Mimicking the extracellular world: from natural to fully synthetic matrices utilizing supramolecular biomaterials.

The extracellular matrix (ECM) has evolved around complex covalent and non-covalent interactions to create impressive function-from cellular signaling to constant remodeling. A major challenge in the biomedical field is the de novo design and control of synthetic ECMs for applications ranging from tissue engineering to neuromodulation to bioelectronics. As we move towards recreating the ECM's complexity in hydrogels, the field has taken several approaches to recapitulate the main important features of the native ECM (i.e. mechanical, bioactive and dynamic properties). In this review, we first describe the wide variety of hydrogel systems that are currently used, ranging from fully natural to completely synthetic to hybrid versions, highlighting the advantages and limitations of each class. Then, we shift towards supramolecular hydrogels that show great potential for their use as ECM mimics due to their biomimetic hierarchical structure, inherent (controllable) dynamic properties and their modular design, allowing for precise control over their mechanical and biochemical properties. In order to make the next step in the complexity of synthetic ECM-mimetic hydrogels, we must leverage the supramolecular self-assembly seen in the native ECM; we therefore propose to use supramolecular monomers to create larger, hierarchical, co-assembled hydrogels with complex and synergistic mechanical, bioactive and dynamic features.

Collagen type I mimicking peptide additives to functionalize synthetic supramolecular hydrogels.

Small bioactive peptide sequences derived from extracellular matrix proteins possess the ability to interact with cell receptors. As such, these peptide additives are excellent mimics to develop materials for 3D cell culture. Two types of supramolecular modified collagen type I mimicking peptide additives are presented; UPy-GFOGER (39 amino acids), with a novel superstructure, and the more simplistic UPy-DGEA (7 amino acids). Here, we studied the impact of the conformational differences between both peptide additives, on their biological performance. Various analyzing techniques demonstrated the ability of the supramolecular UPy-GFOGER to self-assemble into short nanofibers with brush-like outer features, suggesting trimerization into a triple helix. UPy-DGEA is a short additive without a complex structure. Since, collagen type I is a major component of the human corneal stroma, primary keratocytes (PKs) are encapsulated within the functionalized hydrogels to provide insights in the induced bioactivity of both additives. Incorporation of UPy-GFOGER supported an elongated morphology and (re-)differentiation of the encapsulated PKs, while tiny round-shaped cells were observed within the hydrogels functionalized with UPy-DGEA. This difference in biological success between UPy-GFOGER and UPy-DGEA indicates the difficulty of using short peptide additives without a complex structure to mimic the complex structure of natural collagen.

Development of a Fully Synthetic Corneal Stromal Construct via Supramolecular Hydrogel Engineering.

Recent advances in the field of ophthalmology show great potential in the design of bioengineered constructs to mimic the corneal stroma. Hydrogels based on synthetic supramolecular polymers, are attractive synthetic mimics of the natural highly hydrated corneal stroma. Here, a fully synthetic corneal stromal construct is developed via engineering of an injectable supramolecular hydrogel based on ureido-pyrimidinone (UPy) moieties. The hydrogel displays a dynamic and tunable behavior, which allows for control of biochemical and mechanical cues. Two hydrogels are developed, a fully synthetic hydrogel functionalized with a bioactive cyclic arginine-glycine-aspartate UPy (UPy-cRGD) additive, and a hybrid hydrogel based on UPy-moieties mixed with collagen type I fibers. Both hydrogels supported cell encapsulation and associated cellular deposition of extracellular matrix (ECM) proteins after 21 days. Excitingly, the hydrogels support the activation of isolated primary keratocytes into stromal fibroblasts as well as the differentiation toward more quiescent corneal stromal keratocytes, demonstrated by their characteristic long dendritic protrusions and a substantially diminished cytokine secretion. Furthermore, cells survive shear stresses during an injectability test. Together, these findings highlight the development of an injectable supramolecular hydrogel as a synthetic corneal stromal microenvironment able to host primary keratocytes.

Biomaterial screening of protein coatings and peptide additives: towards a simple synthetic mimic of a complex natural coating for a bio-artificial kidney.

Bio-artificial kidneys require conveniently synthesized membranes providing signals that regulate renal epithelial cell function. Therefore, we aimed to find synthetic analogues for natural extracellular matrix (ECM) protein coatings traditionally used for epithelial cell culturing. Two biomaterial libraries, based on natural ECM-coatings and on synthetic supramolecular small molecule additives, were developed. The base material consisted of a bisurea (BU) containing polymer, providing supramolecular BU-additives to be incorporated via specific hydrogen bonding interactions. This system allows for a modular approach and therefore easy fractional factorial based screening. A natural coating on the BU-polymer material with basement membrane proteins, laminin and collagen IV, combined with catechols was shown to induce renal epithelial monolayer formation. Modification of the BU-polymer material with synthetic BU-modified ECM peptide additives did not result in monolayer formation. Unexpectedly, simple BU-catechol additives induced monolayer formation and presented similar levels of epithelial markers and apical transporter function as on the laminin, collagen IV and catechol natural coating. Importantly, when this BU-polymer material was processed into fibrous e-spun membranes the natural coating and the BU-catechol additive were shown to perfectly function. This study clearly indicates that complex natural ECM-coatings can be replaced by simple synthetic additives, and displays the potency of material libraries based on design of experiments in combination with modular, supramolecular chemistry.