Fibrin gels and their clinical and bioengineering applications

Stem cells and nanomaterials

Because of their ability to self-renew and differentiate into many cell types, stem cells offer the potential to be used for tissue regeneration and engineering. Much progress has recently been made in our understanding of the biology of stem cells and our ability to manipulate their proliferation and differentiation to obtain functional tissues. Similarly, nanomaterials have been recently developed that will accelerate discovery of mechanisms driving stem cell fate and their utilization in medicine. Nanoparticles have been developed that allow the labeling and tracking of stem cells and their differentiated phenotype within an organism. Nanosurfaces are engineered that mimic the extracellular matrix to which stem cells adhere and migrate. Scaffolds made of functionalized nanofibers can now be used to grow stem cells and regenerate damaged tissues and organs. However, the small scale of nanomaterials induces changes in their chemical and physical properties that might modify their interactions with cells and tissues, and render them toxic to stem cells. Therefore a thorough understanding of stem cell-nanomaterial interactions is still necessary not only to accelerate the success of medical treatments but also to ensure the safety of the tools provided by these novel technologies.

Cells actively stiffen fibrin networks by generating contractile stress

During wound healing and angiogenesis, fibrin serves as a provisional extracellular matrix. We use a model system of fibroblasts embedded in fibrin gels to study how cell-mediated contraction may influence the macroscopic mechanical properties of their extracellular matrix during such processes. We demonstrate by macroscopic shear rheology that the cells increase the elastic modulus of the fibrin gels. Microscopy observations show that this stiffening sets in when the cells spread and apply traction forces on the fibrin fibers. We further show that the stiffening response mimics the effect of an external stress applied by mechanical shear. We propose that stiffening is a consequence of active myosin-driven cell contraction, which provokes a nonlinear elastic response of the fibrin matrix. Cell-induced stiffening is limited to a factor 3 even though fibrin gels can in principle stiffen much more before breaking. We discuss this observation in light of recent models of fibrin gel elasticity, and conclude that the fibroblasts pull out floppy modes, such as thermal bending undulations, from the fibrin network, but do not axially stretch the fibers. Our findings are relevant for understanding the role of matrix contraction by cells during wound healing and cancer development, and may provide design parameters for materials to guide morphogenesis in tissue engineering.

In vivo ectopic implantation model to assess human mesenchymal progenitor cell potential

Clinical interest on human mesenchymal progenitor cells (hMPC) relies on their potential applicability in cell-based therapies. An in vitro characterization is usually performed in order to define MPC potency. However, in vitro predictions not always correlate with in vivo results and thus there is no consensus in how to really assess cell potency. Our goal was to provide an in vivo testing method to define cell behavior before therapeutic usage, especially for bone tissue engineering applications. In this context, we wondered whether bone marrow stromal cells (hBMSC) would proceed in an osteogenic microenvironment. Based on previous approaches, we developed a fibrin/ceramic/BMP-2/hBMSCs compound. We implanted the compound during only 2 weeks in NOD-SCID mice, either orthotopically to assess its osteoinductive property or subcutaneously to analyze its adequacy as a cell potency testing method. Using fluorescent cell labeling and immunohistochemistry techniques, we could ascertain cell differentiation to bone, bone marrow, cartilage, adipocyte and fibrous tissue. We observed differences in cell potential among different batches of hBMSCs, which did not strictly correlate with in vitro analyses. Our data indicate that the method we have developed is reliable, rapid and reproducible to define cell potency, and may be useful for testing cells destined to bone tissue engineering purposes. Additionally, results obtained with hMPCs from other sources indicate that our method is suitable for testing any potentially implantable mesenchymal cell. Finally, we propose that this model could successfully be employed for bone marrow niche and bone tumor studies.

Three-dimensionally printed biological machines powered by skeletal muscle

Combining biological components, such as cells and tissues, with soft robotics can enable the fabrication of biological machines with the ability to sense, process signals, and produce force. An intuitive demonstration of a biological machine is one that can produce motion in response to controllable external signaling. Whereas cardiac cell-driven biological actuators have been demonstrated, the requirements of these machines to respond to stimuli and exhibit controlled movement merit the use of skeletal muscle, the primary generator of actuation in animals, as a contractile power source. Here, we report the development of 3D printed hydrogel "bio-bots" with an asymmetric physical design and powered by the actuation of an engineered mammalian skeletal muscle strip to result in net locomotion of the bio-bot. Geometric design and material properties of the hydrogel bio-bots were optimized using stereolithographic 3D printing, and the effect of collagen I and fibrin extracellular matrix proteins and insulin-like growth factor 1 on the force production of engineered skeletal muscle was characterized. Electrical stimulation triggered contraction of cells in the muscle strip and net locomotion of the bio-bot with a maximum velocity of ∼ 156 μm s(-1), which is over 1.5 body lengths per min. Modeling and simulation were used to understand both the effect of different design parameters on the bio-bot and the mechanism of motion. This demonstration advances the goal of realizing forward-engineered integrated cellular machines and systems, which can have a myriad array of applications in drug screening, programmable tissue engineering, drug delivery, and biomimetic machine design.

Introduction to cell-hydrogel mechanosensing

The development of hydrogel-based biomaterials represents a promising approach to generating new strategies for tissue engineering and regenerative medicine. In order to develop more sophisticated cell-seeded hydrogel constructs, it is important to understand how cells mechanically interact with hydrogels. In this paper, we review the mechanisms by which cells remodel hydrogels, the influence that the hydrogel mechanical and structural properties have on cell behaviour and the role of mechanical stimulation in cell-seeded hydrogels. Cell-mediated remodelling of hydrogels is directed by several cellular processes, including adhesion, migration, contraction, degradation and extracellular matrix deposition. Variations in hydrogel stiffness, density, composition, orientation and viscoelastic characteristics all affect cell activity and phenotype. The application of mechanical force on cells encapsulated in hydrogels can also instigate changes in cell behaviour. By improving our understanding of cell-material mechano-interactions in hydrogels, this should enable a new generation of regenerative medical therapies to be developed.

Temporal and spatial patterning of transgene expression by near-infrared irradiation

We investigated whether near-infrared (NIR) light could be employed for patterning transgene expression in plasmonic cell constructs. Hollow gold nanoparticles with a plasmon surface band absorption peaking at ∼750 nm, a wavelength within the so called "tissue optical window", were used as fillers in fibrin-based hydrogels. These composites, which efficiently transduce NIR photon energy into heat, were loaded with genetically-modified cells that harbor a heat-activated and ligand-dependent gene switch for regulating transgene expression. NIR laser irradiation in the presence of ligand triggered 3-dimensional patterns of transgene expression faithfully matching the illuminated areas of plasmonic cell constructs. This non-invasive technology was proven useful for remotely controlling in vivo the spatiotemporal bioavailability of transgenic vascular endothelial growth factor. The combination of spatial control by means of NIR irradiation along with safe and timed transgene induction presents a high application potential for engineering tissues in regenerative medicine scenarios.

Artificial microniches for probing mesenchymal stem cell fate in 3D

Droplet microfluidics is combined with bio-orthogonal thiol-ene click chemistry to fabricate micrometer-sized, monodisperse fibrinogen-containing hyaluronic acid hydrogel microbeads in a mild, radical-free procedure in the presence of human mesenchymal stem cells (hMSCs). The gel beads serve as microniches for the 3D culture of single hMSCs, containing hyaluronic acid and additional fibrinogen for cell surface binding, and they are porous and stable in tissue culture medium for up to 4 weeks with mechanical properties right in the range of soft solid tissues (0.9-9.2 kPa). The encapsulation procedure results in 70% viable hMSCs in the microbeads after 24 hours of culture and a very high degree of viability of the cells after long term culture of 2 weeks. hMSCs embedded in the microniches display an overall rounded morphology, consistent with those previously observed in 3D culture. Upon induction, the multipotency and differentiation potential of the hMSCs are characterized by staining of corresponding biomarkers, demonstrating a clear heterogeneity in the cell population. These hydrogel microbeads represent a versatile microstructured material platform with great potential for studying the differences of material cues and soluble factors in stem cell differentiation in a 3D tissue-like environment at the single cell level.

Investigation into the effects of varying frequency of mechanical stimulation in a cycle-by-cycle manner on engineered cardiac construct function

Mechanical stimulation has been used extensively to improve the function of cardiac engineered tissue, as it mimics the physical environment in which the tissue is situated during normal development. However, previous mechanical stimulation has been carried out under a constant frequency that more closely resembles a diseased heart. The goal of this study was to create a bioreactor system that would allow us to control the mechanical stimulation of engineered cardiac tissue on a cycle-by-cycle basis. This unique system allows us to determine the effects on cardiac construct function of introducing variability to the mechanical stretch. To test our bioreactor system, constructs created from neonatal rat cardiomyocytes entrapped in fibrin hydrogels were stimulated under various regimes for 2 weeks and then assessed for functional outcomes. No differences were observed in the final cell number in each condition, indicating that variability in frequency did not have a negative effect on viability. The forces were higher for all mechanical stimulation groups compared to static controls, although no differences were observed between the mechanically stimulated conditions, indicating that variable frequency on a cycle-by-cycle basis has limited effects on the resulting force. Although differences in the observed twitch force were not observed, differences in the protein expression indicate that variable-frequency mechanical stimulation had an effect on cell-cell coupling and growth pathway activation in the constructs. Thus, this bioreactor system provides a valuable tool for further development and optimization of engineered myocardial tissue as a repair or replacement strategy for patients undergoing heart failure. Copyright © 2014 John Wiley & Sons, Ltd.

Estimating the 3D pore size distribution of biopolymer networks from directionally biased data

The pore size of biopolymer networks governs their mechanical properties and strongly impacts the behavior of embedded cells. Confocal reflection microscopy and second harmonic generation microscopy are widely used to image biopolymer networks; however, both techniques fail to resolve vertically oriented fibers. Here, we describe how such directionally biased data can be used to estimate the network pore size. We first determine the distribution of distances from random points in the fluid phase to the nearest fiber. This distribution follows a Rayleigh distribution, regardless of isotropy and data bias, and is fully described by a single parameter--the characteristic pore size of the network. The bias of the pore size estimate due to the missing fibers can be corrected by multiplication with the square root of the visible network fraction. We experimentally verify the validity of this approach by comparing our estimates with data obtained using confocal fluorescence microscopy, which represents the full structure of the network. As an important application, we investigate the pore size dependence of collagen and fibrin networks on protein concentration. We find that the pore size decreases with the square root of the concentration, consistent with a total fiber length that scales linearly with concentration.

Physiology and metabolism of tissue-engineered skeletal muscle

Skeletal muscle is a major target for tissue engineering, given its relative size in the body, fraction of cardiac output that passes through muscle beds, as well as its key role in energy metabolism and diabetes, and the need for therapies for muscle diseases such as muscular dystrophy and sarcopenia. To date, most studies with tissue-engineered skeletal muscle have utilized murine and rat cell sources. On the other hand, successful engineering of functional human muscle would enable different applications including improved methods for preclinical testing of drugs and therapies. Some of the requirements for engineering functional skeletal muscle include expression of adult forms of muscle proteins, comparable contractile forces to those produced by native muscle, and physiological force-length and force-frequency relations. This review discusses the various strategies and challenges associated with these requirements, specific applications with cultured human myoblasts, and future directions.

Mimicking isovolumic contraction with combined electromechanical stimulation improves the development of engineered cardiac constructs

Electrical and mechanical stimulation have both been used extensively to improve the function of cardiac engineered tissue as each of these stimuli is present in the physical environment during normal development in vivo. However, to date, there has been no direct comparison between electrical and mechanical stimulation and current published data are difficult to compare due to the different systems used to create the engineered cardiac tissue and the different measures of functionality studied as outcomes. The goals of this study were twofold. First, we sought to directly compare the effects of mechanical and electrical stimulation on engineered cardiac tissue. Second, we aimed to determine the importance of the timing of the two stimuli in relation to each other in combined electromechanical stimulation. We hypothesized that delaying electrical stimulation after the beginning of mechanical stimulation to mimic the biophysical environment present during isovolumic contraction would improve construct function by improving proteins responsible for cell-cell communication and contractility. To test this hypothesis, we created a bioreactor system that would allow us to electromechanically stimulate engineered tissue created from neonatal rat cardiac cells entrapped in fibrin gel during 2 weeks in culture. Contraction force was higher for all stimulation groups as compared with the static controls, with the delayed combined stimulation constructs having the highest forces. Mechanical stimulation alone displayed increased final cell numbers but there were no other differences between electrical and mechanical stimulation alone. Delayed combined stimulation resulted in an increase in SERCA2a and troponin T expression levels, which did not happen with synchronous combined stimulation, indicating that the timing of combined stimulation is important to maximize the beneficial effect. Increases in Akt protein expression levels suggest that the improvements are at least in part induced by hypertrophic growth. In summary, combined electromechanical stimulation can create engineered cardiac tissue with improved functional properties over electrical or mechanical stimulation alone, and the timing of the combined stimulation greatly influences its effects on engineered cardiac tissue.

Topologically defined composites of collagen types I and V as in vitro cell culture scaffolds

Cell fate is known to be triggered by cues from the extracellular matrix, including its chemical, biological and physical characteristics. Specifically, mechanical and topological properties are increasingly recognized as important signals. The aim of this work was to provide an easily accessible biomimetic in vitro platform of topologically defined collagen I matrices to dissect cell behaviour under various conditions in vitro. We reconstituted covalently bound layers of three-dimensional (3-D) networks of collagen type I and collagen type V with a defined network topology. A new erosion algorithm enabled us to analyse the mean pore diameter and fibril content, while the mean fibril diameter was examined by an autocorrelation method. Different concentrations and ratios of collagen I and V resulted in pore diameters from 2.4 to 4.5μm and fibril diameters from 0.6 to 0.8μm. A comparison of telopeptide intact collagen I to telopeptide deficient collagen I revealed obvious differences in network structure. The good correlation of the topological data to measurements of network stiffness as well as invasion of human dermal fibroblasts proves that the topological analysis provides meaningful measures of the functional characteristics of the reconstituted 3-D collagen matrices.

Structural basis for the nonlinear mechanics of fibrin networks under compression

Fibrin is a protein polymer that forms a 3D filamentous network, a major structural component of protective physiological blood clots as well as life threatening pathological thrombi. It plays an important role in wound healing, tissue regeneration and is widely employed in surgery as a sealant and in tissue engineering as a scaffold. The goal of this study was to establish correlations between structural changes and mechanical responses of fibrin networks exposed to compressive loads. Rheological measurements revealed nonlinear changes of fibrin network viscoelastic properties under dynamic compression, resulting in network softening followed by its dramatic hardening. Repeated compression/decompression enhanced fibrin clot stiffening. Combining fibrin network rheology with simultaneous confocal microscopy provided direct evidence of structural modulations underlying nonlinear viscoelasticity of compressed fibrin networks. Fibrin clot softening in response to compression strongly correlated with fiber buckling and bending, while hardening was associated with fibrin network densification. Our results suggest a complex interplay of entropic and enthalpic mechanisms accompanying structural changes and accounting for the nonlinear mechanical response in fibrin networks undergoing compressive deformations. These findings provide new insight into the fibrin clot structural mechanics and can be useful for designing fibrin-based biomaterials with modulated viscoelastic properties.

Sequential multimodal microscopic imaging and biaxial mechanical testing of living multicomponent tissue constructs

Understanding relationships between mechanical stimuli and cellular responses require measurements of evolving tissue structure and mechanical properties. We developed a 3D tissue bioreactor that couples to both the stage of a custom multimodal microscopy system and a biaxial mechanical testing platform. Time dependent changes in microstructure and mechanical properties of fibroblast seeded cruciform fibrin gels were investigated while cultured under either anchored (1.0:1.0 stretch ratio) or strip biaxial (1.0:1.1) conditions. A multimodal nonlinear optical microscopy-optical coherence microscopy (NLOM-OCM) system was used to delineate noninvasively the relative spatial distributions of original fibrin, deposited collagen, and fibroblasts during month long culture. Serial in-culture mechanical testing was also performed to track the evolution of bulk mechanical properties under sterile conditions. Over the month long time course, seeded cells and deposited collagen were randomly distributed in equibiaxially anchored constructs, but exhibited preferential alignment parallel to the direction of the 10% stretch in constructs cultured under strip biaxial stretch. Surprisingly, both anchored and strip biaxial stretched constructs exhibited isotropic mechanical properties (including progressively increasing stiffness) despite developing a very different collagen microstructural organization. In summary, our biaxial bioreactor system integrating both NLOM-OCM and mechanical testing provided complementary information on microstructural organization and mechanical properties and, thus, may enable greater fundamental understanding of relationships between engineered soft tissue mechanics and mechanobiology.

Biomimetic fibrin–hyaluronan hydrogels for nucleus pulposus regeneration

Aim: To develop a biomimetic polymeric injectable hydrogel that can support nucleus pulposus (NP) regeneration. Materials & methods: Natural polymer-based hydrogels were synthesized using fibrinogen (FBG) and hyaluronic acid (HA), conjugated by a novel two-step procedure. Bovine NP cells were cultured in FBG–HA conjugate-based 3D beads in vitro and in a nucleotomized organ culture model. Results: FBG–HA conjugate-based hydrogels prepared with 235 KDa HA at a FBG/HA w/w ratio of 17:1 showed superior gel stability and mechanical properties and markedly increased glycosaminoglycan synthesis compared with a FBG/HA mixture-based hydrogels or fibrin gels. Gene-expression levels of NP markers were maintained in vitro. In organ culture, NP cells seeded in FBG–HA conjugate-based hydrogels showed better integration with native NP tissue compared with fibrin gels. Moreover, FBG–HA conjugate-based hydrogels restored compressive stiffness and disc height after nucleotomy under dynamic load. Conclusion: Specific FBG–HA conjugate-based hydrogels may be suitable as injectable materials for minimally invasive, biological NP regeneration.

Human mesenchymal stem cell spheroids in fibrin hydrogels exhibit improved cell survival and potential for bone healing

Mesenchymal stem cells (MSCs) have great therapeutic potential for the repair of nonhealing bone defects, because of their proliferative capacity, multilineage potential, trophic factor secretion and lack of immunogenicity. However, a major challenge to the translation of cell-based therapies into clinical practice is ensuring their survival and function upon implantation into the defect site. We hypothesize that forming MSCs into more physiologic three-dimensional spheroids, rather than employing dissociated cells from two-dimensional monolayer culture, will enhance their survival when exposed to a harsh microenvironment but maintain their osteogenic potential. MSC spheroids were formed by using the hanging drop method with increasing cell numbers. Compared with larger spheroids, the smallest spheroids, which contained 15,000 cells, exhibited increased metabolic activity, reduced apoptosis and the most uniform distribution of proliferating cells. Spheroids were then entrapped in fibrin gels and cultured in serum-free medium and 1 % oxygen. Compared with identical numbers of dissociated MSCs in fibrin gels, spheroids exhibited significantly reduced apoptosis and secreted up to 100-fold more vascular endothelial growth factor. Moreover, fibrin gels containing spheroids and those containing an equivalent number of dissociated cells exhibited similar expression levels of early and late markers of osteogenic differentiation. Thus, MSC spheroids exhibit greater resistance to apoptosis and enhanced proangiogenic potential while maintaining similar osteogenic potential to dissociated MSCs entrapped in a clinically relevant biomaterial, supporting the use of MSC spheroids in cell-based approaches to bone repair.

Fibrin-based biomaterials: modulation of macroscopic properties through rational design at the molecular level

Fibrinogen is one of the primary components of the coagulation cascade and rapidly forms an insoluble matrix following tissue injury. In addition to its important role in hemostasis, fibrin acts as a scaffold for tissue repair and provides important cues for directing cell phenotype following injury. Because of these properties and the ease of polymerization of the material, fibrin has been widely utilized as a biomaterial for over a century. Modifying the macroscopic properties of fibrin, such as elasticity and porosity, has been somewhat elusive until recently, yet with a molecular-level rational design approach it can now be somewhat easily modified through alterations of molecular interactions key to the protein's polymerization process. This review outlines the biochemistry of fibrin and discusses methods for modification of molecular interactions and their application to fibrin based biomaterials.

Strategies for directing the structure and function of three-dimensional collagen biomaterials across length scales

Collagen type I is a widely used natural biomaterial that has found utility in a variety of biological and medical applications. Its well-characterized structure and role as an extracellular matrix protein make it a highly relevant material for controlling cell function and mimicking tissue properties. Collagen type I is abundant in a number of tissues, and can be isolated as a purified protein. This review focuses on hydrogel biomaterials made by reconstituting collagen type I from a solubilized form, with an emphasis on in vitro studies in which collagen structure can be controlled. The hierarchical structure of collagen from the nanoscale to the macroscale is described, with an emphasis on how structure is related to function across scales. Methods of reconstituting collagen into hydrogel materials are presented, including molding of macroscopic constructs, creation of microscale modules and electrospinning of nanoscale fibers. The modification of collagen biomaterials to achieve the desired structures and functions is also addressed, with particular emphasis on mechanical control of collagen structure, creation of collagen composite materials and crosslinking of collagenous matrices. Biomaterials scientists have made remarkable progress in rationally designing collagen-based biomaterials and in applying them both to the study of biology and for therapeutic benefit. This broad review illustrates recent examples of techniques used to control collagen structure and thereby to direct its biological and mechanical functions.

Microfluidic generation of gradient hydrogels to modulate hematopoietic stem cell culture environment

The bone marrow provides spatially and temporally variable signals that impact the behavior of hematopoietic stem cells (HSCs). While multiple biomolecular signals and bone marrow cell populations have been proposed as key regulators of HSC fate, new tools are required to probe their importance and mechanisms of action. Here, a novel method based on a microfluidic mixing platform to create small volume, 3D hydrogel constructs containing overlapping patterns of cell and matrix constituents inspired by the HSC niche is described. This approach is used to generate hydrogels containing opposing gradients of fluorescent microspheres, MC3T3-E1 osteoblasts, primary murine hematopoietic stem and progenitor cells (HSPCs), and combinations thereof in a manner independent of hydrogel density and cell/particle size. Three different analytical methods are described to characterize local properties of these hydrogels at multiple scales: 1) whole construct fluorescent analysis; 2) multi-photon imaging of individual cells within the construct; 3) retrieval of discrete sub-regions from the hydrogel post-culture. The approach reported here allows the creation of stable gradients of cell and material cues within a single, optically translucent 3D biomaterial to enable a range of investigations regarding how microenvironmental signals impact cell fate.

Biofunctional hydrogels for skeletal muscle constructs

Hydrogel scaffolds encapsulating C2C12 mouse skeletal muscle cells have been engineered as in vitro constructs towards regenerative medicine therapies for the enhancement and inducement of functional skeletal muscle formation. Previous work has largely involved two-dimensional (2D) muscle strips, naturally occurring hydrogels and incomplete examination of the effects of the scaffold and/or biological functionalization on myogenic differentiation in a controllable manner. The goal of this study was to identify key properties in functionalized poly(ethylene glycol) (PEG)-maleimide (MAL) synthetic hydrogels that promote cell attachment, proliferation and differentiation for the formation of multinucleated myotubes and functional skeletal muscle tissue constructs. Significant differences in myoblast viability were observed as a function of cell seeding density, polymer weight percentage and bioadhesive ligands. The identified optimized conditions for cell survival, required for myotube development, were carried over for differentiation assays. PEG hydrogels (5% weight/volume), functionalized with 2.0 mm RGD adhesive peptide and crosslinked with protease-cleavable peptides, incubated for 3 days before supplementation with 2% horse serum, significantly increased expression of differentiated skeletal muscle markers by 50%; 17% more multinucleated cells and a 40% increase in the number of nuclei/differentiated cell compared to other conditions. Functionality of cell-laden hydrogels was demonstrated by a 20% decrease in the extruded length of the hydrogel when stimulated with a contractile agent, compared to 7% for a saline control. This study provided strategies to engineer a three-dimensional (3D) microenvironment, using synthetic hydrogels to promote the development of differentiated muscle tissue from skeletal muscle progenitor cells to form contractile units. Copyright © 2014 John Wiley & Sons, Ltd.

25th anniversary article: Designer hydrogels for cell cultures: a materials selection guide

Cell culturing, whether for tissue engineering or cell biology studies, always involves placing cells in a non-natural environment and no material currently exist that can mimic the entire complexity of natural tissues and variety of cell-matrix interactions that is found in vivo. Here, we review the vast range of hydrogels, composed of natural or synthetic polymers that provide a route to tailored microenvironments.

Polymicrobial wound infections: pathophysiology and current therapeutic approaches

Acute and chronic wounds represent a very common health problem in the entire world. The dermal wounds are colonized by aerobic and anaerobic bacterial and fungal strains, most of them belonging to the resident microbiota of the surrounding skin, oral cavity and gut, or from the external environment, forming polymicrobial communities called biofilms, which are prevalent especially in chronic wounds. A better understanding of the precise mechanisms by which microbial biofilms delay repair processes together with optimizing methods for biofilm detection and prevention may enhance opportunities for chronic wounds healing. The purpose of this minireview is to assess the role of polymicrobial biofilms in the occurrence and evolution of wound infections, as well as the current and future preventive and therapeutic strategies used for the management of polymicrobial wound infections.

Control of perfusable microvascular network morphology using a multiculture microfluidic system

The mechanical and biochemical microenvironment influences the morphological characteristics of microvascular networks (MVNs) formed by endothelial cells (ECs) undergoing the process of vasculogenesis. The objective of this study was to quantify the role of individual factors in determining key network parameters in an effort to construct a set of design principles for engineering vascular networks with prescribed morphologies. To achieve this goal, we developed a multiculture microfluidic platform enabling precise control over paracrine signaling, cell-seeding densities, and hydrogel mechanical properties. Human umbilical vein endothelial cells (HUVECs) were seeded in fibrin gels and cultured alongside human lung fibroblasts (HLFs). The engineered vessels formed in our device contained patent, perfusable lumens. Communication between the two cell types was found to be critical in avoiding network regression and maintaining stable morphology beyond 4 days. The number of branches, average branch length, percent vascularized area, and average vessel diameter were found to depend uniquely on several input parameters. Importantly, multiple inputs were found to control any given output network parameter. For example, the vessel diameter can be decreased either by applying angiogenic growth factors--vascular endothelial growth factor (VEGF) and sphingosine-1-phsophate (S1P)--or by increasing the fibrinogen concentration in the hydrogel. These findings introduce control into the design of MVNs with specified morphological properties for tissue-specific engineering applications.

First step in developing a 3D biodegradable fibrin scaffold for an artificial ovary

BACKGROUND

Although transplantation of cryopreserved ovarian tissue is a promising approach to restore fertility in cancer patients, it is not advisable for women at risk of ovarian involvement due to the threat of reintroducing malignant cells. The aim of this study was therefore to find an alternative for these patients by development of an artificial ovary.

METHODS

For construction of the artificial ovary matrix, we used a central composite design to investigate nine combinations of fibrinogen (mg/ml) and thrombin (IU/mL) (F/T): F1/T4, F12.5/T1, F12.5/T20, F25/T0.1, F25/T4, F25/T500, F50/T1, F50/T20 and F100/T4. From the first qualitative analyses (handling and matrix size), five combinations (F12.5/T1, F25/T4, F50/T20, F50/T1 and F100/T4) yielded positive results. They were further evaluated in order to assess fibrin matrix degradation and homogeneous cell encapsulation (density), survival and proliferation (Ki67), and atresia (TUNEL) before and after 7 days of in vitro culture. To determine the best compromise between maximizing the dynamic density (Y1) and minimizing the apoptosis rate (Y2), we used the desirability function approach.

RESULTS

Two combinations (F12.5/T1 and F25/T4) showed greater distribution of cells before in vitro culture, reproducible degradation of the fibrin network and adequate support for isolated human ovarian stromal cells, with a high proportion of Ki67-positive cells. SEM analysis revealed a network of fibers with regular pores and healthy stromal cells after in vitro culture with both F/T combinations.

CONCLUSION

This study reports two optimal F/T combinations that allow survival and proliferation of isolated human ovarian cells. Further studies are required to determine if such a scaffold will also be a suitable environment for isolated ovarian follicles.

Macroscale delivery systems for molecular and cellular payloads

Macroscale drug delivery (MDD) devices are engineered to exert spatiotemporal control over the presentation of a wide range of bioactive agents, including small molecules, proteins and cells. In contrast to systemically delivered drugs, MDD systems act as a depot of drug localized to the treatment site, which can increase drug effectiveness while reducing side effects and confer protection to labile drugs. In this Review, we highlight the key advantages of MDD systems, describe their mechanisms of spatiotemporal control and provide guidelines for the selection of carrier materials. We also discuss the combination of MDD technologies with classic medical devices to create multifunctional MDD devices that improve integration with host tissue, and the use of MDD technology in tissue-engineering strategies to direct cell behaviour. As our ever-expanding knowledge of human biology and disease provides new therapeutic targets that require precise control over their application, the importance of MDD devices in medicine is expected to increase.

Fibrinogen and fibrin based micro and nano scaffolds incorporated with drugs, proteins, cells and genes for therapeutic biomedical applications

Over the past two decades, many types of natural and synthetic polymer-based micro- and nanocarriers, with exciting properties and applications, have been developed for application in various types of tissue regeneration, including bone, cartilage, nerve, blood vessels, and skin. The development of suitable polymers scaffold designs to aid the repair of specific cell types have created diverse and important potentials in tissue restoration. Fibrinogen (Fbg)- and fibrin (Fbn)-based micro- and nanostructures can provide suitable natural matrix environments. Since these primary materials are abundantly available in blood as the main coagulation proteins, they can easily interact with damaged tissues and cells through native biochemical interactions. Fbg- and Fbn-based micro and nanostructures can also be consecutively furnished/or encapsulated and specifically delivered, with multiple growth factors, proteins, and stem cells, in structures designed to aid in specific phases of the tissue regeneration process. The present review has been carried out to demonstrate the progress made with micro and nanoscaffold applications and features a number of applications of Fbg- and Fbn-based carriers in the field of biomaterials, including the delivery of drugs, active biomolecules, cells, and genes, that have been effectively used in tissue engineering and regenerative medicine.

S-nitroso-N-acetylpenicillamine (SNAP) derivatization of peptide primary amines to create inducible nitric oxide donor biomaterials

An S-nitroso-N-acetylpenicillamine (SNAP) derivatization approach was used to modify existing free primary amines found in fibrin (a natural protein-based biomaterial) to generate a controlled nitric oxide (NO) releasing scaffold material. The duration of the derivatization reaction affects the NO release kinetics, the induction of controlled NO-release, hydrophobicity, swelling behavior, elastic moduli, rheometric character, and degradation behavior. These properties were quantified to determine changes in fibrin hydrogels following covalent attachment of SNAP. NO-releasing materials exhibited minimal cytotoxicity when cultured with fibroblasts or osteoblasts. Cells maintained viability and proliferative character on derivatized materials as demonstrated by Live/Dead cell staining and counting. In addition, SNAP-derivatized hydrogels exhibited an antimicrobial character indicative of NO-releasing materials. SNAP derivatization of natural polymeric biomaterials containing free primary amines offers a means to generate inducible NO-releasing biomaterials for use as an antimicrobial and regenerative support for tissue engineering.

Fibrin improves beta (INS-1) cell function, proliferation and survival through integrin αvβ3

Extracellular matrix (ECM)-integrin stimulation can promote beta cell differentiation, proliferation and function. However, beta cells lose their insulin secretion function in response to glucose stimulation, and senesce when cultured with ECM proteins for a long time. Fibrin is a provisional ECM protein that is capable of maintaining beta cell function, yet the mechanisms by which this occurs is unknown. The present study examined how fibrin interacts with integrin receptors to promote beta cell cluster formation, proliferation and function. The rat insulinoma cell line, INS-1, was cultured on tissue-culture polystyrene, or with 2-D or 3-D fibrin gels for up to 4 weeks. Cells cultured with fibrin formed islet-like clusters and showed direct contacts with fibrin determined by scanning electron microscopy. Fibrin-cultured INS-1 cells also had significantly increased glucose-stimulated insulin secretion. A significant increase in integrin αvβ3 protein and phosphorylated FAK, Erk1/2 and Akt levels was observed in fibrin-cultured INS-1 cells, which was associated with significantly increased cell proliferation and decreased cell apoptosis. Integrin αvβ3 blockade affected INS-1 cell spreading on fibrin gels, and resulted in significantly decreased FAK phosphorylation and increased cleaved caspase-3 levels. These results show that fibrin promotes beta cell function, proliferation and survival via integrin αvβ3 interactions.

Modeling of fibrin gels based on confocal microscopy and light-scattering data

Fibrin gels are biological networks that play a fundamental role in blood coagulation and other patho/physiological processes, such as thrombosis and cancer. Electron and confocal microscopies show a collection of fibers that are relatively monodisperse in diameter, not uniformly distributed, and connected at nodal points with a branching order of ∼3-4. Although in the confocal images the hydrated fibers appear to be quite straight (mass fractal dimension D(m) = 1), for the overall system 1<D(m)<2. Based on the confocal images, we developed a method to generate three-dimensional (3D) in silico gels made of cylindrical sticks of diameter d, density ρ, and average length <L>, joined at randomly distributed nodal points. The resulting 3D network strikingly resembles real fibrin gels and can be sketched as an assembly of densely packed fractal blobs, i.e., regions of size ξ, where the fiber concentration is higher than average. The blobs are placed at a distance ξ0 between their centers of mass so that they are overlapped by a factor η =ξ/ξ0 and have D(m) ∼1.2-1.6. The in silico gels' structure is quantitatively analyzed by its 3D spatial correlation function g(3D)(r) and corresponding power spectrum I(q) = FFT(3D[g3D(r)]), from which ρ, d, D(m), η, and ξ0 can be extracted. In particular, ξ0 provides an excellent estimate of the gel mesh size. The in silico gels' I(q) compares quite well with real gels' elastic light-scattering measurements. We then derived an analytical form factor for accurately fitting the scattering data, which allowed us to directly recover the gels' structural parameters.

Enhancing osteoconductivity of fibrin gels with apatite-coated polymer microspheres

Fibrin gels are a promising material for use in promoting bone repair and regeneration due to their ease of implant formation, tailorability, biocompatibility, and degradation by natural processes. However, these materials lack necessary osteoconductivity to nucleate calcium, integrate with surrounding bone, and promote bone formation. Polymeric substrata formed from poly(lactide-co-glycolide) (PLG) are widely used in bone tissue engineering. A carbonated apatite layer of bone-like mineral can be successfully grown on the surface of PLG microspheres after a multiday incubation process in modified simulated body fluid. Such coatings improve the osteoconductivity of the polymer, provide nucleation sites for cell-secreted calcium, and enhance the potential osseointegration with host tissue. We examined the capacity of mineralized polymeric microspheres suspended within fibrin hydrogels to enhance the osteoconductivity of fibrin gels and increase the osteogenic potential of these materials. The inclusion of microparticles, both nonmineralized and mineralized, reduced the capacity of mesenchymal stem cells (MSCs) to contract the gel. When cultured in osteogenic media, we detected a near linear increase in both calcium and phosphate incorporation in gels containing mineralized microspheres and entrapped MSCs. The osteoconductivity of acellular fibrin gels with mineralized and nonmineralized microspheres was assessed in a rodent calvarial bone defect over 12 weeks. Compared to untreated rodent calvarial bone defects, we detected significant increases in early vascularization when treated with fibrin gels, with greater vascularization, on average, occurring with gels containing microspheres. We detected a trend for increased bone mineral density in gels containing mineralized microspheres after 12 weeks. These findings demonstrate that the osteoconductivity of fibrin gels can be increased by inclusion of mineralized microspheres, but additional signals may be required to rapidly accelerate bone repair.

Regulating tension in three-dimensional culture environments

The processes of development, repair, and remodeling of virtually all tissues and organs, are dependent upon mechanical signals including external loading, cell-generated tension, and tissue stiffness. Over the past few decades, much has been learned about mechanotransduction pathways in specialized two-dimensional culture systems; however, it has also become clear that cells behave very differently in two- and three-dimensional (3D) environments. Three-dimensional in vitro models bring the ability to simulate the in vivo matrix environment and the complexity of cell-matrix interactions together. In this review, we describe the role of tension in regulating cell behavior in three-dimensional collagen and fibrin matrices with a focus on the effective use of global boundary conditions to modulate the tension generated by populations of cells acting in concert. The ability to control and measure the tension in these 3D culture systems has the potential to increase our understanding of mechanobiology and facilitate development of new ways to treat diseased tissues and to direct cell fate in regenerative medicine and tissue engineering applications.

Fibrin clot structure and mechanics associated with specific oxidation of methionine residues in fibrinogen

Using a combination of structural and mechanical characterization, we examine the effect of fibrinogen oxidation on the formation of fibrin clots. We find that treatment with hypochlorous acid preferentially oxidizes specific methionine residues on the α, β, and γ chains of fibrinogen. Oxidation is associated with the formation of a dense network of thin fibers after activation by thrombin. Additionally, both the linear and nonlinear mechanical properties of oxidized fibrin gels are found to be altered with oxidation. Finally, the structural modifications induced by oxidation are associated with delayed fibrin lysis via plasminogen and tissue plasminogen activator. Based on these results, we speculate that methionine oxidation of specific residues may be related to hindered lateral aggregation of protofibrils in fibrin gels.

Transdermal regulation of vascular network bioengineering using a photopolymerizable methacrylated gelatin hydrogel

The search for hydrogel materials compatible with vascular morphogenesis is an active area of investigation in tissue engineering. One candidate material is methacrylated gelatin (GelMA), a UV-photocrosslinkable hydrogel that is synthesized by adding methacrylate groups to the amine-containing side-groups of gelatin. GelMA hydrogels containing human endothelial colony-forming cells (ECFCs) and mesenchymal stem cells (MSCs) can be photopolymerized ex vivo and then surgically transplanted in vivo as a means to generate vascular networks. However, the full clinical potential of GelMA will be best captured by enabling minimally invasive implantation and in situ polymerization. In this study, we demonstrated the feasibility of bioengineering human vascular networks inside GelMA constructs that were first subcutaneously injected into immunodeficient mice while in liquid form, and then rapidly crosslinked via transdermal exposure to UV light. These bioengineered vascular networks developed within 7 days, formed functional anastomoses with the host vasculature, and were uniformly distributed throughout the constructs. Most notably, we demonstrated that the vascularization process can be directly modulated by adjusting the initial exposure time to UV light (15-45 s range), with constructs displaying progressively less vascular density and smaller average lumen size as the degree of GelMA crosslinking was increased. Our studies support the use of GelMA in its injectable form, followed by in situ transdermal photopolymerization, as a preferable means to deliver cells in applications that require the formation of vascular networks in vivo.

Biologically compatible neural interface to safely couple nanocoated electrodes to the surface of the brain

The ongoing interest in densely packed miniaturized electrode arrays for high-resolution epicortical recordings has induced many researchers to explore the use of nanomaterial coatings to reduce electrode impedance while increasing signal-to-noise ratio and charge injection capability. Although these materials are very effective, their use in clinical practice is strongly inhibited by concerns about the potential risks derived from the use of nanomaterials in direct contact with the human brain. In this work we propose a novel approach to safely couple nanocoated electrodes to the brain surface by encapsulating them with a biocompatible hydrogel. We prove that fibrin hydrogel coating over nanocoated high-density arrays of epicortical microelectrodes is electrically transparent and allows avoiding direct exposure of the brain tissue to the nanocoatings while maintaining all the advantages derived from the nanostructured electrode surface. This method may make available acute and sub-acute neural recordings with nanocoated high-resolution arrays for clinical applications.

Enhanced wound vascularization using a dsASCs seeded FPEG scaffold

The bioengineering of autologous vascular networks is of great importance in wound healing. Adipose-derived stem cells (ASCs) are of interest due to their ability to differentiate toward various cell types, including vascular. We hypothesized that adult human ASCs embedded in a three-dimensional PEG-fibrin (FPEG) gel have the ability to modulate vascularization of a healing wound. Initial in vitro characterization of ASCs isolated from discarded burn skin samples (dsASCs) and embedded in FPEG gels indicated they could express such pericyte/smooth muscle cell markers as α-smooth muscle actin, platelet-derived growth factor receptor-β, NG2 proteoglycan, and angiopoietin-1, suggesting that these cells could potentially be involved in a supportive cell role (i.e., pericyte/mural cell) for blood vessels. Using a rat skin excision model, wounds treated with dsASCs-FPEG gels showed earlier collagen deposition and wound remodeling compared to vehicle FPEG treated wounds. Furthermore, the dsASCs-seeded gels increased the number of vessels in the wound per square millimeter by day 16 (~66.7 vs. ~36.9/mm(2)) in these same studies. dsASCs may support this increase in vascularization through their trophic contribution of vascular endothelial growth factor, as determined by in vitro analysis of mRNA and the protein levels. Immunohistochemistry showed that dsASCs were localized to the surrounding regions of large blood-perfused vessels. Human dsASCs may play a supportive role in the formation of vascular structures in the healing wound through direct mechanisms as well as indirect trophic effects. The merging of autologous grafts or bioengineered composites with the host's vasculature is critical, and the use of autologous dsASCs in these procedures may prove to be therapeutic.

Mechanical stimulation by ultrasound enhances chondrogenic differentiation of mesenchymal stem cells in a fibrin-hyaluronic acid hydrogel

Chondrogenic differentiation and cartilage tissue formation derived from stem cells are highly dependent on both biological and mechanical factors. This study investigated whether or not fibrin-hyaluronic acid (HA) coupled with low-intensity ultrasound (LIUS), a mechanical stimulation, produces an additive or synergistic effect on the chondrogenesis of rabbit mesenchymal stem cells (MSCs) derived from bone marrow. For the purpose of comparison, rabbit MSCs were first cultured in fibrin-HA or alginate hydrogels, and then subjected to chondrogenic differentiation in chondrogenic-defined medium for 4 weeks in the presence of either transforming growth factor-beta3 (TGF-β3) (10 ng/mL) or LIUS treatment (1.0 MHz and 200 mW/cm(2) ). The resulting samples were evaluated at 1 and 4 weeks by histological observation, chemical assays, and mechanical analysis. The fibrin-HA hydrogel was found to be more efficient than alginate in promoting chondrogenesis of the MSCs by producing a larger amount of sulfated glycosaminoglycans (GAGs) and collagen, and engineered constructs made with the hydrogel demonstrated higher mechanical strength. At 4 weeks of tissue culture, the chondrogenesis of the MSCs in fibrin-HA were shown to be further enhanced by treatment with LIUS, as observed by analyses for the amounts of GAGs and collagen, and mechanical strength testing. In contrast, TGF-β3, a well-known chondrogenic inducer, showed a marginal additive effect in the amount of collagen only. These results revealed that LIUS further enhanced chondrogenesis of the MSCs cultured in fibrin-HA, in vitro, and suggested that the combination of fibrin-HA and LIUS is a useful tool in constructing high-quality cartilage tissues from MSCs.

Formation and stability of interpenetrating polymer network hydrogels consisting of fibrin and hyaluronic acid for tissue engineering

Fibrin gel is widely used as a tissue engineering scaffold. However, it has poor mechanical properties, which often result in rapid contraction and degradation of the scaffold. An interpenetrating polymer network (IPN) hydrogel composed of fibrin and hyaluronic acid-tyramine (HA-Tyr) was developed to improve the mechanical properties. The fibrin network was formed by cleaving fibrinogen with thrombin, producing fibrin monomers that rapidly polymerize. The HA network was formed through the coupling of tyramine moieties using horseradish peroxidase (HRP) and hydrogen peroxide (H₂O₂). The degree of crosslinking of the HA-Tyr network can be tuned by varying the H₂O₂ concentration, producing IPN hydrogels with different storage moduli (G'). While fibrin gels were completely degraded in the presence of plasmin and contracted when embedded with cells, the shape of the IPN hydrogels was maintained due to structural support by the HA-Tyr networks. Cell proliferation and capillary formation occurred in IPN hydrogels and were found to decrease with increasing G' of the hydrogels. The results suggest that fibrin-HA-Tyr IPN hydrogels are a potential alternative to fibrin gels as scaffolds for tissue engineering applications that require shape stability.