Gradient Hydrogels for Optimizing Niche Cues to Enhance Cell-Based Cartilage Regeneration

Advancements in hydrogel design for articular cartilage regeneration: A comprehensive review

This review paper explores the cutting-edge advancements in hydrogel design for articular cartilage regeneration (CR). Articular cartilage (AC) defects are a common occurrence worldwide that can lead to joint breakdown at a later stage of the disease, necessitating immediate intervention to prevent progressive degeneration of cartilage. Decades of research into the biomedical applications of hydrogels have revealed their tremendous potential, particularly in soft tissue engineering, including CR. Hydrogels are highly tunable and can be designed to meet the key criteria needed for a template in CR. This paper aims to identify those criteria, including the hydrogel components, mechanical properties, biodegradability, structural design, and integration capability with the adjacent native tissue and delves into the benefits that CR can obtain through appropriate design. Stratified-structural hydrogels that emulate the native cartilage structure, as well as the impact of environmental stimuli on the regeneration outcome, have also been discussed. By examining recent advances and emerging techniques, this paper offers valuable insights into developing effective hydrogel-based therapies for AC repair.

Strategies to engineer articular cartilage with biomimetic zonal features: a review

Articular cartilage (AC) is a highly specialized tissue with restricted ability for self-regeneration, given its avascular and acellular nature. Although a considerable number of surgical treatments is available for the repair, reconstruction, and regeneration of AC defects, most of them do not prioritize the development of engineered cartilage with zonal stratification derived from biomimetic biochemical, biomechanical and topographic cues. In the absence of these zonal elements, engineered cartilage will exhibit increased susceptibility to failure and will neither be able to withstand the mechanical loading to which AC is subjected nor will it integrate well with the surrounding tissue. In this regard, new breakthroughs in the development of hierarchical stratified engineered cartilage are highly sought after. Initially, this review provides a comprehensive analysis of the composition and zonal organization of AC, aiming to enhance our understanding of the significance of the structure of AC for its function. Next, we direct our attention towards the existing in vitro and in vivo studies that introduce zonal elements in engineered cartilage to elicit appropriate AC regeneration by employing tissue engineering strategies. Finally, the advantages, challenges, and future perspectives of these approaches are presented.

Construction of a Decellularized Multicomponent Extracellular Matrix Interpenetrating Network Scaffold by Gelatin Microporous Hydrogel 3D Cell Culture System

Interface tissue repair requires the construction of biomaterials with integrated structures of multiple protein types. Hydrogels that modulate internal porous structures provide a 3D microenvironment for encapsulated cells, making them promise for interface tissue repair. Currently, reduction of intrinsic immunogenicity and increase of bioactive extracellular matrix (ECM) secretion are issues to be considered in these materials. In this study, gelatin methacrylate (GelMA) hydrogel is used to encapsulate chondrocytes and construct a phase transition 3D cell culture system (PTCC) by utilizing the thermosensitivity of gelatin microspheres to create micropores within the hydrogel. The types of bioactive extracellular matrix protein formation by chondrocytes encapsulated in hydrogels are investigated in vitro. After 28 days of culture, GelMA PTCC forms an extracellular matrix predominantly composed of collagen type II, collagen type I, and fibronectin. After decellularization, the protein types and mechanical properties are well preserved, fabricating a decellularized tissue-engineered extracellular matrix and GelMA hydrogel interpenetrating network hydrogel (dECM-GelMA IPN) consisting of GelMA hydrogel as the first-level network and the ECM secreted by chondrocytes as the second-level network. This material has the potential to mediate the repair and regeneration of tendon-bone interface tissues with multiple protein types.

Engineered biomechanical microenvironment of articular chondrocytes based on heterogeneous GelMA hydrogel composites and dynamic mechanical compression

Tissue-engineered articular cartilage constructs are currently not able to equal native tissues in terms of mechanical and biological properties. A major cause lies in the deficiency in engineering the biomechanical microenvironment (BMME) of articular chondrocytes. In this work, to engineer the BMME of articular chondrocytes, heterogeneous hydrogel structures of gelatin methacrylated (GelMA) containing differential-stiffness domains were first fabricated, and then periodic dynamic mechanical stimulations were applied to the hydrogel structures. The chondrocyte phenotype of ATDC5 cells was enhanced as the spatial differentiation in stiffness was increased in the hydrogel structures and was further strengthened by dynamic mechanical stimulation. It was speculated that the mechanical signals generated by the engineered BMME were sensed by the cells through the integrin β1-FAK signaling pathway. This study revealed the key role of the combined effects of differential and dynamic BMME on the chondrocyte phenotype, which could provide theoretical guidance for highly active tissue-engineered articular cartilage.

Recent Advances in "Functional Engineering of Articular Cartilage Zones by Polymeric Biomaterials Mediated with Physical, Mechanical, and Biological/Chemical Cues"

Articular cartilage (AC) plays an unquestionable role in joint movements but unfortunately the healing capacity is restricted due to its avascular and acellular nature. While cartilage tissue engineering has been lifesaving, it is very challenging to remodel the complex cartilage composition and architecture with gradient physio-mechanical properties vital to proper tissue functions. To address these issues, a better understanding of the intrinsic AC properties and how cells respond to stimuli from the external microenvironment must be better understood. This is essential in order to take one step closer to producing functional cartilaginous constructs for clinical use. Recently, biopolymers have aroused much attention due to their versatility, processability, and flexibility because the properties can be tailored to match the requirements of AC. This review highlights polymeric scaffolds developed in the past decade for reconstruction of zonal AC layers including the superficial zone, middle zone, and deep zone by means of exogenous stimuli such as physical, mechanical, and biological/chemical signals. The mimicked properties are reviewed in terms of the biochemical composition and organization, cell fate (morphology, orientation, and differentiation), as well as mechanical properties and finally, the challenges and potential ways to tackle them are discussed.

Gradient Hydrogels-Overview of Techniques Demonstrating the Existence of a Gradient

Gradient hydrogels are promising future materials which could be usable in tissue engineering (scaffolds), pharmaceutical (drug delivery systems with controlled release) and many others related disciplines. These hydrogels exhibit a more complex inner (gradient) structure (e.g., concentration gradient) than simple isotropic hydrogel. Gradient-structured hydrogels could be beneficial in, for example, understanding intercellular interactions. The fabrication of gradient hydrogels has been relatively deeply explored, but a comprehensive description of the physico-chemical techniques demonstrating the existence of a gradient structure is still missing. Here, we summarize the state-of-the-art available experimental techniques applicable in proving and/or describing in physico-chemical terms the inner gradient structure of hydrogels. The aim of this paper is to give the reader an overview of the existing database of suitable techniques for characterizing gradient hydrogels.

Integrins, cadherins and channels in cartilage mechanotransduction: perspectives for future regeneration strategies

Articular cartilage consists of hyaline cartilage, is a major constituent of the human musculoskeletal system and has critical functions in frictionless joint movement and articular homoeostasis. Osteoarthritis (OA) is an inflammatory disease of articular cartilage, which promotes joint degeneration. Although it affects millions of people, there are no satisfying therapies that address this disease at the molecular level. Therefore, tissue regeneration approaches aim at modifying chondrocyte biology to mitigate the consequences of OA. This requires appropriate biochemical and biophysical stimulation of cells. Regarding the latter, mechanotransduction of chondrocytes and their precursor cells has become increasingly important over the last few decades. Mechanotransduction is the transformation of external biophysical stimuli into intracellular biochemical signals, involving sensor molecules at the cell surface and intracellular signalling molecules, so-called mechano-sensors and -transducers. These signalling events determine cell behaviour. Mechanotransducing ion channels and gap junctions additionally govern chondrocyte physiology. It is of great scientific and medical interest to induce a specific cell behaviour by controlling these mechanotransduction pathways and to translate this knowledge into regenerative clinical therapies. This review therefore focuses on the mechanotransduction properties of integrins, cadherins and ion channels in cartilaginous tissues to provide perspectives for cartilage regeneration.

Bioprinting of a Zonal-Specific Cell Density Scaffold: A Biomimetic Approach for Cartilage Tissue Engineering

The treatment of articular cartilage defects remains a significant clinical challenge. This is partially due to current tissue engineering strategies failing to recapitulate native organization. Articular cartilage is a graded tissue with three layers exhibiting different cell densities: the superficial zone having the highest density and the deep zone having the lowest density. However, the introduction of cell gradients for cartilage tissue engineering, which could promote a more biomimetic environment, has not been widely explored. Here, we aimed to bioprint a scaffold with different zonal cell densities to mimic the organization of articular cartilage. The scaffold was bioprinted using an alginate-based bioink containing human articular chondrocytes. The scaffold design included three cell densities, one per zone: 20 × 106 (superficial), 10 × 106 (middle), and 5 × 106 (deep) cells/mL. The scaffold was cultured in a chondrogenic medium for 25 days and analyzed by live/dead assay and histology. The live/dead analysis showed the ability to generate a zonal cell density with high viability. Histological analysis revealed a smooth transition between the zones in terms of cell distribution and a higher sulphated glycosaminoglycan deposition in the highest cell density zone. These findings pave the way toward bioprinting complex zonal cartilage scaffolds as single units, thereby advancing the translation of cartilage tissue engineering into clinical practice.