Highly tough and ultrafast self-healable dual physically crosslinked sulfated alginate-based polyurethane elastomers for vascular tissue engineering

Highlights

• Hard and soft acid and base (HSAB) theory can be used to produce tough bio-elastomers.

• A highly tough physically-crosslinked biomaterial was developed based on sulfated alginate and cationic polyurethane.

• Different lattice energies in supramolecular networks can produce highly-tough biomaterials.

• The resulting supramolecular biomaterial showed appropriate blood- and bio-compatibility.

• This biomaterial is a biodegradable and biocompatible candidate for vascular tissue engineering.

Abstract

Since vascular diseases are regarded as a major cause of death worldwide, developing engineered biomimetic elastomers with physicochemical and biological properties resembling those of the natural vascular tissues, is vital for vascular tissue engineering (VTE). This study reports synthesis of highly tough supramolecular biologically active alginate-based supramolecular polyurethane (BASPU) elastomers that benefit from the presence of two physical networks with different strength of soft tertiary ammonium-soft sulfate pairs, as strong ionic bonds, and soft tertiary ammonium-hard carboxylate groups, as the weak bonds. The presence of sulfate groups resulted in low Young’s modulus, high toughness and stretchability, proper energy dissipation, ultrafast self-healing and complete healing efficiency of BASPU. In vitro studies showed higher endothelial cells attachment, higher anticoagulation ability and significantly less platelet adhesion for BASPUs compared to the commercial vascular prosthesis. The histological studies of subcutaneously implanted scaffolds confirmed their low fibrosis and gradual biodegradation during 2 months of following.

Introduction

Cardiovascular diseases (CVDs) are a major cause of morbidity and mortality worldwide (Hashimoto, Olson, & Bassel-Duby, 2018; Roth et al., 2015). Based on the disease stage, using drugs, angioplasty, bypass surgeries and organ transplantation might be essential to prevent disease progression (Ogle et al., 2016; Seifu, Purnama, Mequanint, & Mantovani, 2013). Due to the side effects of chemicals and the risk of surgeries and transplant rejection, synthetic vascular prostheses made of non-biodegradable polymers were developed (Li, Sengupta, & Chien, 2014). However, long-term complications such as inability to degrade, intima hyperplasia and repetitive surgeries to replace the prosthesis have challenged their use (Rouwkema & Khademhosseini, 2016; Song, Rumma, Ozaki, Edelman, & Chen, 2018). An ideal tissue engineered vascular graft (TEVG) should possess mechanical and biological properties close to natural vessels to make it an appropriate substitute for natural vessels (Shah, Kc, & Zhang, 2019). In this regard, some features including absence of toxicity, immunogenicity, and thrombogenicity, and possessing tunable mechanical properties and proper biodegradation as well as easy processability, are crucial bottlenecks for application of TEVGs (Li et al., 2014).

Using decellularized tissues is a promising approach to restore the normal function of the tissue because they can mediate the attachment and proliferation of new cells. Nevertheless, decellulariztion methods influence the extracellular matrix (ECM) physicochemical and mechanical properties (Badylak, Freytes, & Gilbert, 2009). Besides the decellularized ECM, FDA-approved synthetic polyesters such as polycaprolactone and poly(lactic-co-glycolic acid) possess high potential to be used as temporal scaffolds in TEVGs because of their biocompatible and biodegradable nature. However, poor mechanical properties including low tensile strength, lack of toughness, improper stiffness, and insufficient fatigue resistance against the blood pressure, have limited the usage of such synthetic polyesters (Mano et al., 2007). In addition, polyesters may induce biomaterial failure and inflammatory response in wet conditions due to autocatalytic hydrolysis of ester bonds, and decrement of medium acidity (Rai, Tallawi, Grigore, & Boccaccini, 2012).

Such restrictions have motivated the researchers to employ crosslinked polymeric elastomers with controllable biodegradation and rapid reversible strain in response to a stress (Amsden, 2007; Serrano, Chung, & Ameer, 2010). The ability to recover the initial shape after large deformations, makes them potential supporting flexible matrices for engineering of load-bearing soft tissues such as vascular tissues (Daemi et al., 2016; Ren et al., 2015; Zhang et al., 2019). The physically-crosslinked elastomers strengthened by weak non-covalent interactions, demonstrate proper biodegradation due to ease of bond breakage (Chen et al., 2017; Santoro, Shah, Walker, & Mikos, 2016; Ye, Zhang, Kai, Li, & Loh, 2018). However, they are not perhaps a desirable substitute for load-bearing tissues because of low toughness and weak reversibility of physical interactions. It is important to note that TEVGs must mechanically match with host tissues to prevent or decrease the foreign body reactions and/or implantation failure (Xu, Huang, Tang, & Hong, 2017).

Strengthening of biomaterials using ionic interactions with different lattice energies in supramolecular networks can potentially produce fully physically-crosslinked biomaterials (Luo et al., 2015). It is supposed that a competitive affinity existing between two anions with different charge densities to bind a same cation, would cause tough materials based on hard-soft acid base (HSAB) theory. The HSAB theory describes the qualitative strength of ionic interactions of ion-ligand partners in light of their relative sizes and charge densities (Pearson, 1963). Smaller ions with higher charge densities, known as hard acids, tend preferentially to interact with and bind to ligands of smaller sizes known as hard bases. On the contrary, soft acids (larger ions with lower charge densities) would preferentially interact with soft bases (bulky ligands) (Zare-Gachi et al., 2020). By keeping this concept in mind, we synthesized an isocyanate-terminated cationic urethane prepolymer containing tertiary ammonium groups as soft acid and used in in reaction with sulfated alginate, possessing both carboxylate (hard base) and sulfate (soft base) groups in its chemical structure (Fig. 1).

HSAB theory considers both tertiary ammonium and sulfate groups as soft ions, since they have low charge states and are strongly polarizable. On the other hand, carboxylate group is a hard base due to its high charge density (Jun Lee et al., 2015). Therefore, the difference in strength between soft tertiary ammonium-soft sulfate pairs, as strong bonds, and soft tertiary ammonium-hard carboxylate groups, as the weak bonds, could be used to strengthen and toughen the ionic network. Furthermore, it was shown that not only sulfate groups induce the formation of a tough ionic network and increase its mechanical properties, but the biologically active alginate-based supramolecular polyurethanes (BASPUs) also possess the majority of crucial features required for a favorable TEVG such as appropriate levels of biodegradation and endothelial cells attachment and desired anticoagulant properties.