A fibrin scaffold is a network of protein that holds together and supports a variety of living tissues. It is produced naturally by the body after injury, but also can be engineered as a tissue substitute to speed healing. The scaffold consists of naturally occurring biomaterials composed of a cross-linked fibrin network and has a broad use in biomedical applications.
Fibrin consists of the blood proteins fibrinogen and thrombin which participate in blood clotting. Fibrin glue or fibrin sealant is also referred to as a fibrin based scaffold and used to control surgical bleeding, speed wound healing, seal off hollow body organs or cover holes made by standard sutures, and provide slow-release delivery of medications like antibiotics to tissues exposed. [1] [2]
Fibrin scaffold use is helpful in repairing injuries to the urinary tract, [3] liver [4] lung, [5] spleen, [6] kidney, [7] and heart. [8] In biomedical research, fibrin scaffolds have been used to fill bone cavities, repair neurons, heart valves, [9] vascular grafts [10] and the surface of the eye.
The complexity of biological systems requires customized care to sustain their function. When they are no longer able to perform their purpose, interference of new cells and biological cues is provided by a scaffold material. Fibrin scaffold has many aspects like being biocompatible, biodegradable and easily processable. Furthermore, it has an autologous nature and it can be manipulated in various size and shape. Inherent role in wound healing is helpful in surgical applications. Many factors can be bound to fibrin scaffold and those can be released in a cell-controlled manner. Its stiffness can be managed by changing the concentration according to needs of surrounding or encapsulated cells. Additional mechanical properties can be obtained by combining fibrin with other suitable scaffolds. Each biomedical application has its own characteristic requirement for different kinds of tissues and recent studies with fibrin scaffold are promising towards faster recovery, less complications and long-lasting solutions.
Fibrin scaffold is an important element in tissue engineering approaches as a scaffold material. It is advantageous opposed to synthetic polymers and collagen gels when cost, inflammation, immune response, toxicity and cell adhesion are concerned. [11] When there is a trauma in a body, cells at site start the cascade of blood clotting and fibrin is the first scaffold formed normally. [12] To achieve in clinical use of a scaffold, fast and entire incorporation into host tissue is essential. [13] Regeneration of the tissue and the degradation of the scaffold should be balanced in terms of rate, surface area and interaction so that ideal templating can be achieved. [14] Fibrin satisfies many requirements of scaffold functions. Biomaterials made up of fibrin can attach many biological surfaces with high adhesion. Its biocompatibility comes from being not toxic, allergenic or inflammatory. [14] [15] [16] By the help of fibrinolysis inhibitors [17] or fiber cross-linkers, biodegradation can be managed. [16] [18] Fibrin can be provided from individuals to be treated many times so that gels from autologous fibrin have no undesired immunogenic reactions in addition to be reproducible. [14] [19] [20] Inherently, structure and biochemistry of fibrin has an important role in wound healing. [21] Although there are limitations due to diffusion, exceptional cellular growth and tissue development can be achieved. [14] [22] According to the application, fibrin scaffold characteristics can be adjustable by manipulating concentrations of components. Long-lasting durable fibrin hydrogels are enviable in many applications. [21] [23] [24]
Polymerization time of fibrinogen and thrombin is affected primarily by concentration of thrombin and temperature, while fibrinogen concentration has a minor effect. Fibrin gel characterization by scanning electron microscopy reveals that thick fibers make up a dense structure at lower fibrinogen concentrations (5 mg/ml) and thinner fibers and looser gel can be obtained as fibrinogen concentration (20 mg/ml) increases whereas increase in thrombin concentration (from 0.5 U/ml to 5 U/ml) has no such significant result although the fibers steadily get thinner. [25]
Fibrin gels can be enriched by addition of other extracellular matrix (ECM) components such as fibronectin, vitronectin, laminin and collagen. These can be linked covalently to fibrin scaffold by reactions catalyzed by transglutaminase. [26] Laminin originated substrate amino acid sequences for transglutaminase can be IKVAV, YIGSR or RNIAEIIKDI. Collagen originated sequence is DGEA and many other ECM protein originated RGD sequence can be given as other examples. [26] [27] Heparin binding sequences KβAFAKLAARLYRKA, RβAFARLAARLYRRA, KHKGRDVILKKDVR, YKKIIKKL are from antithrombin III, modified antithrombin III, neural cell adhesion molecule and platelet factor 4, respectively. Heparin-binding growth factors can be attached to heparin binding domains via heparin. As a result, a reservoir can be provided instead of passive diffusion by liberation of growth factors in extended time. [28] [29] Acidic and basic fibroblast growth factor, neurotrophin 3, transforming growth factor beta 1, transforming growth factor beta 2, nerve growth factor, brain derived neurotrophic factor can be given as examples for such growth factors. [18] [28] [29] [30] [31] [32]
For some tissues like cartilage, highly dense polymeric scaffolds such as polyethylene glycol (PEG) are essential due to mechanical stress and that can be achieved by combining them with natural biodegradable cell-adhesive scaffolds since cells can not attach to synthetic polymers and take proper signals for normal cell function. Various scaffold combinations with PEG-based hydrogels are studied to assess the chondrogenic response to dynamic strain stimulation in a recent study. PEG-Proteoglycan, PEG-Fibrinogen, PEG-Albumin conjugates and only PEG including hydrogels are used to evaluate the mechanical effect on bovine chondrocytes by using a pneumatic reactor system. The most substantial increase in stiffness is observed in PEG-Fibrinogen conjugated hydrogel after 28 days of mechanical stimulation. [33]
In orthopedics, methods with minimum invasion are desired and improving injectable systems is a leading aim. Bone cavities can be filled by polymerizing materials when injected and adaptation to the shape of the cavity can be provided. Shorter surgical operation time, minimum large muscle retraction harm, smaller scar size, less pain after operation and consequently faster recovery can be obtained by using such systems. [15] In a study to evaluate if injectable fibrin scaffold is helpful for transplantation of bone marrow stromal cell (BMSC) when central nervous system (CNS) tissue is damaged, Yasuda et al. found that BMSC has extended survival, migration and differentiation after transplantation to rat cortical lesion although there is complete degradation of fibrin matrix after four weeks. [34] Another study to assess if fibrin glue enriched with platelet is better than just platelet rich plasma (PRP) on bone formation was conducted. Each combined with bone marrow mesenchymal stem cells and bone morphogenetic protein 2 (BMP-2) are injected into the subcutaneous space. Results shows that fibrin glue enriched with platelet has better osteogenic properties when compared to PRP. [35] To initiate and speed up tissue repair and regeneration, platelet-rich fibrin gels are ideal since they have a high concentration of platelet releasing growth factors and bioactive proteins. [36] Addition of fibrin glue to calcium phosphate granules has promising results leading to faster bone repair by inducing mineralization and possible effects of fibrin on angiogenesis, cell attachment and proliferation. [37]
Valvular heart disease is a major cause of death globally. Both mechanical valves and fixed biological xenograft or homografts used clinically have many drawbacks. [38] One study focused on fibrin-based heart valves to assess structure and mechanical durability on sheep revealed promising potential for patient originated valve replacements. From autologous arterial-derived cells and fibrin scaffold, tissue engineered heart valves are formed, then mechanically conditioned and transplanted into the pulmonary trunk of the same animals. The preliminary result are potentially hopeful towards autologous heart valve production. [39]
In atherosclerosis, a severe disease in modern society, coronary blood vessels occlude. These vessels have to be freed and held open i.e. by stents. Unfortunately after certain time these vessels close again and have to be bypassed to allow for upkeep of circulation. Usually autologous vessels from the patient or synthetic polymer grafts are used for this purpose. Both options have disadvantages. Firstly there are only few autologous vessels available in a human body that might be of low quality, considering the health status of the patient. The synthetic polymer based grafts on the other hand often have insufficient haemocompatibility and thus rapidly occlude - a problem that is especially prone in small calibre grafts. In this context the fibrin-gel-based tissue engineering of autologous vessel substitutes is a very promising approach to overcome the current problems. Cells and fibrin are isolated by a low invasive procedure from the patient and shaped in individual moulds to meet the required dimensions. Additional pre-cultivation in a specialized bioreactor [40] is inevitable to ensure appropriate properties of the graft. [41] [42] [43]
Bullous keratopathy that is characterized by corneal stromal edema related to cell loss and endothelial decompensation as well as subepithelial fibrosis and corneal vascularization in further cases, results vision problems due to loss of corneal transparency. [44] Fibrin glue is used as a sutureless method onto the corneal surface to fix amniotic membrane that is cryopreserved. Complete re-epithelialization on the ocular surface with no symptom is achieved in 3 weeks. Results show that fibrin glue fixation is easy, reliable and efficient with the corneal surface. [45]
Because fibrin fulfills the mechanical aspects of neuronal growth without initiation of glial proliferation, it can be potentially used in neuronal wound healing even with no need of growth factors or such constituents. [12] Neurons and astrocytes, two major cell type of central nervous system, can show various responses to differences in matrix stiffness. [46] Neuronal development of precursor cells is maintained by gels with low elastic modulus. [47] When stiffness of the matrix is more than that of a normal brain, extension of spinal cord and cortical brain neurons is inhibited since neurite extension and branch forming take place on soft materials (<1000Pa). In a study, fibrins from different species are used to compare the effects on neurite growth of mouse spinal cord neurons. Among salmon, bovine and human fibrin in addition to Matrigel, salmon fibrin promotes the neurite growth best and it is more proteolysis resistant than mammalian fibrins. Because down to 0 °C, salmon fibrinogen can clot whereas polymerization of human fibrinogen occurs slowly below 37 °C, this can be taken as an advantage in surgical settings that are cooler. Therefore, for treatment of central nervous system damages, salmon fibrin can be a useful biomaterial. [12] [48]
For sciatic nerve regeneration, fibrin scaffold is used with glial derived neurotrophic factor (GDNF) in a recent study. Survival of both sensory and motor neurons is promoted by glial-derived neurotrophic factor and its delivery to peripheral nervous system improves regeneration after an injury. GDNF and nerve growth factor (NGF) is sequestered in the gel via a bi-domain peptide. This peptide is composed of heparin binding domain and transglutaminase substrate domain which can be cross-linked into the fibrin matrix by polymerization via transglutaminase activity of factor XIIIa. Many neurotrophic factors can bind to heparin through its sulfated domains. This is the affinity-based delivery system in which growth factors are released by cell-based degradation control. After a 13 mm rat sciatic nerve defect is made, the fibrin matrix delivery system is applied to the gap as a nerve guiding channel. Results show that such a delivery system is efficient to enhance maturity and promote organized architecture of nerve regenerating in presence of GDNF, in addition to expressing the promising treatment variations for peripheral nerve injuries. [49]
The use of fibrin hydrogel in gene delivery (transfection) is studied to address essential factors controlling the delivery process such as fibrinogen and pDNA concentration in addition to significance of cell-mediated fibrin degradation for pursuing the potential of cell-transfection microarray engineering or in vivo gene transfer. Gene transfer is more successful in-gel than on-gel probably because of proximity of lipoplexes and target cells. Less cytotoxicity is observed due to less use of transfection agents like lipofectamine and steady degradation of fibrin. Consequently, each cell type requires optimization of fibrinogen and pDNA concentrations for higher transfection yields and studies towards high-throughput transfection microarray experiments are promising. [50]
Tissue engineering is a biomedical engineering discipline that uses a combination of cells, engineering, materials methods, and suitable biochemical and physicochemical factors to restore, maintain, improve, or replace different types of biological tissues. Tissue engineering often involves the use of cells placed on tissue scaffolds in the formation of new viable tissue for a medical purpose, but is not limited to applications involving cells and tissue scaffolds. While it was once categorized as a sub-field of biomaterials, having grown in scope and importance, it can is considered as a field of its own.
A hydrogel is a biphasic material, a mixture of porous, permeable solids and at least 10% by weight or volume of interstitial fluid composed completely or mainly by water. In hydrogels the porous permeable solid is a water insoluble three dimensional network of natural or synthetic polymers and a fluid, having absorbed a large amount of water or biological fluids. These properties underpin several applications, especially in the biomedical area. Many hydrogels are synthetic, but some are derived from nature. The term 'hydrogel' was coined in 1894.
Articular cartilage, most notably that which is found in the knee joint, is generally characterized by very low friction, high wear resistance, and poor regenerative qualities. It is responsible for much of the compressive resistance and load bearing qualities of the knee joint and, without it, walking is painful to impossible. Osteoarthritis is a common condition of cartilage failure that can lead to limited range of motion, bone damage and invariably, pain. Due to a combination of acute stress and chronic fatigue, osteoarthritis directly manifests itself in a wearing away of the articular surface and, in extreme cases, bone can be exposed in the joint. Some additional examples of cartilage failure mechanisms include cellular matrix linkage rupture, chondrocyte protein synthesis inhibition, and chondrocyte apoptosis. There are several different repair options available for cartilage damage or failure.
Cardiomyoplasty is a surgical procedure in which healthy muscle from another part of the body is wrapped around the heart to provide support for the failing heart. Most often the latissimus dorsi muscle is used for this purpose. A special pacemaker is implanted to make the skeletal muscle contract. If cardiomyoplasty is successful and increased cardiac output is achieved, it usually acts as a bridging therapy, giving time for damaged myocardium to be treated in other ways, such as remodeling by cellular therapies.
Neural tissue engineering is a specific sub-field of tissue engineering. Neural tissue engineering is primarily a search for strategies to eliminate inflammation and fibrosis upon implantation of foreign substances. Often foreign substances in the form of grafts and scaffolds are implanted to promote nerve regeneration and to repair damage caused to nerves of both the central nervous system (CNS) and peripheral nervous system (PNS) by an injury.
A nerve guidance conduit is an artificial means of guiding axonal regrowth to facilitate nerve regeneration and is one of several clinical treatments for nerve injuries. When direct suturing of the two stumps of a severed nerve cannot be accomplished without tension, the standard clinical treatment for peripheral nerve injuries is autologous nerve grafting. Due to the limited availability of donor tissue and functional recovery in autologous nerve grafting, neural tissue engineering research has focused on the development of bioartificial nerve guidance conduits as an alternative treatment, especially for large defects. Similar techniques are also being explored for nerve repair in the spinal cord but nerve regeneration in the central nervous system poses a greater challenge because its axons do not regenerate appreciably in their native environment.
Fibrin glue is a surgical formulation used to create a fibrin clot for hemostasis, cartilage repair surgeries or wound healing. It contains separately packaged human fibrinogen and human thrombin.
Nano-scaffolding or nanoscaffolding is a medical process used to regrow tissue and bone, including limbs and organs. The nano-scaffold is a three-dimensional structure composed of polymer fibers very small that are scaled from a Nanometer scale. Developed by the American military, the medical technology uses a microscopic apparatus made of fine polymer fibers called a scaffold. Damaged cells grip to the scaffold and begin to rebuild missing bone and tissue through tiny holes in the scaffold. As tissue grows, the scaffold is absorbed into the body and disappears completely.
Arginylglycylaspartic acid (RGD) is the most common peptide motif responsible for cell adhesion to the extracellular matrix (ECM), found in species ranging from Drosophila to humans. Cell adhesion proteins called integrins recognize and bind to this sequence, which is found within many matrix proteins, including fibronectin, fibrinogen, vitronectin, osteopontin, and several other adhesive extracellular matrix proteins. The discovery of RGD and elucidation of how RGD binds to integrins has led to the development of a number of drugs and diagnostics, while the peptide itself is used ubiquitously in bioengineering. Depending on the application and the integrin targeted, RGD can be chemically modified or replaced by a similar peptide which promotes cell adhesion.
The Network of Excellence for Functional Biomaterials (NFB) is a multidisciplinary research centre which hosts over sixty biologists, chemists, scientists, engineers and clinicians. It is based at the National University of Ireland, Galway, and is directed by Professor Abhay Pandit.
Decellularization is the process used in biomedical engineering to isolate the extracellular matrix (ECM) of a tissue from its inhabiting cells, leaving an ECM scaffold of the original tissue, which can be used in artificial organ and tissue regeneration. Organ and tissue transplantation treat a variety of medical problems, ranging from end organ failure to cosmetic surgery. One of the greatest limitations to organ transplantation derives from organ rejection caused by antibodies of the transplant recipient reacting to donor antigens on cell surfaces within the donor organ. Because of unfavorable immune responses, transplant patients suffer a lifetime taking immunosuppressing medication. Stephen F. Badylak pioneered the process of decellularization at the McGowan Institute for Regenerative Medicine at the University of Pittsburgh. This process creates a natural biomaterial to act as a scaffold for cell growth, differentiation and tissue development. By recellularizing an ECM scaffold with a patient’s own cells, the adverse immune response is eliminated. Nowadays, commercially available ECM scaffolds are available for a wide variety of tissue engineering. Using peracetic acid to decellularize ECM scaffolds have been found to be false and only disinfects the tissue.
Three dimensional (3D) bioprinting is the utilization of 3D printing–like techniques to combine cells, growth factors, bio-inks, and biomaterials to fabricate functional structures that were traditionally used for tissue engineering applications but in recent times have seen increased interest in other applications such as biosensing, and environmental remediation. Generally, 3D bioprinting utilizes a layer-by-layer method to deposit materials known as bio-inks to create tissue-like structures that are later used in various medical and tissue engineering fields. 3D bioprinting covers a broad range of bioprinting techniques and biomaterials. Currently, bioprinting can be used to print tissue and organ models to help research drugs and potential treatments. Nonetheless, translation of bioprinted living cellular constructs into clinical application is met with several issues due to the complexity and cell number necessary to create functional organs. However, innovations span from bioprinting of extracellular matrix to mixing cells with hydrogels deposited layer by layer to produce the desired tissue. In addition, 3D bioprinting has begun to incorporate the printing of scaffolds which can be used to regenerate joints and ligaments. Apart from these, 3D bioprinting has recently been used in environmental remediation applications, including the fabrication of functional biofilms that host functional microorganisms that can facilitate pollutant removal.
Self-healing hydrogels are a specialized type of polymer hydrogel. A hydrogel is a macromolecular polymer gel constructed of a network of crosslinked polymer chains. Hydrogels are synthesized from hydrophilic monomers by either chain or step growth, along with a functional crosslinker to promote network formation. A net-like structure along with void imperfections enhance the hydrogel's ability to absorb large amounts of water via hydrogen bonding. As a result, hydrogels, self-healing alike, develop characteristic firm yet elastic mechanical properties. Self-healing refers to the spontaneous formation of new bonds when old bonds are broken within a material. The structure of the hydrogel along with electrostatic attraction forces drive new bond formation through reconstructive covalent dangling side chain or non-covalent hydrogen bonding. These flesh-like properties have motivated the research and development of self-healing hydrogels in fields such as reconstructive tissue engineering as scaffolding, as well as use in passive and preventive applications.
The in vivo bioreactor is a tissue engineering paradigm that uses bioreactor methodology to grow neotissue in vivo that augments or replaces malfunctioning native tissue. Tissue engineering principles are used to construct a confined, artificial bioreactor space in vivo that hosts a tissue scaffold and key biomolecules necessary for neotissue growth. Said space often requires inoculation with pluripotent or specific stem cells to encourage initial growth, and access to a blood source. A blood source allows for recruitment of stem cells from the body alongside nutrient delivery for continual growth. This delivery of cells and nutrients to the bioreactor eventually results in the formation of a neotissue product.
Platelet-rich fibrin (PRF) or leukocyte- and platelet-rich fibrin (L-PRF) is a derivative of PRP where autologous platelets and leukocytes are present in a complex fibrin matrix to accelerate the healing of soft and hard tissue and is used as a tissue-engineering scaffold in oral and maxillofacial surgeries. PRF falls under FDA Product Code KST, labeling it as a blood draw/Hematology product classifying it as 510(k) exempt.
Muscle tissue engineering is a subset of the general field of tissue engineering, which studies the combined use of cells and scaffolds to design therapeutic tissue implants. Within the clinical setting, muscle tissue engineering involves the culturing of cells from the patient's own body or from a donor, development of muscle tissue with or without the use of scaffolds, then the insertion of functional muscle tissue into the patient's body. Ideally, this implantation results in full regeneration of function and aesthetic within the patient's body. Outside the clinical setting, muscle tissue engineering is involved in drug screening, hybrid mechanical muscle actuators, robotic devices, and the development of engineered meat as a new food source.
Bio-inks are materials used to produce engineered/artificial live tissue using 3D printing. These inks are mostly composed of the cells that are being used, but are often used in tandem with additional materials that envelope the cells. The combination of cells and usually biopolymer gels are defined as a bio-ink. They must meet certain characteristics, including such as rheological, mechanical, biofunctional and biocompatibility properties, among others. Using bio-inks provides a high reproducibility and precise control over the fabricated constructs in an automated manner. These inks are considered as one of the most advanced tools for tissue engineering and regenerative medicine (TERM).
Antonios Georgios Mikos is a Greek-American biomedical engineer who is the Louis Calder Professor of Bioengineering and Chemical and Biomolecular Engineering at Rice University. He specialises in biomaterials, drug delivery, and tissue engineering.
Nasal chondrocytes (NC) are present in the hyaline cartilage of the nasal septum and in fact are the only cell type within the tissue. Similar to chondrocytes present in articular cartilage, NC express extracellular matrix proteins such as glycosaminoglycans and collagen.
Tissue engineered heart valves (TEHV) offer a new and advancing proposed treatment of creating a living heart valve for people who are in need of either a full or partial heart valve replacement. Currently, there are over a quarter of a million prosthetic heart valves implanted annually, and the number of patients requiring replacement surgeries is only suspected to rise and even triple over the next fifty years. While current treatments offered such as mechanical valves or biological valves are not deleterious to one's health, they both have their own limitations in that mechanical valves necessitate the lifelong use of anticoagulants while biological valves are susceptible to structural degradation and reoperation. Thus, in situ (in its original position or place) tissue engineering of heart valves serves as a novel approach that explores the use creating a living heart valve composed of the host's own cells that is capable of growing, adapting, and interacting within the human body's biological system.