found that the hydrogel expelled water and shrunk.
These results indicate that the specific material behaviors of PNIPAAM to be temperature dependent on their ability to swell and shrink in certain environments can be spatially encoded and distributed within a layer as desired by the manufacturer. Additionally, the researchers found that by utilizing the PμSL technique, they were able to specify the swelling of the material in either lateral or vertical direction depending on the layer thickness that they originally designed in the CAD software. The results of this study provide additional behavior on the unique temperature responsive swelling characteristics of PNIPAAM, which indicate its potential application in soft robots, microfluidic devices, drug delivery vehicles and added structural rigidity for organs.
References:
- “Micro 3D Printing of a Temperature Responsive Hydrogel Using Projection Micro-Stereolithography” Han, D., Lu, Z., Chester, S., & Lee, H. Scientific Reports. (2018). DOI: 10.1038/s41598-20385-2.
Image credit: FabrikaSimf/shutterstock
By FabrikaSimf
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