Recycling of Load-Bearing 3D Printable Double Network Granular Hydrogels

With millions of tons of plastic waste being released annually, plastic pollution has become one of the biggest environmental threats because most cannot degrade. In the last decade, the recycling of soft and plastic materials has been widely used to reduce plastic waste. A good example is the fabrication of recyclable and degradable elastomers and hydrogels that have opened up new applications like controlled drug release and tissue regeneration. However, degradable hydrogels are mechanically soft and are not suitable for load-bearing applications.

Lately, using novel processing technologies that allow more localized and shorter value chains has emerged as a promising strategy for reducing the environmental impact of hydrogel and plastic wastes. Additive manufacturing, popularly known as 3D printing, has gained interest owing to its remarkable advantages, such as minimal material waste and fast prototyping capabilities. Although hydrogels can be 3D printed, the inks typically do not exhibit ideal rheological properties such that the printing resolution is compromised.

Other solutions to aid 3D printing of hydrogels include functionalization of inks using polymer additives, granular materials and polymeric nanoparticles. These solutions, however, fail to address the intrinsic tradeoff between the high mechanical performance of these structures and their ability to degrade at the end of their life cycle. This could exacerbate their already compelling environmental impact. Thus, it is important to fabricate soft and sustainable materials that satisfy all these needs.

Herein, Swiss scientists: Dr. Alvaro Charlet, PhD candidate Matteo Hirsch, Mr. Sanjay Schreiber and led by Professor Esther Amstad from École Polytechnique Fédérale de Lausanne developed a 3D printable and recyclable double network granular hydrogels (rDNGH) to simultaneously satisfy the above requirements. This was achieved by combining double-network hydrogels with high mechanical performance, suitable 3D printed jammed microgels and degradable covalent reversible bonds. Polymerization reaction of the second precursor containing the cleavage crosslinks was initiated to stabilize the granular structure. To this end, a hydrogel comprised poly(2-acrylamido-2-methylpropane sulfonic acid) microparticles crosslinked covalently via a disulfide-based percolating network. The work is published in the journal, Small.

The research team demonstrated the ability to degrade the percolating network independently without harming the primary networks within the microgels. As a result, the printed rDNGH exhibited sufficiently tough and stiff properties to bear significant loads. Indeed, it reached an ultimate tensile strength as high as 0.7 MPa. At the end of its life cycle, the rDNGH could be disassembled by exposing it to an aqueous solution containing an agent for degrading the second percolating network. Thereby, the second network was selectively decomposed resulting in constituent microgel components These microgels were recovered at a yield close to 1 purified, loaded with new reagents and recycled to prepare new rDNGH.

In summary, the authors reported the fabrication of 3D printable rDNGH with excellent recyclability and remarkable mechanical properties comparable to those of pristine materials. The ability to recover the constituent microgel components that could be recycled to prepare a new rDNGH rendered this process efficient and sustainable. The degradation process was not limited to disulfide-based linkers or hydrogels discussed in this study but was generalizable and could be extended to cover a broader range of materials. A proof of concept for translating rDNGH to fabricate recyclable hard plastics was shown. In a statement to Advances in Engineering, Professor Esther Amstad explained that their findings would advance the design and fabrication of sustainable and recyclable materials and plastics as one fundamental way of combating the growing plastic pollution.