Monitoring sinew contraction during formation of tissue-engineered fibrin-based ligament constructs

Synthetic and natural polymer hydrogels: A review of 3D spheroids and drug delivery

This review centers on the synthesis and characterization of both natural and synthetic hydrogels, highlighting their diverse applications across various fields. We will delve into the evolution of hydrogels, focusing on the importance of polysaccharide-based and synthetic variants, which have been particularly chosen for 3D spheroid development in cancer research and drug delivery. A detailed background on the research and specific methodologies, including the in-situ free radical polymerization used for synthesizing these hydrogels, will be extensively discussed. Additionally, the review will explore various applications of these hydrogels, such as their self-healing properties, swelling ratios, pH responsiveness, and cell viability. A comprehensive literature review will support this investigation. Ultimately, this review aims to clearly outline the objectives and significance of hydrogel synthesis and their applications.

Anatomical design and production of a novel three-dimensional co-culture system replicating the human flexor digitorum profundus enthesis

The enthesis, the specialized junction between tendon and bone, is a common site of injury. Although notoriously difficult to repair, advances in interfacial tissue engineering techniques are being developed for restorative function. Most notably are 3D in vitro co-culture models, built to recreate the complex heterogeneity of the native enthesis. While cell and matrix properties are often considered, there has been little attention given to native enthesis anatomical morphometrics and replicating these to enhance clinical relevance. This study focuses on the flexor digitorum profundus (FDP) tendon enthesis and, by combining anatomical morphometrics with computer-aided design, demonstrates the design and construction of an accurate and scalable model of the FDP enthesis. Bespoke 3D-printed mould inserts were fabricated based on the size, shape and insertion angle of the FDP enthesis. Then, silicone culture moulds were created, enabling the production of bespoke anatomical culture zones for an in vitro FDP enthesis model. The validity of the model has been confirmed using brushite cement scaffolds seeded with osteoblasts (bone) and fibrin hydrogel scaffolds seeded with fibroblasts (tendon) in individual studies with cells from either human or rat origin. This novel approach allows a bespoke anatomical design for enthesis repair and should be applied to future studies in this area.

Applications and Advances of Multicellular Tumor Spheroids: Challenges in Their Development and Analysis

Biomedical research requires both in vitro and in vivo studies in order to explore disease processes or drug interactions. Foundational investigations have been performed at the cellular level using two-dimensional cultures as the gold-standard method since the early 20th century. However, three-dimensional (3D) cultures have emerged as a new tool for tissue modeling over the last few years, bridging the gap between in vitro and animal model studies. Cancer has been a worldwide challenge for the biomedical community due to its high morbidity and mortality rates. Various methods have been developed to produce multicellular tumor spheroids (MCTSs), including scaffold-free and scaffold-based structures, which usually depend on the demands of the cells used and the related biological question. MCTSs are increasingly utilized in studies involving cancer cell metabolism and cell cycle defects. These studies produce massive amounts of data, which demand elaborate and complex tools for thorough analysis. In this review, we discuss the advantages and disadvantages of several up-to-date methods used to construct MCTSs. In addition, we also present advanced methods for analyzing MCTS features. As MCTSs more closely mimic the in vivo tumor environment, compared to 2D monolayers, they can evolve to be an appealing model for in vitro tumor biology studies.

Investigating materials and orientation parameters for the creation of a 3D musculoskeletal interface co-culture model

Musculoskeletal tissue interfaces are a common site of injury in the young, active populations. In particular, the interface between the musculoskeletal tissues of tendon and bone is often injured and to date, no single treatment has been able to restore the form and function of damaged tissue at the bone–tendon interface. Tissue engineering and regeneration hold great promise for the manufacture of bespoke in vitro models or implants to be used to advance repair and so this study investigated the material, orientation and culture choices for manufacturing a reproducible 3D model of a musculoskeletal interface between tendon and bone cell populations. Such models are essential for future studies focussing on the regeneration of musculoskeletal interfaces in vitro. Cell-encapsulated fibrin hydrogels, arranged in a horizontal orientation though a simple moulding procedure, were shown to best support cellular growth and migration of cells to form an in vitro tendon–bone interface. This study highlights the importance of acknowledging the material and technical challenges in establishing co-cultures and suggests a reproducible methodology to form 3D co-cultures between tendon and bone, or other musculoskeletal cell types, in vitro.

Dendritic Hydrogel Bioink for 3D Printing of Bacterial Microhabitat

A glucose-modified dendritic hydrogel is used as a bioink for bacterial encapsulation. This biocompatible hydrogel is a potentially suitable alternative to conventional alginate hydrogel for bacterial encapsulation, as it readily forms gel in the presence of Na+ or K+ ions without any additional stimuli such as pH, temperature, sonication, or the presence of divalent metal ions. We created a bacterial microhabitat by adding the gelator to phosphate-buffered saline containing live bacteria at physiological pH and using an additive three-dimensional (3D) printing technique. The bacteria remained viable and metabolically active within the 3D printed bacterial microhabitat, as shown with confocal laser scanning microscopy (CLSM) and scanning electrochemical microscopy (SECM).

Fibrin Matrices as (Injectable) Biomaterials: Formation, Clinical Use, and Molecular Engineering

This review focuses on fibrin, starting from biological mechanisms (its production from fibrinogen and its enzymatic degradation), through its use as a medical device and as a biomaterial, and finally discussing the techniques used to add biological functions and/or improve its mechanical performance through its molecular engineering. Fibrin is a material of biological (human, and even patient's own) origin, injectable, adhesive, and remodellable by cells; further, it is nature's most common choice for an in situ forming, provisional matrix. Its widespread use in the clinic and in research is therefore completely unsurprising. There are, however, areas where its biomedical performance can be improved, namely achieving a better control over mechanical properties (and possibly higher modulus), slowing down degradation or incorporating cell-instructive functions (e.g., controlled delivery of growth factors). The authors here specifically review the efforts made in the last 20 years to achieve these aims via biomimetic reactions or self-assembly, as much via formation of hybrid materials.

Treatment of Ligament Constructs with Exercise-conditioned Serum: A Translational Tissue Engineering Model

In vitro experiments are essential to understand biological mechanisms; however, the gap between monolayer tissue culture and human physiology is large, and translation of findings is often poor. Thus, there is ample opportunity for alternative experimental approaches. Here we present an approach in which human cells are isolated from human anterior cruciate ligament tissue remnants, expanded in culture, and used to form engineered ligaments. Exercise alters the biochemical milieu in the blood such that the function of many tissues, organs and bodily processes are improved. In this experiment, ligament construct culture media was supplemented with experimental human serum that has been 'conditioned' by exercise. Thus the intervention is more biologically relevant since an experimental tissue is exposed to the full endogenous biochemical milieu, including binding proteins and adjunct compounds that may be altered in tandem with the activity of an unknown agent of interest. After treatment, engineered ligaments can be analyzed for mechanical function, collagen content, morphology, and cellular biochemistry. Overall, there are four major advantages versus traditional monolayer culture and animal models, of the physiological model of ligament tissue that is presented here. First, ligament constructs are three-dimensional, allowing for mechanical properties (i.e., function) such as ultimate tensile stress, maximal tensile load, and modulus, to be quantified. Second, the enthesis, the interface between boney and sinew elements, can be examined in detail and within functional context. Third, preparing media with post-exercise serum allows for the effects of the exercise-induced biochemical milieu, which is responsible for the wide range of health benefits of exercise, to be investigated in an unbiased manner. Finally, this experimental model advances scientific research in a humane and ethical manner by replacing the use of animals, a core mandate of the National Institutes of Health, the Center for Disease Control, and the Food and Drug Administration.

Proteomic differences between native and tissue-engineered tendon and ligament

Tendons and ligaments (T/Ls) play key roles in the musculoskeletal system, but they are susceptible to traumatic or age‐related rupture, leading to severe morbidity as well as increased susceptibility to degenerative joint diseases such as osteoarthritis. Tissue engineering represents an attractive therapeutic approach to treating T/L injury but it is hampered by our poor understanding of the defining characteristics of the two tissues. The present study aimed to determine differences in the proteomic profile between native T/Ls and tissue engineered (TE) T/L constructs. The canine long digital extensor tendon and anterior cruciate ligament were analyzed along with 3D TE fibrin‐based constructs created from their cells. Native tendon and ligament differed in their content of key structural proteins, with the ligament being more abundant in fibrocartilaginous proteins. 3D T/L TE constructs contained less extracellular matrix (ECM) proteins and had a greater proportion of cellular‐associated proteins than native tissue, corresponding to their low collagen and high DNA content. Constructs were able to recapitulate native T/L tissue characteristics particularly with regard to ECM proteins. However, 3D T/L TE constructs had similar ECM and cellular protein compositions indicating that cell source may not be an important factor for T/L tissue engineering.

A practical guide to hydrogels for cell culture

There is growing appreciation of the role that the extracellular environment plays in regulating cell behavior. Mechanical, structural, and compositional cues, either alone or in concert, can drastically alter cell function. Biomaterials, and particularly hydrogels, have been developed and implemented to present defined subsets of these cues for investigating countless cellular processes as a means of understanding morphogenesis, aging, and disease. Although most scientists concede that standard cell culture materials (tissue culture plastic and glass) do a poor job of recapitulating native cellular milieus, there is currently a knowledge barrier for many researchers in regard to the application of hydrogels for cell culture. Here, we introduce hydrogels to those who may be unfamiliar with procedures to culture and study cells with these systems, with a particular focus on commercially available hydrogels.

The potential applications of fibrin-coated electrospun polylactide nanofibers in skin tissue engineering

Fibrin plays an important role during wound healing and skin regeneration. It is often applied in clinical practice for treatment of skin injuries or as a component of skin substitutes. We prepared electrospun nanofibrous membranes made from poly(l-lactide) modified with a thin fibrin nanocoating. Fibrin surrounded the individual fibers in the membrane and also formed a thin fibrous mesh on several places on the membrane surface. The cell-free fibrin nanocoating remained stable in the cell culture medium for 14 days and did not change its morphology. On membranes populated with human dermal fibroblasts, the rate of fibrin degradation correlated with the degree of cell proliferation. The cell spreading, mitochondrial activity, and cell population density were significantly higher on membranes coated with fibrin than on nonmodified membranes, and this cell performance was further improved by the addition of ascorbic acid in the cell culture medium. Similarly, fibrin stimulated the expression and synthesis of collagen I in human dermal fibroblasts, and this effect was further enhanced by ascorbic acid. The expression of beta₁-integrins was also improved by fibrin, and on pure polylactide membranes, it was slightly enhanced by ascorbic acid. In addition, ascorbic acid promoted deposition of collagen I in the form of a fibrous extracellular matrix. Thus, the combination of nanofibrous membranes with a fibrin nanocoating and ascorbic acid seems to be particularly advantageous for skin tissue engineering.

Exploiting cell-mediated contraction and adhesion to structure tissues in vitro

Progress in tissue engineering is now impacting beyond the field of regenerative medicine. Engineered tissues are now used as tools to evaluate the toxicity of compounds or even to enable the modelling of disease. While many of the materials that are used to facilitate tissue growth are designed to enable cell attachment, many researchers consider that the contraction and modification of these matrices by attached cells is not desirable and take measures to prevent this from occurring. Where substantial alignment of the molecules within tissues, however, is a feature of structure the process of contraction can be exploited to guide new matrix deposition. In this paper, we will demonstrate how we have used the cell contraction process to generate tissues with high levels of organization. The tissues that have been grown in the laboratory have been characterized using a suite of analytical techniques to demonstrate significant levels of matrix organization and mechanical behaviour analogous to natural tissues. This paper provides an overview of research that has been undertaken to determine how tissues have been grown in vitro with structuring from the molecular, right through to the macroscopic level.

Combined decellularisation and dehydration improves the mechanical properties of tissue-engineered sinews

Novel sources of replacement sinews are needed to repair damaged tissue after injury. The current methods of repair ultilise autografts, allografts or xenografts, although each method has distinct disadvantages that limit their success. Decellularisation of harvested tissues has been previously investigated for sinew repair with the long-term aim of repopulating the structure with autologous cells. Although this procedure shows promise, the demand for donor scaffolds will always outweigh supply. Here, we report the fabrication of fibrin-based tissue-engineered sinews, which can be decellularised, dehydrated and stored. The sinews may then be rehydrated and repopulated with an autologous cell population. In addition to enabling production of patient-specific implants, interestingly, the process of combined decellularisation, dehydration and rehydration enhanced the mechanical properties of the sinew. The treated sinews exhibited a 2.6-fold increase in maximum load and 8-fold increase in ultimate tensile strength when compared with the control group (p < 0.05 in both cases).

The effect of growth factors on both collagen synthesis and tensile strength of engineered human ligaments

Growth factors play a central role in the development and remodelling of musculoskeletal tissues. To determine which growth factors optimized in vitro ligament formation and mechanics, a Box-Behnken designed array of varying concentrations of growth factors and ascorbic acid were applied to engineered ligaments and the collagen content and mechanics of the grafts were determined. Increasing the amount of transforming growth factor (TGF) β1 and insulin-like growth factor (IGF)-1 led to an additive effect on ligament collagen and maximal tensile load (MTL). In contrast, epidermal growth factor (EGF) had a negative effect on both collagen content and MTL. The predicted optimal growth media (50 μg/ml TGFβ, IGF-1, and GDF-7 and 200 μM ascorbic acid) was then validated in two separate trials: showing a 5.7-fold greater MTL and 5.2-fold more collagen than a minimal media. Notably, the effect of the maximized growth media was scalable such that larger constructs developed the same material properties, but larger MTL. These results show that optimizing the interactions between growth factors and engineered ligament volume results in an engineered ligament of clinically relevant function.