Myron Spector

In the contemporary era of minimally invasive surgery, injectable biomaterial scaffolds have demonstrated significant potential in bone tissue engineering (BTE). Completely injectable hydrogel substratum with microscale bone graft particulates delivered through a needle-shaped orifice shifts a new paradigm for surgical interventions in clinical settings. Despite growing interest in biopolymer-based BTE systems, clinically applicable delivery platforms and a mechanistic understanding of cell–material interactions remain limited. This study developed a dual-syringe auto-mix system capable of generating an in situ cross-linking hydrogel–mineral construct composed of gelatin–hydroxyphenyl propionic acid, hyaluronic acid–tyramine, horseradish peroxidase, hydrogen peroxide, and calcium phosphate particles of varying sizes. Material distribution, rheological and mechanical properties, and swelling were characterized. Goat bone marrow-derived mesenchymal stem cells served as the basis for examining how the composite affected cell viability, morphology, proliferation, contractility, osteogenic differentiation, mineralization, and chemotactic behavior. To determine whether these biological findings were supported mechanically, an ex vivo cone-beam computed tomography model was used to evaluate volumetric stability and resistance to deformation at the graft–host interface. Cross-linking established a stable internal microarchitecture while remaining compatible with cell viability and nutrient-dependent survival. Formation of the gelatin–hyaluronan (GH) network significantly increased the storage modulus relative to gelatin (G) alone, whereas subsequent calcium phosphate incorporation (GH-CP) preserved this mechanical competence while attenuating the volumetric expansion of GH. These physical characteristics were accompanied by more organized cell morphology, enhanced osteogenic differentiation, and mineral deposition throughout a larger portion of the matrix. Heterogeneous interpenetrating gap striation (HIGS) appeared in regions of cellular aggregation and matrix deposition, and a new conceptualization of the osteogenic phenomenon, termed cling osteogenesis, has been proposed. These outcomes support an intricate relationship between early mechanical stabilization, mechanotransduction, and osteogenesis in injectable hydrogel–mineral systems.

Engineering matrices for cell therapy requires design criteria that include the ability of these materials to support, protect and enhance cellular behavior in vivo. The chemical and mechanical formulation of the biomaterials can influence not only target cell phenotype but also cellular differentiation. In this study, we have demonstrated the effect of a gelatin (Gtn)—hyaluronic acid (HA) hydrogel on human retinal progenitor cells (hRPCs) and show that by altering the mechanical properties of the materials, cellular behavior is altered as well. We have created an interpenetrating network polymer capable of encapsulating hRPCs. By manipulating the stiffness of the hydrogel, the differentiation potential of the hRPCs was controlled. Interpenetrating network 75 (IPN 75; 75% HA) allowed higher expression of rod photoreceptor markers, whereas cone photoreceptor marker expression was found to be higher in IPN 50. In vivo testing of these living matrices performed in Long–Evans rats showed higher levels of rod photoreceptor marker expression when IPN 75 was injected versus IPN 50. These biomaterials mimic biological cues that are required to simulate the dynamic complexity of natural retinal ECM. These hydrogels can be used as a vehicle for cell delivery in vivo as well as for expansion and differentiation in an in vitro 3D system in a highly reproducible manner.

In implant dentistry, large vertical and horizontal alveolar ridge deficiencies in mandibular and maxillary bone are challenges that clinicians continue to face. One of the limitations of porous blocks for reconstruction of bone in large defects in the oral cavity, and in the musculoskeletal system, is that fibrin clot does not adequately fill the interior pores and does not persist long enough to accommodate cell migration into the center of the block. The objective of our work was to develop a gelatin-based gel incorporating platelet-rich plasma (PRP) lysate, to mimic the role that a blood clot would normally play to attract and accommodate the migration of host osteoprogenitor and endothelial cells into the scaffold, thereby facilitating bone reconstruction. A conjugate of gelatin (Gtn) and hydroxyphenyl propionic acid (HPA), an amino-acid-like molecule, was commended for this application because of its ability to undergo enzyme-mediated covalent cross-linking to form a hydrogel in vivo, after being injected as a liquid. The initiation and propagation of cross-linking were under the control of horseradish peroxidase and hydrogen peroxide, respectively. The objectives of this in vitro study were directed toward evaluating: (1) the migration of rat mesenchymal stem cells (MSCs) into Gtn–HPA gel under the influence of rat PRP lysate or recombinant platelet-derived growth factor (PDGF)-BB incorporated into the gel; (2) the differentiation of MSCs, incorporated into the gel, into osteogenic cells under the influence of PRP lysate and PDGF-BB; and (3) the release kinetics of PDGF-BB from gels incorporating two formulations of PRP lysate and recombinant PDGF-BB. Results: The number of MSCs migrating into the hydrogel was significantly (3-fold) higher in the hydrogel group incorporating PRP lysate compared to the PDGF-BB and the blank gel control groups. For the differentiation/osteogenesis assay, the osteocalcin-positive cell area percentage was significantly higher in both the gel/PRP and gel/PDGF-BB groups, compared to the two control groups: cells in the blank gels grown in cell expansion medium and in osteogenic medium. Results of the ELISA release assay indicated that Gtn–HPA acted as an effective delivery vehicle for the sustained release of PDGF-BB from two different PRP lysate batches, with about 60% of the original PDGF-BB amount in the two groups remaining in the gel at 28 days. Conclusions: Gtn–HPA accommodates MSC migration. PRP-lysate-incorporating hydrogels chemoattract increased MSC migration into the Gtn–HPA compared to the blank gel. PRP-lysate- and the PDGF-BB-incorporating gels stimulate osteogenic differentiation of the MSCs. The release of the growth factors from Gtn–HPA containing PRP lysate can extend over the period of time (weeks) necessary for bone reconstruction. The findings demonstrate that Gtn–HPA can serve as both a scaffold for cell migration and a delivery vehicle that allows sustained and controlled release of the incorporated therapeutic agent over extended periods of time. These findings commend Gtn–HPA incorporating PRP lysate for infusion into porous calcium phosphate blocks for vertical and horizontal ridge reconstruction, and for other musculoskeletal applications.

Zwitterionic surfaces are increasingly explored as antifouling coatings due to their propensity to resist protein, bacterial, and cell adhesion and are typically applied as polymeric systems. Here, the self‐assembly of strongly interacting small molecule amphiphiles is reported to produce nanoribbons for antifouling applications. Synthesized amphiphiles spontaneously form micrometers‐long nanoribbons with nanometer‐scale cross‐sections and intrinsically display a dense coating of zwitterionic moieties on their surfaces. Substrates coated with nanoribbons demonstrate concentration‐dependent thicknesses and near superhydrophilicity. These surface coatings are then probed for antifouling properties and substantial reductions are demonstrated in protein adsorption, bacterial biofilm formation, and cell adhesion relative to uncoated controls. Harnessing cohesive small molecule self‐assembling nanomaterials for surface coatings offers a facile route to effective antifouling surfaces.

This study develops a novel strategy for regenerative therapy of musculoskeletal soft tissue defects using a dual‐phase multifunctional injectable gelatin‐hydroxyphenyl propionic acid (Gtn‐HPA) composite. The dual‐phase gel consists of stiff, degradation‐resistant, ≈2‐mm diameter spherical beads made from 8 wt% Gtn‐HPA in a 2 wt% Gtn‐HPA matrix. The results of a 3D migration assay show that both the cell number and migration distance in the dual‐phase gel system are comparable with the 2 wt% mono‐phase Gtn‐HPA, but notably significantly higher than for 8 wt% mono‐phase Gtn‐HPA (into which few cells migrated). The results also show that the dual phase gel system has degradation resistance and a prolonged growth factor release profile comparable with 8 wt% mono‐phase Gtn‐HPA. In addition, the compressive modulus of the 2 wt% dual‐phase gel system incorporating the 8 wt% bead phase is nearly four‐fold higher than the 2 wt% mono‐phase gel (5.3 ± 0.4 kPa versus 1.5 ± 0.06 kPa). This novel injectable dual‐phase Gtn‐HPA composite thus combines the advantages of low‐concentration Gtn‐HPA (cell migration) with high‐concentration Gtn‐HPA (stiffness, degradation resistance, slower chemical release kinetics) to facilitate effective reparative/regenerative processes in musculoskeletal soft tissue.

Recent advancements in the delivery of therapeutics for retinal diseases include the development of injectable hydrogels, networks of one or more hydrophilic polymers that contain a high-volume fraction of water. These systems are of particular interest due to their biocompatibility, permeability to water-soluble metabolites, and function as minimally invasive injectable delivery vehicles. Recently, hydrogels for ophthalmic applications have been developed that display a controlled release of factors necessary for cellular survival and proliferation. Understanding the relationship between the volume water fraction and the physical, chemical, and diffusion properties of the hydrogel scaffold could aid in the improvement of existing drug delivery treatments for retinal regeneration. In this study, we compared the diffusion and release of human epidermal growth factor (hEGF) encapsulated in different injectable homogenous and heterogenous hydrogels, namely Gelatin (Gtn-HPA) and Hyaluronic acid (HA-Tyr)-based hydrogels. These experimental results were compared to the measured stiffness and water content of these hydrogels and applied to different diffusion theories of polymers to determine the model of best fit. We find that the normalized diffusion and release of hEGF increases with free water content in injectable hydrogels: ranging from 0.176 at 41% free water in HA-Tyr to 0.2 at 53% free water in Gtn-HPA, while it is decreasing with hydrogel stiffness: 600 Pa for Gtn-HPA and 1440 Pa for HA-Tyr. Furthermore, we compared our experimental data to theoretical diffusion models. We found that homogeneous theoretical models, notably the hydrodynamic model (giving a normalized diffusion close to 0.2), provide the most suitable explanation for the measured solute diffusion coefficient.

Bone marrow mesenchymal stem cells (bMSCs) are responsible in the repair of injured tissue through differentiation into multiple cell types and secretion of paracrine factors, and thus have a broad application profile in tissue engineering/regenerative medicine, especially for the musculoskeletal system. The lesion due to injury or disease may be a closed irregular-shaped cavity deep within tissue necessitating an injectable biomaterial permissive of host (endogenous) cell migration, proliferation and differentiation. Gelatin-hydroxyphenyl propionic acid (Gtn-HPA) is a natural biopolymer hydrogel which is covalently cross-linked by horseradish peroxidase (HRP) and hydrogen peroxide (H₂O₂) in situ and can be delivered to the lesion by needle injection. Growth factors and cytokines can be directly incorporated into the gel or into nano- and micro-particles, which can be employed for sustained release of biomolecules while maintaining their bioactivity. In this study, we selected polyelectrolyte complex nanoparticles (PCNs) prepared with dextran sulfate and chitosan as the carrier for platelet-derived growth factor (PDGF)-BB and stromal cell-derived factor (SDF)-1α, which have been tested effectively in recruiting stem cells. Our in vitro results showed a high degree of viability of bMSCs through the process of Gtn-HPA covalent cross-linking gelation. The Gtn-HPA matrix was highly permissive of bMSC migration, proliferation, and differentiation. PDGF-BB (20 ng/mL) directly incorporated into the gel and, alternatively, released from PCNs stimulated bMSC migration and proliferation. There were only small differences in the results for the direct incorporation of PDGF into the gel compared with its release from PCNs, and for increased doses of the growth factor (200 ng/mL and 2 µg/mL). In contrast, SDF-1α elicited an increase in migration and proliferation only when released from PCNs; its effect on migration was notably less than PDGF-BB. The in vitro results demonstrate that PDGF-BB substantially increases migration of bMSCs into Gtn-HPA and their proliferation in the gel, and that these benefits can be derived from incorporation of a relatively low dose of the growth factor directly into the gel. These findings commend the use of Gtn-HPA/PDGF-BB as an injectable therapeutic agent to treat defects in musculoskeletal tissues.

An interview with the new Editor-in-Chief of Biomedical Materials.