Engineering Hydrogels beyond a Hydrated Network

Mechanically tough, adhesive, self-healing hydrogel promotes annulus fibrosus repair via autologous cell recruitment and microenvironment regulation

Annulus fibrosus (AF) defect is an important cause of disc re-herniation after discectomy. The self-regeneration ability of the AF is limited, and AF repair is always hindered by the inflammatory microenvironment after injury. Hydrogels represent one of the most promising materials for AF tissue engineering strategies. However, currently available commercial hydrogels cannot withstand the harsh mechanical load within intervertebral disc. In the present study, an innovative triple cross-linked oxidized hyaluronic acid (OHA)-dopamine (DA)- polyacrylamide (PAM) composite hydrogel, modified with collagen mimetic peptide (CMP) and supplied with transforming growth factor beta 1 (TGF-β1) (OHA-DA-PAM/CMP/TGF-β1 hydrogel) was developed for AF regeneration. The hydrogel exhibited robust mechanical strength, strong bioadhesion, and significant self-healing capabilities. Modified with collagen mimetic peptide, the hydrogel exhibited extracellular-matrix-mimicking properties and sustained the AF cell phenotype. The sustained release of TGF-β1 from the hydrogel was pivotal in recruiting AF cells and promoting extracellular matrix production. Furthermore, the composite hydrogel attenuated LPS-induced inflammatory response and promote ECM synthesis in AF cells via suppressing NFκB/NLRP3 pathway. In vivo, the composite hydrogel successfully sealed AF defects and alleviated intervertebral disk degeneration in a rat tail AF defect model. Histological evaluation showed that the hydrogel integrated well with host tissue and facilitated AF repair. The strategy of recruiting endogenous cells and providing an extracellular-matrix-mimicking and anti-inflammatory microenvironment using the mechanically tough composite OHA-DA-PAM/CMP/TGF-β1 hydrogel may be applicable for AF defect repair in the clinic. STATEMENT OF SIGNIFICANCE: Annulus fibrosus (AF) repair is challenging due to its limited self-regenerative capacity and post-injury inflammation. In this study, a mechanically tough and highly bioadhesive triple cross-linked composite hydrogel, modified with collagen mimetic peptide (CMP) and supplemented with transforming growth factor beta 1 (TGF-β1), was developed to facilitate AF regeneration. The sustained release of TGF-β1 enhanced AF cell recruitment, while both TGF-β1 and CMP could modulate the microenvironment to promote AF cell proliferation and ECM synthesis. In vivo, this composite hydrogel effectively promoted the AF repair and mitigated the intervertebral disc degeneration. This research indicates the clinical potential of the OHA-DA-PAM/CMP/TGF-β1 composite hydrogel for repairing AF defects.

A self-assembled dynamic extracellular matrix-like hydrogel system with multi-scale structures for cell bioengineering applications

Extracellular matrix (ECM) provides various types of direct interactions with cells and a dynamic environment, which can be remodeled through different assembly/degradation mechanisms to adapt to different biological processes. Herein, through introducing polyphosphate-modified hyaluronic acid and bioactive glass (BG) nano-fibril into a self-assembled hydrogel system with peptide-polymer conjugate, we can realize many new ECM-like functions in a synthetic polymer network. The hydrogel network formation is mediated by coacervation, followed by a gradual transition of peptide structure from  α-helix to β-sheet. The ECM-like hydrogels can be degraded through a number of orthogonal mechanisms, including treatments with protease, hyaluronidase, alkaline phosphatase, and calcium ion. As 2D coating, the ECM-like hydrogels can be used to modify the planar surface to promote the adhesion of mesenchymal stromal cells, or to coat the cell surface in a layer-by-layer fashion to shield the interaction with the substrate. As ECM-like hydrogels for 3D cell culture, the system is compatible with injection and cell encapsulation. Upon incorporating fragmented electrospun bioactive glass nano-fibril into the hydrogels, the synergetic effects of soft hydrogel and stiff reinforcement nanofibers on recapitulating ECM functions result in reduced cell circularity in 3D. Finally, by injecting the ECM-like hydrogels into mice, gradual degradations over a time period of one month and high biocompatibility have been shown in vivo. The contribution of complex network dynamics and hierarchical structures to cell-biomatrix interaction can be investigated multi-dimensionally, as many mechanisms are orthogonal to each other and can be regulated individually. STATEMENT OF SIGNIFICANCE: A list of native ECM features has attracted the most interest and attention in the research of synthetic biomaterials. In this research, we have described a simple ECM-like hydrogel system in which the complex and elegant functions of native ECM can be recapitulated in a chemically defined synthetic system. The ECM-like hydrogel systems were developed to avoid undesired features of biological substances (e.g., ethical concerns, batch-to-batch variation, immunogenicity, and potential risk of contamination), as well as gaining new functions to facilitate bioengineering applications (e.g., 3D cell culture, injection, and high stability). To this end, we have developed an ECM-like hydrogel system and provide evidence that this purely synthetic biomaterial is a promising candidate for cell bioengineering applications.

Thermoresponsive and antibacterial two-dimensional polyglycerol-interlocked-polynipam for targeted drug delivery

Two-dimensional polymeric networks are a new class of polymers with interesting physicochemical and biological properties. They promise a wide range of future biomedical applications including pathogen interactions, drug delivery, bioimaging, photothermal, and photodynamic therapy, owing to their unique features, such as high surface area and multivalent interactions at nano-biointerfaces. In this work, a thermosensitive two-dimensional polymeric network consisting poly(N-isopropylacrylamide) (pNIPAM) chains that are mechanically interlocked by a polyglycerol platform was synthesized and used for bacteria incapacitation. Two-dimensional hyperbranched polyglycerol (2D-hPG) was synthesized by a graphene-assisted strategy and used for encapsulation of azobisisobutyronitrile (AIBN). Radical polymerization of N-isopropylacrylamide by encapsulated AIBN resulted in thermoresponsive platforms with ~ 500 nm lateral size and 20-50 nm thickness. Due to its porous structure, 2D-PNPG was able to efficiently load antibiotics, such as tetracycline (TC) and amoxicillin (AMX). The rate of release of antibiotics from 2D-PNPG and the antibacterial activity of the system correlated with the variation of temperature as a result of the thermosensitivity of 2D-PNPG. This study shows that two-dimensional polymers are efficient platforms for future biomedical applications including drug delivery and bacteria incapacitation.

Graphical abstract

Thermoresponsive two-dimensional nanomaterials with the ability of loading therapeutic agents and antibacterial activity are synthesized and characterized.

Microfluidic Fabrication of Click Chemistry-Mediated Hyaluronic Acid Microgels: A Bottom-Up Material Guide to Tailor a Microgel's Physicochemical and Mechanical Properties

The demand for tailored, micrometer-scaled biomaterials in cell biology and (cell-free) biotechnology has led to the development of tunable microgel systems based on natural polymers, such as hyaluronic acid (HA). To precisely tailor their physicochemical and mechanical properties and thus to address the need for well-defined microgel systems, in this study, a bottom-up material guide is presented that highlights the synergy between highly selective bio-orthogonal click chemistry strategies and the versatility of a droplet microfluidics (MF)-assisted microgel design. By employing MF, microgels based on modified HA-derivates and homobifunctional poly(ethylene glycol) (PEG)-crosslinkers are prepared via three different types of click reaction: Diels-Alder [4 + 2] cycloaddition, strain-promoted azide-alkyne cycloaddition (SPAAC), and UV-initiated thiol-ene reaction. First, chemical modification strategies of HA are screened in-depth. Beyond the microfluidic processing of HA-derivates yielding monodisperse microgels, in an analytical study, we show that their physicochemical and mechanical properties-e.g., permeability, (thermo)stability, and elasticity-can be systematically adapted with respect to the type of click reaction and PEG-crosslinker concentration. In addition, we highlight the versatility of our HA-microgel design by preparing non-spherical microgels and introduce, for the first time, a selective, hetero-trifunctional HA-based microgel system with multiple binding sites. As a result, a holistic material guide is provided to tailor fundamental properties of HA-microgels for their potential application in cell biology and (cell-free) biotechnology.