The degradation and performance of electrospun supramolecular vascular scaffolds examined upon in vitro enzymatic exposure.

To maintain functionality during in situ vascular regeneration, the rate of implant degradation should be closely balanced by neo-tissue formation. It is unknown, however, how the implant's functionality is affected by the degradation of the polymers it is composed of. We therefore examined the macro- and microscopic features as well as the mechanical performance of vascular scaffolds upon in vitro enzymatic degradation. Three candidate biomaterials with supramolecularly interacting bis-urea (BU) hard blocks ('slow-degrading' polycarbonate-BU (PC-BU), 'intermediate-degrading' polycarbonate-ester-BU (PC(e)-BU), and 'fast-degrading' polycaprolactone-ester-BU (PCL-BU)) were synthesized and electrospun into microporous scaffolds. These materials possess a sequence-controlled macromolecular structure, so their susceptibility to degradation is tunable by controlling the nature of the polymer backbone. The scaffolds were incubated in lipase and monitored for changes in physical, chemical, and mechanical properties. Remarkably, comparing PC-BU to PC(e)-BU, we observed that small changes in macromolecular structure led to significant differences in degradation kinetics. All three scaffold types degraded via surface erosion, which was accompanied by fiber swelling for PC-BU scaffolds, and some bulk degradation and a collapsing network for PCL-BU scaffolds. For the PC-BU and PC(e)-BU scaffolds this resulted in retention of mechanical properties, whereas for the PCL-BU scaffolds this resulted in stiffening. Our in vitro study demonstrates that vascular scaffolds, electrospun from sequence-controlled supramolecular materials with varying ester contents, not only display different susceptibilities to degradation, but also degrade via different mechanisms. STATEMENT OF SIGNIFICANCE: One of the key elements to successfully engineer vascular tissues in situ, is to balance the rate of implant degradation and neo-tissue formation. Due to their tunable properties, supramolecular polymers can be customized into attractive biomaterials for vascular tissue engineering. Here, we have exploited this tunability and prepared a set of polymers with different susceptibility to degradation. The polymers, which were electrospun into microporous scaffolds, displayed not only different susceptibilities to degradation, but also obeyed different degradation mechanisms. This study illustrates how the class of supramolecular polymers continues to represent a promising group of materials for tissue engineering approaches.

Modulation of macrophage phenotype and protein secretion via heparin-IL-4 functionalized supramolecular elastomers.

Hallmark of the in situ tissue engineering approach is the use of bioresorbable, synthetic, acellular scaffolds, which are designed to modulate the inflammatory response and actively trigger tissue regeneration by the body itself at the site of implantation. Much research is devoted to the design of synthetic materials modulating the polarization of macrophages, which are essential mediators of the early stages of the inflammatory response. Here, we present a novel method for the functionalization of elastomers based on synthetic peptide chemistry, supramolecular self-assembly, and immobilization of heparin and interleukin 4 (IL-4), which is known to skew the polarization of macrophages into the wound healing "M2" phenotype. Ureido-pyrimidinone (UPy)-modified chain extended polycaprolactone (CE-UPy-PCL) was mixed with a UPy-modified heparin binding peptide (UPy-HBP) to allow for immobilization of heparin, and further functionalization with IL-4 via its heparin binding domain. As a first proof of principle, CE-UPy-PCL and UPy-HBP were premixed in solution, dropcast and exposed to primary human monocyte-derived macrophages, in the presence or absence of IL-4-heparin functionalization. It was demonstrated that the supramolecular IL-4-heparin functionalization effectively promoted macrophage polarization into an anti-inflammatory phenotype, in terms of morphology, immunohistochemistry and cytokine secretion. Moreover, the supramolecular functionalization approach used was successfully translated to 3D electrospun scaffolds for in situ tissue engineering purposes, where UPy-HBP retention, and heparin and IL-4 attachment to the supramolecular scaffolds were proven over 7 days. Lastly, human monocyte-derived macrophages were cultured on 3D scaffolds, which, in case of IL-4-heparin functionalization, were proven to promote of an anti-inflammatory environment on protein level. This study presents a novel method in designing a versatile class of functionalized elastomers that effectively harness the anti-inflammatory behavior of macrophages in vitro, and as such, may be instrumental for the development of a new class of synthetic materials for in situ tissue engineering purposes. STATEMENT OF SIGNIFICANCE: Macrophages and their phenotypic and functional plasticity play a pivotal role in metabolic homeostasis and tissue repair. Based on this notion, bioactivated materials modulating macrophage polarization were extensively investigated in the past. Here, we designed immunomodulating, synthetic materials based on supramolecular immobilization of a heparin binding peptide, and further bioactivation with heparin and IL-4, an anti-inflammatory cytokine responsible for M2 activation and polarization. Human monocyte-derived macrophages cultured on heparin-IL-4 bioactivated materials displayed an elongated morphology and an anti-inflammatory phenotype, with downregulation of pro-inflammatory cytokines and promotion of anti-inflammatory cytokines over time. This study represents the first step in designing a novel class of synthetic, bioactivated materials that harness the regenerative behavior of host macrophages towards in situ tissue regeneration.

Supramolecular surface functionalization via catechols for the improvement of cell-material interactions.

Optimization of cell-material interactions is crucial for the success of synthetic biomaterials in guiding tissue regeneration. To do so, catechol chemistry is often used to introduce adhesiveness into biomaterials. Here, a supramolecular approach based on ureido-pyrimidinone (UPy) modified polymers is combined with catechol chemistry in order to achieve improved cellular adhesion onto supramolecular biomaterials. UPy-modified hydrophobic polymers with non-cell adhesive properties are developed that can be bioactivated via a modular approach using UPy-modified catechols. It is shown that successful formulation of the UPy-catechol additive with the UPy-polymer results in surfaces that induce cardiomyocyte progenitor cell adhesion, cell spreading, and preservation of cardiac specific extracellular matrix production. Hence, by functionalizing supramolecular surfaces with catechol functionalities, an adhesive supramolecular biomaterial is developed that allows for the possibility to contribute to biomaterial-based regeneration.