Development of fibroblast-seeded collagen gels under planar biaxial mechanical constraints: a biomechanical study

Dynamic biaxial loading of vascular smooth muscle cell seeded tissue equivalents

An intricate reciprocal relationship exists between adherent synthetic cells and their extracellular matrix (ECM). These cells deposit, organize, and degrade the ECM, which in turn influences cell phenotype via responses that include sensitivity to changes in the mechanical state that arises from changes in external loading. Collagen-based tissue equivalents are commonly used as simple but revealing model systems to study cell-matrix interactions. Nevertheless, few quantitative studies report changes in the forces that the cells establish and maintain in such gels under dynamic loading. Moreover, most prior studies have been limited to uniaxial experiments despite many soft tissues, including arteries, experiencing multiaxial loading in vivo. To begin to close this gap, we use a custom biaxial bioreactor to subject collagen gels seeded with primary aortic smooth muscle cells to different biaxial loading conditions. These conditions include cyclic loading with different amplitudes as well as different mechanical constraints at the boundaries of a cruciform sample. Irrespective of loading amplitude and boundary condition, similar mean steady-state biaxial forces emerged across all tests. Additionally, stiffness-force relationships assessed via intermittent equibiaxial force-extension tests showed remarkable similarity for ranges of forces to which the cells adapted during periods of cyclic loading. Taken together, these findings are consistent with a load-mediated homeostatic response by vascular smooth muscle cells.

Mechanical Factors Regulate Annulus Fibrosus (AF) Injury Repair and Remodeling: A Review

Low back pain is a common chronic disease that can severely affect the patient's work and daily life. The breakdown of spinal mechanical homeostasis caused by intervertebral disc (IVD) degeneration is a leading cause of low back pain. Annulus fibrosus (AF), as the outer layer structure of the IVD, is often the first affected part. AF injury caused by consistent stress overload will further accelerate IVD degeneration. Therefore, regulating AF injury repair and remodeling should be the primary goal of the IVD repair strategy. Mechanical stimulation has been shown to promote AF regeneration and repair, but most studies only focus on the effect of single stress on AF, and lack realistic models and methods that can mimic the actual mechanical environment of AF. In this article, we review the effects of different types of stress stimulation on AF injury repair and remodeling, suggest possible beneficial load combinations, and explore the underlying molecular mechanisms. It will provide the theoretical basis for designing better tissue engineering therapy using mechanical factors to regulate AF injury repair and remodeling in the future.

Mechano-Activated Cell Therapy for Accelerated Diabetic Wound Healing

Chronic diabetic wounds are a significant global healthcare challenge. Current strategies, such as biomaterials, cell therapies, and medical devices, however, only target a few pathological features and have limited efficacy. A powerful platform technology combining magneto-responsive hydrogel, cells, and wireless magneto-induced dynamic mechanical stimulation (MDMS) is developed to accelerate diabetic wound healing. The hydrogel encapsulates U.S. Food and Drug Administration (FDA)-approved fibroblasts and keratinocytes to achieve ∼3-fold better wound closure in a diabetic mouse model. MDMS acts as a nongenetic mechano-rheostat to activate fibroblasts, resulting in ∼240% better proliferation, ∼220% more collagen deposition, and improved keratinocyte paracrine profiles via the Ras/MEK/ERK pathway to boost angiogenesis. The magneto-responsive property also enables on-demand insulin release for spatiotemporal glucose regulation through increasing network deformation and interstitial flow. By mining scRNAseq data, a mechanosensitive fibroblast subpopulation is identified that can be mechanically tuned for enhanced proliferation and collagen production, maximizing therapeutic impact. The "all-in-one" system addresses major pathological factors associated with diabetic wounds in a single platform, with potential applications for other challenging wound types.

Dynamic Stimulations with Bioengineered Extracellular Matrix-Mimicking Hydrogels for Mechano Cell Reprogramming and Therapy

Cells interact with their surrounding environment through a combination of static and dynamic mechanical signals that vary over stimulus types, intensity, space, and time. Compared to static mechanical signals such as stiffness, porosity, and topography, the current understanding on the effects of dynamic mechanical stimulations on cells remains limited, attributing to a lack of access to devices, the complexity of experimental set-up, and data interpretation. Yet, in the pursuit of emerging translational applications (e.g., cell manufacturing for clinical treatment), it is crucial to understand how cells respond to a variety of dynamic forces that are omnipresent in vivo so that they can be exploited to enhance manufacturing and therapeutic outcomes. With a rising appreciation of the extracellular matrix (ECM) as a key regulator of biofunctions, researchers have bioengineered a suite of ECM-mimicking hydrogels, which can be fine-tuned with spatiotemporal mechanical cues to model complex static and dynamic mechanical profiles. This review first discusses how mechanical stimuli may impact different cellular components and the various mechanobiology pathways involved. Then, how hydrogels can be designed to incorporate static and dynamic mechanical parameters to influence cell behaviors are described. The Scopus database is also used to analyze the relative strength in evidence, ranging from strong to weak, based on number of published literatures, associated citations, and treatment significance. Additionally, the impacts of static and dynamic mechanical stimulations on clinically relevant cell types including mesenchymal stem cells, fibroblasts, and immune cells, are evaluated. The aim is to draw attention to the paucity of studies on the effects of dynamic mechanical stimuli on cells, as well as to highlight the potential of using a cocktail of various types and intensities of mechanical stimulations to influence cell fates (similar to the concept of biochemical cocktail to direct cell fate). It is envisioned that this progress report will inspire more exciting translational development of mechanoresponsive hydrogels for biomedical applications.

Collagen gel contraction assays: From modelling wound healing to quantifying cellular interactions with three-dimensional extracellular matrices

Cells respond to and actively remodel the extracellular matrix (ECM). The dynamic and bidirectional interaction between cells and ECM, especially their mechanical interactions, has been found to play an essential role in triggering a series of complex biochemical and biomechanical signal pathways and in regulating cellular functions and behaviours. The collagen gel contraction assay (CGCA) is a widely used method to investigate cell-ECM interactions in 3D environments and provides a mechanically associated readout reflecting 3D cellular contractility. In this review, we summarize various versions of CGCA, with an emphasis on recent high-throughput and low-consumption CGCA techniques. More importantly, we focus on the technique of force monitoring during the contraction of collagen gel, which provides a quantitative characterization of the overall forces generated by all the resident cells in the collagen hydrogel. Accordingly, we present recent biological applications of the CGCA, which have expanded from the initial wound healing model to other studies concerning cell-ECM interactions, including fibrosis, cancer, tissue repair and the preparation of biomimetic microtissues.

Mechanical homeostasis in tissue equivalents: a review

There is substantial evidence that growth and remodeling of load bearing soft biological tissues is to a large extent controlled by mechanical factors. Mechanical homeostasis, which describes the natural tendency of such tissues to establish, maintain, or restore a preferred mechanical state, is thought to be one mechanism by which such control is achieved across multiple scales. Yet, many questions remain regarding what promotes or prevents homeostasis. Tissue equivalents, such as collagen gels seeded with living cells, have become an important tool to address these open questions under well-defined, though limited, conditions. This article briefly reviews the current state of research in this area. It summarizes, categorizes, and compares experimental observations from the literature that focus on the development of tension in tissue equivalents. It focuses primarily on uniaxial and biaxial experimental studies, which are well-suited for quantifying interactions between mechanics and biology. The article concludes with a brief discussion of key questions for future research in this field.

Computer-Controlled Biaxial Bioreactor for Investigating Cell-Mediated Homeostasis in Tissue Equivalents

Soft biological tissues consist of cells and extracellular matrix (ECM), a network of diverse proteins, glycoproteins, and glycosaminoglycans that surround the cells. The cells actively sense the surrounding ECM and regulate its mechanical state. Cell-seeded collagen or fibrin gels, so-called tissue equivalents, are simple but powerful model systems to study this phenomenon. Nevertheless, few quantitative studies document the stresses that cells establish and maintain in such gels; moreover, most prior data were collected via uniaxial experiments whereas soft tissues are mainly subject to multiaxial loading in vivo. To begin to close this gap between existing experimental data and in vivo conditions, we describe here a computer-controlled bioreactor that enables accurate measurements of the evolution of mechanical tension and deformation of tissue equivalents under well-controlled biaxial loads. This device allows diverse studies, including how cells establish a homeostatic state of biaxial stress and if they maintain it in response to mechanical perturbations. It similarly allows, for example, studies of the impact of cell and matrix density, exogenous growth factors and cytokines, and different types of loading conditions (uniaxial, strip-biaxial, and biaxial) on these processes. As illustrative results, we show that NIH/3T3 fibroblasts establish a homeostatic mechanical state that depends on cell density and collagen concentration. Following perturbations from this homeostatic state, the cells were able to recover biaxial loading similar to homeostatic. Depending on the precise loads, however, they were not always able to fully maintain that state.

Fibroblast-seeded collagen gels in response to dynamic equibiaxial mechanical stimuli: A biomechanical study

The remodeling of fibroblast-seeded collagen gels in response to dynamic mechanical stimuli was investigated by using a newly developed biaxial culture system capable of cyclically stretching planar soft tissues. Fibroblast-seeded collagen gels were subjected to three distinct dynamic mechanical conditions for six days: Cyclic Equibiaxial Stretching at two constant strain magnitudes (CES-7% and CES-20%), and Cyclic Equibiaxial Stretching with incrementally Increasing stain magnitude (ICES, 7% → 15% → 20% each for two days). The frequency of cyclic stretching was set at 1 Hz. At the end of culture, mechanical properties of the gels were examined by biaxial mechanical testing and checked again upon the removal of seeded cells. Collagen microstructure within the gels was illustrated by multiphoton microscopy. The mRNA levels of collagen type I and type III and fibronectin in the cells were examined by reverse transcription PCR. The protein expression of α-smooth muscle actin was detected by immunohistochemistry. We found that the gels cultured under cyclic stretching were stiffer than those cultured under static stretching. Particularly, the stiffness appeared to be significantly enhanced when the ICES was employed. The enhancement of mechanical properties by cyclic stretching appeared to persist upon cell removal, suggesting an irreversible remodeling of extracellular matrix. Second harmonic generation images showed that collagen fibers became thicker and more compact in the gels cultured under cyclic stretching, which may explain the mechanical findings. The mRNA expression of collagen type I in the cells of the ICES was significantly greater than that of the other groups except for the CES-20%. This study suggests that when cyclic stretching is to be used in engineering soft tissues, incrementally increasing strain magnitude may prove useful in the development of the tissue.

Device-Based In Vitro Techniques for Mechanical Stimulation of Vascular Cells: A Review

The most common cause of death in the developed world is cardiovascular disease. For decades, this has provided a powerful motivation to study the effects of mechanical forces on vascular cells in a controlled setting, since these cells have been implicated in the development of disease. Early efforts in the 1970 s included the first use of a parallel-plate flow system to apply shear stress to endothelial cells (ECs) and the development of uniaxial substrate stretching techniques (Krueger et al., 1971, “An in Vitro Study of Flow Response by Cells,” J. Biomech., 4(1), pp. 31–36 and Meikle et al., 1979, “Rabbit Cranial Sutures in Vitro: A New Experimental Model for Studying the Response of Fibrous Joints to Mechanical Stress,” Calcif. Tissue Int., 28(2), pp. 13–144). Since then, a multitude of in vitro devices have been designed and developed for mechanical stimulation of vascular cells and tissues in an effort to better understand their response to in vivo physiologic mechanical conditions. This article reviews the functional attributes of mechanical bioreactors developed in the 21st century, including their major advantages and disadvantages. Each of these systems has been categorized in terms of their primary loading modality: fluid shear stress (FSS), substrate distention, combined distention and fluid shear, or other applied forces. The goal of this article is to provide researchers with a survey of useful methodologies that can be adapted to studies in this area, and to clarify future possibilities for improved research methods.