Ordered, adherent layers of nanofibers enabled by supramolecular interactions

Improvement in Cohesive Properties of Adhesion Systems Using Movable Cross-Linked Materials with Stress Relaxation Properties

A strong, tough, and stable adhesion system used in various environments must be developed. A long-lasting adhesion system should effectively perform in the following five aspects: adhesion strength, toughness, energy dissipation property, self-restoration property, and creep resistance property. However, these properties are difficult to balance using conventional adhesives. Here, a new topological adhesion system using single-movable cross-network (SC) materials [SC(DMAAm) Adh] was designed. 3-(Trimethoxysilyl) propyl acrylate was used as the anchor, N,N-dimethyl acrylamide (DMAAm) was used as the main chain monomer, and γ-cyclodextrin (γ-CD) units acted as movable cross-links. The movable cross-links provided SC(DMAAm) Adh with energy dissipation properties, thereby improving its toughness. The γ-CD units also acted as bulky stoppers that provided a high adhesion strength and self-restoration properties. Moreover, the combination of the movable cross-links and bulky stoppers provided creep resistance to SC(DMAAm) Adh. The performance of the adhesion systems under different mobilities of the polymer chains was examined by adjusting the water content. In proper water-containing states, all mechanical properties of SC(DMAAm) Adh were better than those of the adhesion systems using homopolymers [P(DMAAm) Adh] and polymers with covalent cross-linking points [CP(DMAAm) Adh].

Recent advances in electrospinning supramolecular systems

Electrospinning is one of the simple, versatile, and convenient techniques for producing nanofibers that have found numerous applications in the fields of biomedical engineering, surface materials, and catalysis. Despite the great achievements, the electrospinning compounds are still limited to the utilization of polymers with high molar mass which are regarded as an indispensable element for the production of nanofibers. It is found that electrospinning chemicals based on supramolecular systems can avoid the use of high molecular weight polymers, and it is emerging as a powerful route to generate fibers in the nano-scale size. The presence of strong intermolecular interactions that function as chain entanglements allows for the formation of nanofibers during the process of electrospinning. This article provides recent impressive developments concerning nanofiber preparation made by the combination of electrospinning and supramolecular chemistry, which enables easy access to tailor-made nanofibers. Electrospinning supramolecular systems consisting of phospholipids, surfactants, crown ether derivatives as well as cyclodextrins will be highlighted in this review. Moreover, we will pay particular attention to the functionalities of electrospun nanofibers obtained from supramolecular systems.

Electrospun hydrogels for dynamic culture systems: advantages, progress, and opportunities

The extracellular matrix (ECM) is a water-swollen, tissue-specific material environment in which biophysiochemical signals are organized and influence cell behaviors. Electrospun nanofibrous substrates have been pursued as platforms for tissue engineering and cell studies that recapitulate features of the native ECM, in particular its fibrous nature. In recent years, progress in the design of electrospun hydrogel systems has demonstrated that molecular design also enables unique studies of cellular behaviors. In comparison to the use of hydrophobic polymeric materials, electrospinning hydrophilic materials that crosslink to form hydrogels offer the potential to achieve the water-swollen, nanofibrous characteristics of endogenous ECM. Although electrospun hydrogels require an additional crosslinking step to stabilize the fibers (allowing fibers to swell with water instead of dissolving) in comparison to their hydrophobic counterparts, researchers have made significant advances in leveraging hydrogel chemistries to incorporate biochemical and dynamic functionalities within the fibers. Consequently, dynamic biophysical and biochemical properties can be engineered into hydrophilic nanofibers that would be difficult to engineer in hydrophobic systems without strategic and sometimes intensive post-processing techniques. This Review describes common methodologies to control biophysical and biochemical properties of both electrospun hydrophobic and hydrogel nanofibers, with an emphasis on highlighting recent progress using hydrogel nanofibers with engineered dynamic complexities to develop culture systems for the study of biological function, dysfunction, development, and regeneration.

User-defined, temporal presentation of bioactive molecules on hydrogel substrates using supramolecular coiled coil complexes

The ability to spatiotemporally control the presentation of relevant biomolecules in synthetic culture systems has gained significant attention as researchers strive to recapitulate the endogenous extracellular matrix (ECM) in vitro. With the biochemical composition of the ECM constantly in flux, the development of platforms that allow for user-defined control of bioactivity is desired. Here, we reversibly conjugate bioactive molecules to hydrogel-based substrates through supramolecular coiled coil complexes that form between complementary peptides. Our system employs a thiolated peptide for tethering to hydrogel surfaces (T-peptide) through a spatially-controlled photomediated click reaction. The complementary association peptide (A-peptide), containing the bioactive domain, forms a heterodimeric coiled coil complex with the T-peptide. Addition of a disruptor peptide (D-peptide) engineered specifically to target the A-peptide outcompetes the T-peptide for binding, and removes the A-peptide and the attached bioactive motif from the scaffold. We use this platform to demonstrate spatiotemporal control of biomolecule presentation within hydrogel systems in a repeatable process that can be extended to adhesive motifs for cell culture. NIH 3T3 fibroblasts seeded on hyaluronic acid hydrogels and polyethylene glycol-based fibrous substrates supramolecularly functionalized with an RGD motif demonstrated significant cell spreading over their nonfunctionalized counterparts. Upon displacement of the RGD motif, fibroblasts occupied less area and clustured on the substrates. Taken together, this platform enables facile user-defined incorporation and removal of biomolecules in a repeatable process for controlled presentation of bioactivity in engineered culture systems.

Guest-Host Supramolecular Assembly of Injectable Hydrogel Nanofibers for Cell Encapsulation

The fibrous architecture of the extracellular matrix (ECM) is recognized as an integral regulator of cell function. However, there is an unmet need to develop mechanically robust biomaterials mimicking nanofibrous tissue topography that are also injectable to enable minimally invasive delivery. In this study, we have developed a fibrous hydrogel composed of supramolecularly assembled hyaluronic acid (HA) nanofibers that exhibits mechanical integrity, shear-thinning behavior, rapid self-healing, and cytocompatibility. HA was modified with methacrylates to permit fiber photo-cross-linking following electrospinning and either "guest" adamantane or "host" β-cyclodextrin groups to guide supramolecular fibrous hydrogel assembly. Analysis of fibrous hydrogel rheological properties showed that the mixed guest-host fibrous hydrogel was more mechanically robust (6.6 ± 2.0 kPa, storage modulus (G')) than unmixed guest hydrogel fibers (1.0 ± 0.1 kPa) or host hydrogel fibers (1.1 ± 0.1 kPa) separately. The reversible nature of the guest-host supramolecular interactions also allowed for shear-thinning and self-healing behavior as demonstrated by cyclic deformation testing. Human mesenchymal stromal cells (hMSCs) encapsulated in fibrous hydrogels demonstrated satisfactory viability following injection and after 7 days of culture (>85%). Encapsulated hMSCs were more spread and elongated when cultured in viscoelastic guest-host hydrogels compared to nonfibrous elastic controls, with hMSCs also showing significantly decreased circularity in fibrous guest-host hydrogels compared to nonfibrous guest-host hydrogels. Together, these data highlight the potential of this injectable fibrous hydrogel platform for cell and tissue engineering applications requiring minimally invasive delivery.

Engineering Hydrogel Adhesion for Biomedical Applications via Chemical Design of the Junction

Hydrogel adhesion inherently relies on engineering the contact surface at soft and hydrated interfaces. Upon contact, adhesion normally occurs through the formation of chemical or physical interactions between the disparate surfaces. The ability to form these adhesion junctions is challenging for hydrogels as the interfaces are wet and deformable and often contain low densities of functional groups. In this Review, we link the design of the binding chemistries or adhesion junctions, whether covalent, dynamic covalent, supramolecular, or physical, to the emergent adhesive properties of soft and hydrated interfaces. Wet adhesion is useful for bonding to or between tissues and implants for a range of biomedical applications. We highlight several recent and emerging adhesive hydrogels for use in biomedicine in the context of efficient junction design. The main focus is on engineering hydrogel adhesion through molecular design of the junctions to tailor the adhesion strength, reversibility, stability, and response to environmental stimuli.

Adhesive supramolecular polymeric materials constructed from macrocycle-based host–guest interactions

This review describes recent progress in adhesive supramolecular polymeric materials constructed from macrocycle-based host–guest interactions.

Cyclodextrins as versatile building blocks for regenerative medicine

Cyclodextrins (CDs) are one of the most versatile substances produced by nature, and it is in the aqueous biological environment where the multifaceted potential of CDs can be completely unveiled. CDs form inclusion complexes with a variety of guest molecules, including polymers, producing very diverse biocompatible supramolecular structures. Additionally, CDs themselves can trigger cell differentiation to distinct lineages depending on the substituent groups and also promote salt nucleation. These features together with the affinity-driven regulated release of therapeutic molecules, growth factors and gene vectors explain the rising interest for CDs as building blocks in regenerative medicine. Supramolecular poly(pseudo)rotaxane structures and zipper-like assemblies exhibit outstanding viscoelastic properties, performing as syringeable implants. The sharp shear-responsiveness of the supramolecular assemblies is opening new avenues for the design of bioinks for 3D printing and also of electrospun fibers. CDs can also be transformed into polymerizable monomers to prepare alternative nanostructured materials. The aim of this review is to analyze the role that CDs may play in regenerative medicine through the analysis of the last decade research. Most applications of CD-based scaffolds are focussed on non-healing bone fractures, cartilage reparation and skin recovery, but also on even more challenging demands such as neural grafts. For the sake of clarity, main sections of this review are organized according to the architecture of the CD-based scaffolds, mainly syringeable supramolecular hydrogels, 3D printed scaffolds, electrospun fibers, and composites, since the same scaffold type may find application in different tissues.

Electrospun Fibers of Cyclodextrins and Poly(cyclodextrins)

Cyclodextrins (CDs) can endow electrospun fibers with outstanding performance characteristics that rely on their ability to form inclusion complexes. The inclusion complexes can be blended with electrospinnable polymers or used themselves as main components of electrospun nanofibers. In general, the presence of CDs promotes drug release in aqueous media, but they may also play other roles such as protection of the drug against adverse agents during and after electrospinning, and retention of volatile fragrances or therapeutic agents to be slowly released to the environment. Moreover, fibers prepared with empty CDs appear particularly suitable for affinity separation. The interest for CD-containing nanofibers is exponentially increasing as the scope of applications is widening. The aim of this review is to provide an overview of the state-of-the-art on CD-containing electrospun mats. The information has been classified into three main sections: (i) fibers of mixtures of CDs and polymers, including polypseudorotaxanes and post-functionalization; (ii) fibers of polymer-free CDs; and (iii) fibers of CD-based polymers (namely, polycyclodextrins). Processing conditions and applications are analyzed, including possibilities of development of stimuli-responsive fibers.

Processing and surface modification of polymer nanofibers for biological scaffolds: a review

Polymeric fibrous constructs possess high surface area-to-volume ratios when compared with solid substrates and are quite commonly used as tissue engineering and cell growth scaffolds. An overview of important design and material considerations for fibrous scaffolds as well as an outline of both established and emerging solution- and melt-based fabrication techniques is provided. Innovative post-process surface modification avenues using "click" chemistry with both single and dual active cues as well as gradient cues, which maintain the fibrous structure are described. By combining process parameters with post-process surface modification, researchers have been able to selectively tune cellular response after seeding and culturing on fibrous constructs.

Biomaterials approaches to modeling macrophage-extracellular matrix interactions in the tumor microenvironment

Tumors are characterized by aberrant extracellular matrix (ECM) remodeling and chronic inflammation. While advances in biomaterials and tissue engineering strategies have led to important new insights regarding the role of ECM composition, structure, and mechanical properties in cancer in general, the functional link between these parameters and macrophage phenotype is poorly understood. Nevertheless, increasing experimental evidence suggests that macrophage behavior is similarly controlled by physicochemical properties of the ECM and consequential changes in mechanosignaling. Here, we will summarize the current knowledge of macrophage biology and ECM-mediated differences in mechanotransduction and discuss future opportunities of biomaterials and tissue engineering platforms to interrogate the functional relationship between these parameters and their relevance to cancer.

Supramolecular polymer adhesives: advanced materials inspired by nature

Due to their dynamic, stimuli-responsive nature, non-covalent interactions represent versatile design elements that can be found in nature in many molecular processes or materials, where adaptive behavior or reversible connectivity is required. Examples include molecular recognition processes, which trigger biological responses or cell-adhesion to surfaces, and a broad range of animal secreted adhesives with environment-dependent properties. Such advanced functionalities have inspired researchers to employ similar design approaches for the development of synthetic polymers with stimuli-responsive properties. The utilization of non-covalent interactions for the design of adhesives with advanced functionalities such as stimuli responsiveness, bonding and debonding on demand capability, surface selectivity or recyclability is a rapidly emerging subset of this field, which is summarized in this review.