Load-bearing and self-healing properties of ASPU biomaterials. (a) 100 successive loading–unloading cycles of tensile loading on ASPU (PU 4) show that the elastomer maintains its dimensional stability. (b) Perfect recovery after rupture of PU 4 versus weak recovery of neat cationic polyurethane. (c) Self-healing of two freshly cut surfaces of PU 4 revealing the high affinity of dissimilar ionic components of the ASPU network.
Load-bearing and self-healing properties of ASPU biomaterials. (a) 100 successive loading–unloading cycles of tensile loading on ASPU (PU 4) show that the elastomer maintains its dimensional stability. (b) Perfect recovery after rupture of PU 4 versus weak recovery of neat cationic polyurethane. (c) Self-healing of two freshly cut surfaces of PU 4 revealing the high affinity of dissimilar ionic components of the ASPU network.

A stretchy, rubber-like biomaterial based on alginate derived from seaweed could overcome the shortcomings of conventional polyurethanes, which are used in the repair of damaged or diseased cardiac and vascular tissue [Daemi et al., Biomaterials 84 (2016) 54].

Polyurethanes represent an important biomaterial but are not biodegradable, are derived from petroleum-based raw materials, and do not promote the adherence and growth of cells. So a team of researchers from Iran Polymer & Petrochemical Institute, Royan Institute, University of Science and Culture in Iran, University of the Basque Country, and Harvard Medical School has come up with a novel approach.

“We used alginate as a green, easily available and low-cost polysaccharide and combined it chemically with polyurethane to obtain a novel bio-based supramolecular ionic polymeric network,” explains Mehdi Barikani of the Iran Polymer & Petrochemical Institute.

The novel biomaterial, known as alginate-based supramolecular ionic polyurethane (or ASPU), has tunable mechanical properties that depend upon the amount of alginate. Unlike previously reported bio-elastomers, ASPU contains physical crosslinks instead of chemical ones between its constituent parts that make it much more biodegradable in physiological conditions.

Even though ASPU is biodegradable, it is exceptionally strong and tough – showing up to ten times the tensile strength of most synthetic biodegradable polymers. In fact, its toughness (190 kJ/m3) and tensile strength (50 MPa) is comparable to that of human tendons, ligaments, and cartilage. The novel biomaterial is also self-healing, the researchers have found, and can rapidly recover almost completely after rupture.

Barikani, Hossein Baharvand and their colleagues believe that the outstanding mechanical properties of ASPU are down to the alterations that alginate makes to the microstructure of the elastomer. The amorphous nature of alginate reduces the overall crystallinity of polyurethane, which is demonstrated by the increased transparency of the material.

“All of the interesting features of this biodegradable elastomer, including tunable biodegradation and strange mechanical properties combined with fast self-healing, make it ideal for future tissue engineering applications,” says Baharvand of the Royan Institute. “In addition, this material is biocompatible and… shows a minimal immune response in physiological conditions.”

The researchers believe that there are no major obstacles to the adoption of ASPU for tissue engineering since alginate is already approved by the FDA for some applications, all the raw materials are commercially available and cost effective, and synthesis is performed under mild conditions.

“[We] expect that our biodegradable elastomers will result in new applications for tissue engineering of load-bearing tissues,” Baharvand told Materials Today.

The biomaterial has been tested in animal models and the researchers hope to move onto human clinical trials in the near future.