Cell-generated forces influence the viability, metabolism and mechanical properties of fibroblast-seeded collagen gel constructs

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.

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

Prior studies indicated that mechanical loading influences cell turnover and matrix remodeling in tissues, suggesting that mechanical stimuli can play an active role in engineering artificial tissues. While most tissue culture studies focus on influence of uniaxial loading or constraints, effects of multi-axial loading or constraints on tissue development are far from clear. In this study, we examined the biaxial mechanical properties of fibroblast-seeded collagen gels cultured under four different mechanical constraints for 6 days: free-floating, equibiaxial stretching (with three different stretch ratios), strip-biaxial stretching, and uniaxial stretching. Passive mechanical behavior of the cell-seeded gels was also examined after decellularization. A continuum-based two-dimensional Fung model was used to quantify the mechanical behavior of the gel. Based on the model, the value of stored strain energy and the ratio of stiffness in the stretching directions were calculated at prescribed strains for each gel, and statistical comparisons were made among the gels cultured under the various mechanical constraints. Results showed that gels cultured under the free-floating and equibiaxial stretching conditions exhibited a nearly isotropic mechanical behavior, while gels cultured under the strip-biaxial and uniaxial stretching conditions developed a significant degree of mechanical anisotropy. In particular, gels cultured under the equibiaxial stretching condition with a greater stretch ratio appeared to be stiffer than those with a smaller stretch ratio. Also, a decellularized gel was stiffer than its non-decellularized counterpart. Finally, the retained mechanical anisotropy in gels cultured under the strip-biaxial stretching and uniaxial stretching conditions after cell removal reflected an irreversible matrix remodeling.

A hydrogel derived from decellularized dermal extracellular matrix

The ECM of mammalian tissues has been used as a scaffold to facilitate the repair and reconstruction of numerous tissues. Such scaffolds are prepared in many forms including sheets, powders, and hydrogels. ECM hydrogels provide advantages such as injectability, the ability to fill an irregularly shaped space, and the inherent bioactivity of native matrix. However, material properties of ECM hydrogels and the effect of these properties upon cell behavior are neither well understood nor controlled. The objective of this study was to prepare and determine the structure, mechanics, and the cell response in vitro and in vivo of ECM hydrogels prepared from decellularized porcine dermis and urinary bladder tissues. Dermal ECM hydrogels were characterized by a more dense fiber architecture and greater mechanical integrity than urinary bladder ECM hydrogels, and showed a dose dependent increase in mechanical properties with ECM concentration. In vitro, dermal ECM hydrogels supported greater C2C12 myoblast fusion, and less fibroblast infiltration and less fibroblast mediated hydrogel contraction than urinary bladder ECM hydrogels. Both hydrogels were rapidly infiltrated by host cells, primarily macrophages, when implanted in a rat abdominal wall defect. Both ECM hydrogels degraded by 35 days in vivo, but UBM hydrogels degraded more quickly, and with greater amounts of myogenesis than dermal ECM. These results show that ECM hydrogel properties can be varied and partially controlled by the scaffold tissue source, and that these properties can markedly affect cell behavior.

Single cell viability measurements in 3D scaffolds using in situ label free imaging by optical coherence microscopy

The focus on creating tissue engineered constructs of clinically relevant sizes requires new approaches for monitoring construct health during tissue development. A few key requirements are that the technology be in situ, non-invasive, and provide temporal and spatial information. In this work, we demonstrate that optical coherence microscopy (OCM) can be used to assess cell viability without the addition of exogenous probes in three-dimensional (3D) tissue scaffolds maintained under standard culture conditions. This is done by collecting time-lapse images of speckle generated by sub-cellular features. Image cross-correlation is used to calculate the number of features the final image has in common with the initial image. If the cells are live, the number of common features is low. The number of common features approaches 100% if the cells are dead. In control experiments, cell viability is verified by the addition of a two-photon fluorescence channel to the OCM. Green fluorescent protein transfected human bone marrow stromal cells cultured in a transparent poly(ethylene glycol) tetramethacrylate hydrogel scaffold is used as the control system. Then, the utility of this approach is demonstrated by determining L929 fibroblast cell viability in a more challenging matrix, collagen, an optical scatterer. These results demonstrate a new technique for in situ mapping of single cell viability without any exogenous probes that is capable of providing continuous monitoring of construct health.

Working together: the combined application of a magnetic field and penetratin for the delivery of magnetic nanoparticles to cells in 3D

Nanoparticles (NPs) are currently being developed as vehicles for in vivo drug delivery. Two of the biggest barriers facing this therapy are the site-specific targeting and consequent cellular uptake of drug-loaded NPs(1). In vitro studies in 2D cell cultures have shown that an external magnetic field (MF) and functionalization with cell-penetrating peptides (CPPs) have the capacity to overcome these barriers. This study aimed to investigate if the potential of these techniques, which has been reported in 2D, can be successfully applied to cells growing in a 3D environment. As such, this study provides a more realistic assessment of how these techniques might perform in future clinical settings. The effect of a MF and/or penetratin attachment on the uptake of 100 and 200 nm fluorescent iron oxide magnetic NPs (mNPs) into a fibroblast-seeded 3D collagen gel was quantified by inductively coupled plasma mass spectrometry. The most suitable mNP species was further investigated by fluorescence microscopy, histology, confocal microscopy, and TEM. Results show that gel mNP uptake occurred on average twice as fast in the presence of a MF and up to three times faster with penetratin attachment. In addition, a MF increased the distance of mNP travel through the gel, while penetratin increased mNP cell localization. This work is one of the first to demonstrate that MFs and CPPs can be effectively translated for use in 3D systems and, if applied together, will make excellent partners to achieve therapeutic drug delivery in vivo.

In vitro studies and preliminary in vivo evaluation of silicified concentrated collagen hydrogels

Hybrid and nanocomposite silica-collagen materials derived from concentrated collagen hydrogels were evaluated in vitro and in vivo to establish their potentialities for biological dressings. Silicification significantly improved the mechanical and thermal stability of the collagen network within the hybrid systems. Nanocomposites were found to favor the metabolic activity of immobilized human dermal fibroblasts while decreasing the hydrogel contraction. Cell adhesion experiments suggested that in vitro cell behavior was dictated by mechanical properties and surface structure of the scaffold. First-to-date in vivo implantation of bulk hydrogels in subcutaneous sites of rats was performed over the vascular inflammatory period. These materials were colonized and vascularized without inducing strong inflammatory response. These data raise reasonable hope for the future application of silica-collagen biomaterials as biological dressings.

Dynamic quantitative visualization of single cell alignment and migration and matrix remodeling in 3-D collagen hydrogels under mechanical force

We developed a live imaging system enabling dynamic visualization of single cell alignment induced by external mechanical force in a 3-D collagen matrix. The alignment dynamics and migration of smooth muscle cells (SMCs) were studied by time lapse differential interference contrast and/or phase contrast microscopy. Fluorescent and reflection confocal microcopy were used to study the SMC morphology and the microscale collagen matrix remodeling induced by SMCs. A custom developed program was used to quantify the cell migration and matrix remodeling. Our system enables cell concentration-independent alignment eliminating cell-to-cell interference and enables dynamic cell tracking, high magnification observation and rapid cell alignment accomplished in a few hours compared to days in traditional models. We observed that cells sense and response to the mechanical signal before cell spreading. Under mechanical stretch the migration directionality index of SMCs is 46.3% more than those cells without external stretch; the dynamic direction of cell protrusion is aligned to that of the mechanical force; SMCs showed directional matrix remodeling and the alignment index calculated from the matrix in front of cell protrusions is about 3 fold of that adjacent to cell bodies. Our results indicate that the mechanism of cell alignment is directional cell protrusion. Mechano-sensing, directionality in cell protrusion dynamics, cell migration and matrix remodeling are highly integrated. Our system provides a platform for studying the role of mechanical force on the cell matrix interactions and thus finds strategies to optimize selected properties of engineered tissues.

Silica-collagen bionanocomposites as three-dimensional scaffolds for fibroblast immobilization

Silica-collagen bionanocomposite hydrogels were obtained by addition of silica nanoparticles to a protein suspension followed by neutralization. Electron microscopy studies indicated that larger silica nanoparticles (80 nm) do not interact strongly with collagen, whereas smaller ones (12 nm) form rosaries along the protein fibers. However, the composite network structurally evolved with time due to the contraction of the cells and the dissolution of the silica nanoparticles. When compared to classical collagen hydrogels, these bionanocomposite materials showed lower surface contraction in the short term (1 week) and higher viability of entrapped cells in the long term (3 weeks). A low level of gelatinase MMP2 enzyme expression was also found after this period. Several proteins involved in the catabolic and anabolic activity of the cells could also be observed by immunodetection techniques. All these data suggest that the bionanocomposite matrices constitute a suitable environment for fibroblast adhesion, proliferation and biological activity and therefore constitute an original three-dimensional environment for in vitro cell culture and in vivo applications, in particular as biological dressings.

Ligament-derived matrix stimulates a ligamentous phenotype in human adipose-derived stem cells

Human adipose stem cells (hASCs) can differentiate into a variety of phenotypes. Native extracellular matrix (e.g., demineralized bone matrix or small intestinal submucosa) can influence the growth and differentiation of stem cells. The hypothesis of this study was that a novel ligament-derived matrix (LDM) would enhance expression of a ligamentous phenotype in hASCs compared to collagen gel alone. LDM prepared using phosphate-buffered saline or 0.1% peracetic acid was mixed with collagen gel (COL) and was evaluated for its ability to induce proliferation, differentiation, and extracellular matrix synthesis in hASCs over 28 days in culture at different seeding densities (0, 0.25 x 10(6), 1 x 10(6), or 2 x 10(6) hASC/mL). Biochemical and gene expression data were analyzed using analysis of variance. Fisher's least significant difference test was used to determine differences between treatments following analysis of variance. hASCs in either LDM or COL demonstrated changes in gene expression consistent with ligament development. hASCs cultured with LDM demonstrated more dsDNA content, sulfated-glycosaminoglycan accumulation, and type I and III collagen synthesis, and released more sulfated-glycosaminoglycan and collagen into the medium compared to hASCs in COL (p <or= 0.05). Increased seeding density increased DNA content incrementally over 28 days in culture for LDM but not COL constructs (p <or= 0.05). These findings suggest that LDM can stimulate a ligament phenotype by hASCs, and may provide a novel scaffold material for ligament engineering applications.