Grayscale mask‐assisted photochemical crosslinking for a dense collagen construct with stiffness gradient

Collagen-Based Biomimetic Systems to Study the Biophysical Tumour Microenvironment

The extracellular matrix (ECM) is a pericellular network of proteins and other molecules that provides mechanical support to organs and tissues. ECM biophysical properties such as topography, elasticity and porosity strongly influence cell proliferation, differentiation and migration. The cell's perception of the biophysical microenvironment (mechanosensing) leads to altered gene expression or contractility status (mechanotransduction). Mechanosensing and mechanotransduction have profound implications in both tissue homeostasis and cancer. Many solid tumours are surrounded by a dense and aberrant ECM that disturbs normal cell functions and makes certain areas of the tumour inaccessible to therapeutic drugs. Understanding the cell-ECM interplay may therefore lead to novel and more effective therapies. Controllable and reproducible cell culturing systems mimicking the ECM enable detailed investigation of mechanosensing and mechanotransduction pathways. Here, we discuss ECM biomimetic systems. Mainly focusing on collagen, we compare and contrast structural and molecular complexity as well as biophysical properties of simple 2D substrates, 3D fibrillar collagen gels, cell-derived matrices and complex decellularized organs. Finally, we emphasize how the integration of advanced methodologies and computational methods with collagen-based biomimetics will improve the design of novel therapies aimed at targeting the biophysical and mechanical features of the tumour ECM to increase therapy efficacy.

Biomaterials with Stiffness Gradient for Interface Tissue Engineering

Interface tissue engineering is a rapidly growing field that aims to develop engineered tissue alternates with the goal of promoting integration between multiple tissue types. Engineering interface tissues is a complex process, which requires a specialized biomaterials with organized material composition, stiffness, cell types, and signaling molecules. Among these, stiffness-controllable substrates have been developed to investigate the effect of stiffness on cell behavior. Especially these substrates with graded stiffness are advantageous since they allow the differentiation of multiple cell phenotypes and subsequent tissue development. In this review, we highlight the various types of manufacturing techniques that can be leveraged to fabricate scaffolds with stiffness gradient, discuss methods to characterize them, and gradient biomaterials for controlling cellular behavior including attachment, migration, proliferation, and differentiation. We also address fundamentals of interface tissue organization, and stiffness gradient biomaterials for interface tissue regeneration. Potential challenges and future directions in this emerging field are also discussed.

Collagen- and hyaluronic acid-based hydrogels and their biomedical applications

Hydrogels have been widely investigated in biomedical fields due to their similar physical and biochemical properties to the extracellular matrix (ECM). Collagen and hyaluronic acid (HA) are the main components of the ECM in many tissues. As a result, hydrogels prepared from collagen and HA hold inherent advantages in mimicking the structure and function of the native ECM. Numerous studies have focused on the development of collagen and HA hydrogels and their biomedical applications. In this extensive review, we provide a summary and analysis of the sources, features, and modifications of collagen and HA. Specifically, we highlight the fabrication, properties, and potential biomedical applications as well as promising commercialization of hydrogels based on these two natural polymers.

Mechanical Properties of Bioengineered Corneal Stroma

For the majority of patients with severe corneal injury or disease, corneal transplantation is the only suitable treatment option. Unfortunately, the demand for donor corneas greatly exceeds the availability. To overcome shortage issues, a myriad of bioengineered constructs have been developed as mimetics of the corneal stroma over the last few decades. Despite the sheer number of bioengineered stromas developed , these implants fail clinical trials exhibiting poor tissue integration and adverse effects in vivo. Such shortcomings can partially be ascribed to poor biomechanical performance. In this review, existing approaches for bioengineering corneal stromal constructs and their mechanical properties are described. The information collected in this review can be used to critically analyze the biomechanical properties of future stromal constructs, which are often overlooked, but can determine the failure or success of corresponding implants.

Ultra-stiff compressed collagen for corneal perforation patch graft realized by in situ photochemical crosslinking

Despite the potential of a collagen construct, consisting of a major extracellular matrix component of the native cornea, as a patch graft to treat the corneal perforation, there has still been difficulty in acquiring sufficient mechanical properties for clinical availability. This study developed a novel in situ photochemical crosslinking (IPC)-assisted collagen compression process, namely, the IPC-C² process, to significantly enhance the mechanical properties of the collagen construct for the development of a collagenous patch graft. For the first time, we found that compressed collagen construct was rapidly rehydrated in an aqueous solution, which inhibited effective riboflavin-mediated photochemical crosslinking for mechanical improvement. The IPC-C² process was designed to concurrently induce the physical compaction and photochemical crosslinking of a compressed collagen construct, thereby avoiding the loosening of collagen fibrillar structure during rehydration and ultimately improving crosslinking efficiency. Hence, the suggested IPC-C² process could fabricate a collagen construct with a high collagen density (∼120-280 mg ml^-1) and ∼10³-fold increased mechanical properties (an elastic modulus of up to ∼29 MPa and ultimate tensile strength of ∼8 MPa) compared with collagen gel. This construct can then be used as a clinically applicable collagenous patch graft. With sufficient mechanical strength for surgical suture and the controllable thickness for patient specificity, the potential of the fabricated IPC-compressed collagen construct for clinical applications was demonstrated by using an in vivo rabbit corneal perforation model. It effectively protected aqueous humor leakage and maintained the integrity of the eye globe without an additional complication.