Jennifer Patterson

Skeletal tissue regeneration: where can hydrogels play a role?

The emerging field of tissue engineering reveals promising approaches for the repair and regeneration of skeletal tissues including the articular cartilage, bone, and the entire joint. Amongst the myriad of biomaterials available to support this strategy, hydrogels are highly tissue mimicking substitutes and thus of great potential for the regeneration of functional tissues. This review comprises an overview of the novel and most promising hydrogels for articular cartilage, osteochondral and bone defect repair. Chondro- and osteo-conductive and -instructive hydrogels are presented, highlighting successful combinations with inductive signals and cell sources. Moreover, advantages, drawbacks, and future perspectives of the role of hydrogels in skeletal regeneration are addressed, pointing out the current state of this rising approach.

Computational model-informed design and bioprinting of cell-patterned constructs for bone tissue engineering

Three-dimensional (3D) bioprinting is a rapidly advancing tissue engineering technology that holds great promise for the regeneration of several tissues, including bone. However, to generate a successful 3D bone tissue engineering construct, additional complexities should be taken into account such as nutrient and oxygen delivery, which is often insufficient after implantation in large bone defects. We propose that a well-designed tissue engineering construct, that is, an implant with a specific spatial pattern of cells in a matrix, will improve the healing outcome. By using a computational model of bone regeneration we show that particular cell patterns in tissue engineering constructs are able to enhance bone regeneration compared to uniform ones. We successfully bioprinted one of the most promising cell-gradient patterns by using cell-laden hydrogels with varying cell densities and observed a high cell viability for three days following the bioprinting process. In summary, we present a novel strategy for the biofabrication of bone tissue engineering constructs by designing cell-gradient patterns based on a computational model of bone regeneration, and successfully bioprinting the chosen design. This integrated approach may increase the success rate of implanted tissue engineering constructs for critical size bone defects and also can find a wider application in the biofabrication of other types of tissue engineering constructs.

RGD-Functionalized Polyethylene Glycol Hydrogels Support Proliferation and In Vitro Chondrogenesis of Human Periosteum-Derived Cells

The combination of progenitor cells with appropriate scaffolds and in vitro culture regimes is a promising area of research in bone and cartilage tissue engineering. Mesenchymal stem cells (MSCs), when encapsulated within hydrogels composed of the necessary cues and/or preconditioned using suitable culture conditions, have been shown to differentiate into bone or cartilage. Here, we utilized human periosteum-derived cells (hPDCs), a progenitor cell population with MSC characteristics, paired with protease-degradable, functionalized polyethylene glycol (PEG) hydrogels to create tissue-engineered constructs. The objective of this study was to investigate the effects of scaffold composition, exploring the addition of the cell-binding motif Arginine-Glycine-Aspartic Acid (RGD), in combination with various in vitro culture conditions on the proliferation, chondrogenic gene expression, and matrix production of encapsulated hPDCs. In growth medium, the hPDCs in the RGD-functionalized hydrogels maintained high levels of viability and demonstrated an enhanced proliferation when compared with hPDCs in non-functionalized hydrogels. Additionally, the RGD-containing hydrogels promoted higher glycosaminoglycan (GAG) synthesis and chondrogenic gene expression of the encapsulated hPDCs, as opposed to the non-functionalized constructs, when cultured in two different chondrogenic media. These results demonstrate the potential of hPDCs in combination with enzymatically degradable PEG hydrogels functionalized with adhesion ligands for cartilage regenerative applications.

Molecularly engineered polymer-based systems in drug delivery and regenerative medicine

BACKGROUND Polymer-based systems are attractive in drug delivery and regenerative medicine due to the possibility of tailoring their properties and functions to a specific application. METHODS The present review provides several examples of molecularly engineered polymer systems, including stimuli responsive polymers and supramolecular polymers. RESULTS The advent of controlled polymerization techniques has enabled the preparation of polymers with controlled molecular weight and well-defined architecture. By using these techniques coupled to orthogonal chemical modification reactions, polymers can be molecularly engineered to incorporate functional groups able to respond to small changes in the local environment or to a specific biological signal. This review highlights the properties and applications of stimuli-responsive systems and polymer therapeutics, such as polymer-drug conjugates, polymer-protein conjugates, polymersomes, and hyperbranched systems. The applications of polymeric membranes in regenerative medicine are also discussed. CONCLUSION The examples presented in this review suggest that the combination of membranes with polymers that are molecularly engineered to respond to specific biological functions could be relevant in the field of regenerative medicine.

Initiating human articular chondrocyte re-differentiation in a 3D system after 2D expansion

Cartilage damage affects a large population via acute and chronic injury and disease. Since native cartilage does not self-renew, cartilage tissue engineering has gained traction as a potential treatment. However, a limiting factor is that the primary cell type in cartilage, the articular chondrocyte, tends to de-differentiate when grown on 2D surfaces for in vitro expansion. Thus, 3D systems are being developed and used to counter this loss of chondrogenic capabilities. We hypothesize that a 3D matrix that can be remodeled may be more supportive of the chondrogenic phenotype of encapsulated articular chondrocytes than a 2D surface and may allow for the re-differentiation of chondrocytes after 2D expansion. Hence, in this study, enzymatically degradable polyethylene glycol (PEG) hydrogels containing two different protease degradable peptide segments, with different degradation rates, were tested in combination with chondrogenic medium as a 3D in vitro culture system to better recapitulate the native environment of human articular chondrocytes (hACs). In addition, the effect of incorporation of the integrin binding ligand Arg-Gly-Asp (RGD) in the hydrogels was explored. Hydrogels crosslinked with a slower degrading crosslinker and not functionalized with RGD maintained hAC viability and led to increased GAG production and chondrogenic gene expression over time, suggesting that this system can initiate hAC re-differentiation after 2D expansion.Graphical abstract

Fibrin structural and diffusional analysis suggests that fibers are permeable to solute transport

Fibrin hydrogels are promising carrier materials in tissue engineering. They are biocompatible and easy to prepare, they can bind growth factors and they can be prepared from a patients own blood. While fibrin structure and mechanics have been extensively studied, not much is known about the relation between structure and diffusivity of solutes within the network. This is particularly relevant for solutes with a size similar to that of growth factors. A novel methodological approach has been used in this study to retrieve quantitative structural characteristics of fibrin hydrogels, by combining two complementary techniques, namely confocal fluorescence microscopy with a fiber extraction algorithm and turbidity measurements. Bulk rheological measurements were conducted to determine the impact of fibrin hydrogel structure on mechanical properties. From these measurements it can be concluded that variations in the fibrin hydrogel structure have a large impact on the rheological response of the hydrogels (up to two orders of magnitude difference in storage modulus) but only a moderate influence on the diffusivity of dextran solutes (up to 25% difference). By analyzing the diffusivity measurements by means of the Ogston diffusion model we further provide evidence that individual fibrin fibers can be semi-permeable to solute transport, depending on the average distance between individual protofibrils. This can be important for reducing mass transport limitations, for modulating fibrinolysis and for growth factor binding, which are all relevant for tissue engineering. STATEMENT OF SIGNIFICANCE Fibrin is a natural biopolymer that has drawn much interest as a biomimetic carrier in tissue engineering applications. We hereby use a novel combined approach for the structural characterization of fibrin networks based on optical microscopy and light scattering methods that can also be applied to other fibrillar hydrogels, like collagen. Furthermore, our findings on the relation between solute transport and fibrin structural properties can lead to the optimized design of fibrin hydrogel constructs for controlled release applications. Finally, we provide new evidence for the fact that fibrin fibers may be permeable for solutes with a molecular weight comparable to that of growth factors. This finding may open new avenues for tailoring mass transport properties of fibrin carriers.

Synthesis and Peptide Functionalization of Hyperbranched Poly(arylene oxindole) towards Versatile Biomaterials

Hyperbranched polymers (HBPs) are dendritic macromolecules with a three-dimensional globular nanostructure. Due to their unique properties, HBPs are interesting for biomedical applications such as drug/protein delivery and tissue engineering, and functionalized HBPs can be tailored to exhibit specific biological responses. Superelectrophilic arylation of isatin is one strategy to synthesize hydrophobic hyperbranched poly(arylene oxindole)s from A2 and B3 monomers in one step. Herein, an azide derivative of this HBP, which allows postgrafting with a cell adhesive RGD peptide derivative via copper assisted azide–alkyne cycloaddition (CuAAC), is prepared. The resulting materials are shown to be cytocompatible and support cell attachment and proliferation; moreover, the amount of postgrafted peptide can be controlled. Overall, the demonstrated postgrafting strategy is compatible with different biomolecules, and the functionalized HBPs show high potential as biomaterials.

In Vitro Screening of Molecularly Engineered Polyethylene Glycol Hydrogels for Cartilage Tissue Engineering using Periosteum-Derived and ATDC5 Cells

The rapidly growing field of tissue engineering and regenerative medicine has brought about an increase in demand for biomaterials that mimic closely the form and function of biological tissues. Therefore, understanding the cellular response to the changes in material composition moves research one step closer to a successful tissue-engineered product. With this in mind, polyethylene glycol (PEG) hydrogels comprised of different concentrations of polymer (2.5%, 4%, 6.5%, or 8% (w/v)); different protease sensitive, peptide cross-linkers (VPMSMRGG or GPQGIWGQ); and the incorporation or lack of a peptide cell adhesion ligand (RGD) were screened for their ability to support in vitro chondrogenesis. Human periosteum-derived cells (hPDCs), a mesenchymal stem cell (MSC)-like primary cell source, and ATDC5 cells, a murine carcinoma-derived chondrogenic cell line, were encapsulated within the various hydrogels to assess the effects of the different formulations on cellular viability, proliferation, and chondrogenic differentiation while receiving exogenous growth factor stimulation via the medium. Through the results of this screening process, the 6.5% (w/v) PEG constructs, cross-linked with the GPQGIWGQ peptide and containing the RGD cell binding molecule, demonstrated an environment that consistently supported cellular viability and proliferation as well as chondrogenic differentiation.