Functional peptide presentation on different hydrogen bonding biomaterials using supramolecular additives

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.

Using Chemistry To Recreate the Complexity of the Extracellular Matrix: Guidelines for Supramolecular Hydrogel-Cell Interactions

Hydrogels have emerged as a promising class of extracellular matrix (ECM)-mimicking materials in regenerative medicine. Here, we briefly describe current state-of-the-art of ECM-mimicking hydrogels, ranging from natural to hybrid to completely synthetic versions, giving the prelude to the importance of supramolecular interactions to make true ECM mimics. The potential of supramolecular interactions to create ECM mimics for cell culture is illustrated through a focus on two different supramolecular hydrogel systems, both developed in our laboratories. We use some recent, significant findings to present important design principles underlying the cell-material interaction. To achieve cell spreading, we propose that slow molecular dynamics (monomer exchange within fibers) is crucial to ensure the robust incorporation of cell adhesion ligands within supramolecular fibers. Slow bulk dynamics (stress-relaxation─fiber rearrangements, τ1/2 ≈ 1000 s) is required to achieve cell spreading in soft gels (<1 kPa), while gel stiffness overrules dynamics in stiffer gels. Importantly, this resonates with the findings of others which specialize in different material types: cell spreading is impaired in case substrate relaxation occurs faster than clutch binding and focal adhesion lifetime. We conclude with discussing considerations and limitations of the supramolecular approach as well as provide a forward thinking perspective to further understand supramolecular hydrogel-cell interactions. Future work may utilize the presented guidelines underlying cell-material interactions to not only arrive at the next generation of ECM-mimicking hydrogels but also advance other fields, such as bioelectronics, opening up new opportunities for innovative applications.

Merging Modular Molecular Design with High Throughput Screening of Cell Adhesion on Antimicrobial Supramolecular Biomaterials

A polymer microarray based on the supramolecular ureido-pyrimidinone (UPy) moiety is fabricated to screen antimicrobial materials for their ability to support cell adhesion. UPy-functionalized additives, either cell-adhesive, antimicrobial or control peptides, are used, and investigated in different combinations at different concentrations, resulting in a library of 194 spots. These are characterized on composition and morphology to evaluate the microarray fabrication. Normal human dermal fibroblasts are cultured on the microarrays and cell adhesion to the spots is systematically analyzed. Results demonstrate enhanced cell adhesion on spots with combinations including the antimicrobial peptides. This study clearly proves the power of the high throughput approach in combination with supramolecular molecules, to screen additive libraries for desired biological response.

Thermoresponsive Injectable Hydrogel To Mimic the Heat- and Strain-Stiffening Behavior of Biopolymers toward Muscle Cell Proliferation

Injectable hydrogels with nonlinear mechanical attributes to emulate natural biopolymers hold paramount significance in tissue engineering, offering the potential to create scaffolds that seamlessly mimic the biomechanical intricacies of living tissues. Herein, we unveil a synthetic design strategy employing Schiff base chemistry to furnish a peptide-polymer hierarchical contractile injectable hydrogel network. This innovative design demonstrates cross-linking of supramolecular peptide nanostructures such as nanofibers, 1NF, and twisted bundles, 1TB, with a thermosensitive aldehyde-functionalized polymer, PCHO. These networks exhibit interesting nonlinear mechanical stiffening responses to temperature and external stress. Furthermore, the hydrogels transform into a gel state at physiological temperature to exhibit injectable behavior and demonstrate compression load-bearing capabilities. Finally, the hydrogel network exhibits excellent biocompatibility and cell proliferation toward fibroblast, L929, and myoblast, C2C12, to validate their use as potential extracellular matrix mimetic injectable scaffolds.

Optimization of Surface-Engineered Micropatterns on Bacterial Cellulose for Guided Scar-Free Skin Wound Healing

Bacterial cellulose (BC) has been widely used in tissue engineering due to its unique spatial structure and suitable biological properties. In this study, a small biologically active Arginine-Glycine-Aspartic acid-Serine (RGDS) tetrapeptide was incorporated on the porous BC surface followed by a low-energy CO₂ laser etching operation. As a result, different micropatterns were established on the BC surface with RGDS only anchored on the raised platform surface of the micropatterned BC (MPBC). Material characterization showed that all micropatterned structures exhibited platforms with a width of ~150 μm and grooves with a width of ~100 μm and a depth of ~300 μm, which displayed distinct hydrophilic and hydrophobic properties. The resulting RGDS-MPBC could hold the material integrity, as well as the microstructure morphology under a humid environment. In-vitro and in-vivo assays on cell migration, collagen deposition, and histological analysis revealed that micropatterns led to significant impacts on wound healing progress compared to the BC without surface-engineered micropatterns. Specifically, the basket-woven micropattern etched on the BC surface exhibited the optimal wound healing outcome with the presence of fewer macrophages and the least scar formation. This study further addresses the potential of adopting surface micropatterning strategies to promote skin wounds towards scar-free outcomes.

Modular mixing of benzene-1,3,5-tricarboxamide supramolecular hydrogelators allows tunable biomimetic hydrogels for control of cell aggregation in 3D

Few synthetic hydrogels can mimic both the viscoelasticity and supramolecular fibrous structure found in the naturally occurring extracellular matrix (ECM). Furthermore, the ability to control the viscoelasticity of fibrous supramolecular hydrogel networks to influence cell culture remains a challenge. Here, we show that modular mixing of supramolecular architectures with slow and fast exchange dynamics can provide a suitable environment for multiple cell types and influence cellular aggregation. We employed modular mixing of two synthetic benzene-1,3,5-tricarboxamide (BTA) architectures: a small molecule water-soluble BTA with slow exchange dynamics and a telechelic polymeric BTA-PEG-BTA with fast exchange dynamics. Copolymerisation of these two supramolecular architectures was observed, and all tested formulations formed stable hydrogels in water and cell culture media. We found that rational tuning of mechanical and viscoelastic properties is possible by mixing BTA with BTA-PEG-BTA. These hydrogels showed high viability for both chondrocyte (ATDC5) and human dermal fibroblast (HDF) encapsulation (>80%) and supported neuronal outgrowth (PC12 and dorsal root ganglion, DRG). Furthermore, ATDC5s and human mesenchymal stem cells (hMSCs) were able to form spheroids within these viscoelastic hydrogels, with control over cell aggregation modulated by the dynamic properties of the material. Overall, this study shows that modular mixing of supramolecular architectures enables tunable fibrous hydrogels, creating a biomimetic environment for cell encapsulation. These materials are suitable for the formation and culture of spheroids in 3D, critical for upscaling tissue engineering approaches towards cell densities relevant for physiological tissues.

Covalent attachment of three derivatives of pegylated RGD peptides on the NH2-terminated silicon surfaces: Impact on fibroblast cell behavior

This paper describes a simple strategy for covalent immobilization of the NHS-PEG-RGD peptide with the three different PEG lengths (8, 13, and 22) onto the amine-terminated monolayers with the subsequent investigation of fibroblast cellular response to the three derivatives of pegylated RGD peptides-modified substrates. First, acetamide-terminated monolayers were prepared on the hydride terminated silicon surface to protect NH2-terminated monolayers. This was followed by the removal of the protective groups, and the reaction of NHS-PEG8-RGD, NHS-PEG13-RGD and NHS-PEG22-RGD peptides with the NH2-terminated monolayers while reducing nonspecific protein adsorption. Analysis of X-ray photoelectron spectroscopy (XPS), Fourier Transform Infrared (ATR-FTIR) spectroscopy, and Ellipsometry measurements demonstrated that PEG13-RGD peptide forms relatively a more homogenous, thicker and stable structure compared with those of PEG8-RGD and PEG22-RGD peptide. The quantitative and qualitative assessment of cell adhesion, spreading, and proliferation indicated that relatively further elongated fibroblast cells attached on the PEG13-RGD peptide relative to those on the PEG8-RGD and PEG22-RGD peptide. The results presented here may offer a developed strategy based on the length of the spacer to regulate cellular behavior on the surface substrates.

Introducing Hyaluronic Acid into Supramolecular Polymers and Hydrogels

The use of supramolecular polymers to construct functional biomaterials is gaining more attention due to the tunable dynamic behavior and fibrous structures of supramolecular polymers, which resemble those found in natural systems, such as the extracellular matrix. Nevertheless, to obtain a biomaterial capable of mimicking native systems, complex biomolecules should be incorporated, as they allow one to achieve essential biological processes. In this study, supramolecular polymers based on water-soluble benzene-1,3,5-tricarboxamides (BTAs) were assembled in the presence of hyaluronic acid (HA) both in solution and hydrogel states. The coassembly of BTAs bearing tetra(ethylene glycol) at the periphery (BTA-OEG₄) and HA at different ratios showed strong interactions between the two components that led to the formation of short fibers and heterogeneous hydrogels. BTAs were further covalently linked to HA (HA-BTA), resulting in a polymer that was unable to assemble into fibers or form hydrogels due to the high hydrophilicity of HA. However, coassembly of HA-BTA with BTA-OEG₄ resulted in the formation of long fibers, similar to those formed by BTA-OEG₄ alone, and hydrogels were produced with tunable stiffness ranging from 250 to 700 Pa, which is 10-fold higher than that of hydrogels assembled with only BTA-OEG₄. Further coassembly of BTA-OEG₄ fibers with other polysaccharides showed that except for dextran, all polysaccharides studied interacted with BTA-OEG₄ fibers. The possibility of incorporating polysaccharides into BTA-based materials paves the way for the creation of dynamic complex biomaterials.

Effectiveness of cell adhesive additives in different supramolecular polymers

Supramolecular motifs in elastomeric biomaterials facilitate the modular incorporation of additives with corresponding motifs. The influence of the elastomeric supramolecular base polymer on the presentation of additives has been sparsely examined, limiting the knowledge of transferability of effective functionalization between polymers. Here it was investigated if the polymer backbone and the additive influence biomaterial modification in two different types of hydrogen bonding supramolecular systems, that is, based on ureido-pyrimidinone or bis-urea units. Two different cell-adhesive additives, that is, catechol or cyclic RGD, were incorporated into different elastomeric polymers, that is, polycaprolactone, priplast or polycarbonate. The additive effectiveness was evaluated with three different cell types. AFM measurements showed modest alterations on nano-scale assembly in ureido-pyrimidinone materials modified with additives. On the contrary, additive addition was highly intrusive in bis-urea materials. Detailed cell adhesive studies revealed additive effectiveness varied between base polymers and the supramolecular platform, with bis-urea materials more potently affecting cell behavior. This research highlights that additive transposition might not always be as evident. Therefore, additive effectiveness requires re-evaluation in supramolecular biomaterials when altering the polymer backbone to suit the biomaterial application.

Realizing tissue integration with supramolecular hydrogels

Biomaterial matrices must permit tissue growth and maturation for the success of tissue regeneration strategies. Naturally, this accommodation is achieved via the dynamic remodeling of a cell's extracellular matrix (ECM). Synthetically, hydrolytic or enzymatic degradation are often engineered into materials for this purpose. More recently, supramolecular interactions have been used to provide a biomimetic and tunable mechanism to facilitate tissue formation via their dynamic and reversible non-covalent interactions. By engineering the mechanical and bioactive properties of a material, supramolecular chemists are able to design permissivity into the construct and facilitate tissue integration in-vivo. Furthermore, via the reversibility of non-covalent interactions, injectability and responsiveness can be designed for enhanced delivery and spatio-temporal control. In this review, we delineate the basic considerations needed when designing permissive supramolecular hydrogels for tissue engineering with an eye toward tissue growth and integration. We highlight three archetypal hydrogel systems that have shown well-documented tissue integration in vivo, and provide avenues to assess tissue in-growth. Careful design and assessment of the biomedical potential of a supramolecular hydrogels can inspire the creation of robust and dynamic implants for new tissue engineering applications.

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.

Towards understanding the messengers of extracellular space: Computational models of outside-in integrin reaction networks

The interactions between cells and their extracellular matrix (ECM) are critically important for homeostatic control of cell growth, proliferation, differentiation and apoptosis. Transmembrane integrin molecules facilitate the communication between ECM and the cell. Since the characterization of integrins in the late 1980s, there has been great advancement in understanding the function of integrins at different subcellular levels. However, the versatility in molecular pathways integrins are involved in, the high diversity in their interaction partners both outside and inside the cell as well as on the cell membrane and the short lifetime of events happening at the cell-ECM interface make it difficult to elucidate all the details regarding integrin function experimentally. To overcome the experimental challenges and advance the understanding of integrin biology, computational modeling tools have been used extensively. In this review, we summarize the computational models of integrin signaling while we explain the function of integrins at three main subcellular levels (outside the cell, cell membrane, cytosol). We also discuss how these computational modeling efforts can be helpful in other disciplines such as biomaterial design. As such, this review is a didactic modeling summary for biomaterial researchers interested in complementing their experimental work with computational tools or for seasoned computational scientists that would like to advance current in silico integrin models.

Host-Guest-Mediated Epitope Presentation on Self-Assembled Peptide Amphiphile Hydrogels

A key feature in biomaterial design is the incorporation of bioactive signals into artificial constructs to stimulate tissue regeneration. Most currently used hydrogel cell culture systems depend on the covalent attachment of extracellular matrix (ECM)-derived peptides to either macromolecular units or smaller self-assembling building blocks, thereby restricting biosignal presentation and adaptability. However, new ways to rationally incorporate adhesion epitopes through noncovalent interactions would offer opportunities to better recreate the dynamic and reversible nature of the native ECM. Here, we report on a noncovalent epitope presentation approach mediated by host-guest interactions. Using peptide amphiphile hydrogels, we demonstrate that the adamantane/β-cyclodextrin pair can be used to anchor RGDS cell adhesion signals onto self-assembled hydrogels via host-guest interactions. We evaluate hydrogel morphological and rheological properties as well as fibroblast attachment, organization, and spreading when cultured atop these scaffolds. This host-guest-mediated epitope display might lead to new self-assembling hydrogels for improved cell culture applications in fields such as tissue engineering and regenerative medicine.

Combinatorial functionalization with bisurea-peptides and antifouling bisurea additives of a supramolecular elastomeric biomaterial

The bioactive additive toolbox to functionalize supramolecular elastomeric materials expands rapidly. Here we have set an explorative step toward screening of complex combinatorial functionalization with antifouling and three peptide-containing additives in a bisurea-based supramolecular system. Thorough investigation of surface properties of thin films with contact angle measurements, X-ray photoelectron spectroscopy and atomic force microscopy, was correlated to cell-adhesion of endothelial and smooth muscle cells to apprehend their respective predictive values for functional biomaterial development. Peptides were presented at the surface alone, and in combinatorial functionalization with the oligo(ethylene glycol)-based non-cell adhesive additive. The bisurea-RGD additive was cell-adhesive in all conditions, whereas the endothelial cell-specific bisurea-REDV showed limited bioactive properties in all chemical nano-environments. Also, aspecific functionality was observed for a bisurea-SDF1α peptide. These results emphasize that special care should be taken in changing the chemical nano-environment with peptide functionalization. © 2019 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1725-1735.