Draft:Fibrin Nanoparticles

Fibrin plays a critical role in the clotting cascade by forming a fibrin mesh that induces clotting at the site of an injury to promote wound healing1. Fibrin is a fibrous protein that comes from fibrinogen when it is cleaved at a wound site by the enzyme thrombin (Figure). Commercially available fibrin-based products have focused on the use of high concentrations of fibrinogen and thrombin to make high-density fibrin glues for wound healing. Such high-concentration, high-density materials are not conducive to cell infiltration and healing.2,3 Current research has focused on the development of fibrin nanoparticles at physiologically relevant concentrations to be drug carriers for accelerating wound healing and to serve as vehicles for regenerative medicine applications.

Materials For Making Fibrin Nanoparticles Fibrin is an insoluble protein that is a major component of blood clots1. Fibrin is made when fibrinogen, a glycoprotein that plays an integral role in the wound healing process, gets cleaved by the clotting enzyme thrombin in response to tissue damage1. Fibrin can be isolated from patients for investigation of their wound healing applications. There are various types of fibrin, but most notably there are significant age-related differences between adult and neonatal fibrin4. Neonatal fibrin is associated with significantly advanced wound healing capabilities compared to adult fibrin4. The increased regenerative properties of neonatal fibrin are associated with the higher sialic acid content in neonatal fibrinogen5,6. Fibrin can then be used to make nanoparticles, which could be Additionally, fibrin nanoparticles can be synthesized with a variety of polymers as co-monomers7. Most commonly natural polymers such as alginate, chitosan, polyvinyl acetate (PVA), and polylactic glycolic acid (PLGA) can be used to stabilize fibrin to create more robust nanoparticle systems8–10. Using co-monomers can be beneficial to create in-situ gelling materials at fibrin concentrations that are physiologically relevant11.

Methods for Makin Fibrin Nanoparticles There are two typical ways that fibrin nanoparticles are made. These particles can either be synthesized via emulsions or by mechanical disruption. In general, emulsions fall into two overarching categories: water-in-oil (W/O) emulsions or emulsions made using microfluidic devices12. For water-in-oil emulsions, the aqueous phase, the fibrin material, gets dispersed in the continuous oil phase. This results in small droplets made from the aqueous phase. W/O emulsions are beneficial because several factors such as spin speed, aqueous phase drop rate, and temperatures of reagents can be minutely controlled to specifically change the size and physical properties of the nanoconstructs12,13. Microfluidic devices can also be used to synthesize fibrin nanoparticles. Similar to W/O emulsions, two phases are used; in this case, a secondary phase is used to disrupt the flow of a fibrin solution through a microfluidic device. This results in the formation of fibrin nanoconstructs14. Changing the flow rate results in particles of different sizes. Finally, fibrin nanoconstructs can be made via simple mechanical stimulation. Fibrin clots can be physically disturbed via mechanical agitation, filtration, or even sonication15. The physical disruption of a fibrin clot results in the formation of nanoconstructs. Each mode of physical disruption results in fibrin nanoparticles with widely varying physical characteristics, shapes, and sizes. Fibrin Nanoparticles as Cell Delivery Vehicles The novelty of fibrin nanoparticles is the biodegradability and biocompatibility of the particles16. Since fibrin is degradable and acellular, it can be easily remodeled by the body. For this reason, fibrin nanoparticles are great vehicles for cell delivery16. Cells can easily remodel fibrin and lay down their own matrix. This is integral for wound healing and regenerative purposes because ideally, the fibrin nanoparticles would integrate seamlessly into the native tissue. Not only are fibrin nanoparticles good for delivery cells, but they can also be leveraged to deliver cellular growth factors. Growth factors like basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), and keratinocyte growth factor (KGF) can easily be loaded into fibrin nanoconstructs9,14. Loading and delivery of growth factors are beneficial because they allow for a more seamless integration into native tissue. Delivering growth factors has the potential to allow native cells to better infiltrate the nanoparticles and integrate the particles into the overall matrix. Healing and Regenerative Applications of Fibrin Nanoparticles Current work has focused on the development of fibrin nanoparticles and nanoconstructs for regenerative medicine and healing applications. A lot of therapeutics have been studied to see how fibrin nanoparticles can be used as scaffolds for wound healing purposes. Fibrin nanoparticles can be applied to full-thickness dermal wounds to increase the rate of wound healing and augment the quality of wound healing14. Fibrin hydrogels can also be used for tissue regeneration purposes. Fibrin nanoparticles have also been shown to improve cellular infiltration into dermal wounds14. Fibrin nanoconstructs have also been applied to chronic non-healing wounds. Application of fibrin nanoparticles to diabetic dermal wounds resulted in advanced wound healing outcomes and better quality of wound healing9. Not only can these particles improve wound healing outcomes, but fibrin nanoparticles have been shown to increase vascularization and angiogenesis within wounds17.

Limitations Some limitations for the applicability of fibrin nanoparticles include uncontrolled degradation rate for drug delivery and limited durotaxis of cells in the wound environment. Fibrin nanoparticles have shown promise as delivery vesicles due to their biodegradability and biocompatibility, however, these same characteristics make it hard to control how long this degradation will take to occur within the body. Control of the degradation rate is important to ensure the component being carried is delivered to the right spot and is released in a manner conducive to wound healing and remodeling18. Fibrin nanoparticles are also a relatively soft material, which doesn’t actively recruit cells as well as stiffer particles are able to do. Therefore, the integration of cells into the wound environment that are needed for tissue remodeling could be limited19.

Future Directions Fibrin nanoparticles are a novel material currently being investigated for other applications within wound healing and delivery. One such application is drug delivery, specifically the delivery of antimicrobial drugs for treating infected wounds. Chronic wounds and post-traumatic wounds can often become infected, inhibiting the wound-healing process20. Fibrin nanoparticles already show promise in the treatment of some chronic wounds, such as diabetic ulcers, leading to the examination of their applicability in microbially infected wounds18. The incorporation of antimicrobial drugs into fibrin nanoparticles to be delivered to the wound site would allow for the restoration of the wound healing process by simultaneously fighting the infection and promoting angiogenesis20. Another application for fibrin nanoparticles is their incorporation into composites as a wound dressing for large, exposed wounds. Composites loaded with growth factors, cells, or synthetic components would act as a matrix to promote wound healing, which would be especially applicable to chronic wounds17. Incorporation of fibrin nanoparticles into these composites would induce the clotting cascade, promoting hemostasis and initiating progression through the wound healing phases. They could also act to add structure to the composite themselves through the fiber presence of fibrin within the nanoparticle17.