Incorporation of heparin into biomaterials

Structure-Activity Relationships of Bioengineered Heparin/Heparan Sulfates Produced in Different Bioreactors

Heparin and heparan sulfate are structurally-related carbohydrates with therapeutic applications in anticoagulation, drug delivery, and regenerative medicine. This study explored the effect of different bioreactor conditions on the production of heparin/heparan sulfate chains via the recombinant expression of serglycin in mammalian cells. Tissue culture flasks and continuously-stirred tank reactors promoted the production of serglycin decorated with heparin/heparan sulfate, as well as chondroitin sulfate, while the serglycin secreted by cells in the tissue culture flasks produced more highly-sulfated heparin/heparan sulfate chains. The serglycin produced in tissue culture flasks was effective in binding and signaling fibroblast growth factor 2, indicating the utility of this molecule in drug delivery and regenerative medicine applications in addition to its well-known anticoagulant activity.

Controlled delivery of platelet-derived proteins enhances porcine wound healing

Platelet-rich plasma (PRP) is widely used for many clinical indications including wound healing due to the high concentrations of growth factors. However, the short half-life of these therapeutic proteins requires multiple large doses, and their efficacy is highly debated among clinicians. Here we report a method of protecting these proteins and releasing them in a controlled manner via a heparin-based coacervate delivery vehicle to improve wound healing in a porcine model. Platelet-derived proteins incorporated into the coacervate were protected and slowly released over 3weeks in vitro. In a porcine model, PRP coacervate significantly accelerated the healing response over 10days, in part by increasing the rate of wound reepithelialization by 35% compared to control. Additionally, PRP coacervate doubled the rate of wound contraction compared to all other treatments, including that of free PRP proteins. Wounds treated with PRP coacervate exhibited increased collagen alignment and an advanced state of vascularity compared to control treatments. These results suggest that this preparation of PRP accelerates healing of cutaneous wounds only as a controlled release formulation. The coacervate delivery vehicle is a simple and effective tool to improve the therapeutic efficacy of platelet-derived proteins for wound healing.

Heparin coatings for improving blood compatibility of medical devices

Blood contact with biomaterials triggers activation of multiple reactive mechanisms that can impair the performance of implantable medical devices and potentially cause serious adverse clinical events. This includes thrombosis and thromboembolic complications due to activation of platelets and the coagulation cascade, activation of the complement system, and inflammation. Numerous surface coatings have been developed to improve blood compatibility of biomaterials. For more than thirty years, the anticoagulant drug heparin has been employed as a covalently immobilized surface coating on a variety of medical devices. This review describes the fundamental principles of non-eluting heparin coatings, mechanisms of action, and clinical applications with focus on those technologies which have been commercialized. Because of its extensive publication history, there is emphasis on the CARMEDA^® BioActive Surface (CBAS^® Heparin Surface), a widely used commercialized technology for the covalent bonding of heparin.

Device thrombosis and pre-clinical blood flow models for assessing antithrombogenic efficacy of drug-device combinations

Thrombosis associated with blood-contacting devices is a complex process involving several component interactions that have eluded precise definition. Extensive investigations of individual biological modules such as protein adsorption, coagulation cascade activation and platelet activation/adhesion/aggregation have provided an initial foundation for developing biomaterials for blood-contacting devices, but a material that is intrinsically non-thrombogenic is yet to be developed. The well-recognized association between fluid dynamics parameters such as shear stress, vortices, stagnation and thrombotic processes such as platelet aggregation and coagulation aggravate thrombosis on most device geometries that elicit these flow disturbances. Thus, antithrombotic drugs that were developed to treat thrombosis associated with vascular diseases such as atherosclerosis have also been adapted to mitigate the risk of device thrombosis. However, balancing the risk of bleeding with the antithrombotic efficacy of these drugs continues to be a challenge, and surface modification of devices with these drug molecules to mitigate device thrombosis locally has been explored. Pre-clinical blood flow models to test the effectiveness of these drug-device combinations have also evolved and several in-vitro, ex-vivo, and in-vivo test configurations are available with their attendant merits and limitations. Despite considerable efforts toward iterative design and testing of blood contacting devices and antithrombogenic surface modifications, device thrombosis remains an unsolved problem.

Hemocompatibility studies on a degradable polar hydrophobic ionic polyurethane (D-PHI)

Biomaterial blood compatibility is a complex process that involves four key pathways, including the coagulation cascade, the complement system, platelets, and leukocytes. While many studies have addressed the initial contact of blood with homopolymeric (e.g. Teflon) or simple copolymeric (e.g. Dacron) biomaterials, relatively less attention has been given to investigating blood coagulation with respect to complex copolymeric systems containing well defined and diverse function. The current study sought to assess the hemocompatibility of a complex polyurethane (PU) containing a unique combination of polar, hydrophobic, and ionic domains (D-PHI). This included a whole blood (WB) study, followed by tests on the intrinsic and extrinsic coagulation pathways, complement activation, platelet activation, and an assessment of the effect of leukocytes on platelet-biomaterial interactions. A small increase in blood clot formation was observed on D-PHI in WB; however, there was no significant increase in clotting via the intrinsic coagulation cascade. No significant increase in platelet adhesion and only a very slight increase in platelet activation were observed in comparison to albumin-coated substrates (negative control). D-PHI showed mild complement activation and increased initiation of the extrinsic pathway of coagulation, along with the observation that leukocytes were important in mediating platelet-biomaterial interactions. It is proposed that complement is responsible for activating coagulation by inciting leukocytes to generate tissue factor (TF), which causes extrinsic pathway activation. This low level of blood clotting on D-PHI's surface may be necessary for the beneficial wound healing of vascular constructs that has been previously reported for this material.

STATEMENT OF SIGNIFICANCE

Understanding the hemocompatibility of devices intended for blood-contacting applications is important for predicting device failure. Hemocompatibility is a complex parameter (affected by at least four different mechanisms) that measures the level of thrombus generation and immune system activation resulting from blood-biomaterial contact. The complexity of hemocompatibility implies that homopolymers are unlikely to solve the clotting challenges that face most biomaterials. Diversity in surface chemistry (containing hydrophobic, ionic, and polar domains) obtained from engineered polyurethanes can lead to favourable interactions with blood. The current research considered the effect of a highly functionalized polyurethane biomaterial on all four mechanisms in order to provide a comprehensive in vitro measure of the hemocompatibility of this unique material and the important mechanisms at play.

A multifunctional surface for blood contact with fibrinolytic activity, ability to promote endothelial cell adhesion and inhibit smooth muscle cell adhesion

A multifunctional surface with fibrinolytic activity, the ability to promote endothelial cell and inhibit smooth muscle cell adhesion was realized.

Recent advances in thromboresistant and antimicrobial polymers for biomedical applications: just say yes to nitric oxide (NO)

Biomedical devices are essential for patient diagnosis and treatment; however, when blood comes in contact with foreign surfaces or homeostasis is disrupted, complications including thrombus formation and bacterial infections can interrupt device functionality, causing false readings and/or shorten device lifetime. Here, we review some of the current approaches for developing antithrombotic and antibacterial materials for biomedical applications. Special emphasis is given to materials that release or generate low levels of nitric oxide (NO). Nitric oxide is an endogenous gas molecule that can inhibit platelet activation as well as bacterial proliferation and adhesion. Various NO delivery vehicles have been developed to improve NO's therapeutic potential. In this review, we provide a summary of the NO releasing and NO generating polymeric materials developed to date, with a focus on the chemistry of different NO donors, the polymer preparation processes, and in vitro and in vivo applications of the two most promising types of NO donors studied thus far, N-diazeniumdiolates (NONOates) and S-nitrosothiols (RSNOs).

Sulfated hydrogel matrices direct mitogenicity and maintenance of chondrocyte phenotype through activation of FGF signaling

Deciphering the roles of chemical and physical features of the extracellular matrix (ECM) is vital for developing biomimetic materials with desired cellular responses in regenerative medicine. Here, we demonstrate that sulfation of biopolymers, mimicking the proteoglycans in native tissues, induces mitogenicity, chondrogenic phenotype, and suppresses catabolic activity of chondrocytes, a cell type that resides in a highly sulfated tissue. We show through tunable modification of alginate that increased sulfation of the microenvironment promotes FGF signaling-mediated proliferation of chondrocytes in a three-dimensional (3D) matrix independent of stiffness, swelling, and porosity. Furthermore, we show for the first time that a biomimetic hydrogel acts as a 3D signaling matrix to mediate a heparan sulfate/heparin-like interaction between FGF and its receptor leading to signaling cascades inducing cell proliferation, cartilage matrix production, and suppression of de-differentiation markers. Collectively, this study reveals important insights on mimicking the ECM to guide self-renewal of cells via manipulation of distinct signaling mechanisms.

Local release from affinity-based polymers increases urethral concentration of the stem cell chemokine CCL7 in rats

The protein chemokine (C-C motif) ligand 7 (CCL7) is significantly over-expressed in urethral and vaginal tissues immediately following vaginal distention in a rat model of stress urinary incontinence. Further evidence, in this scenario and other clinical scenarios, indicates CCL7 stimulates stem cell homing for regenerative repair. This CCL7 gradient is likely absent or compromised in the natural repair process of women who continue to suffer from SUI into advanced age. We evaluated the feasibility of locally providing this missing CCL7 gradient by means of an affinity-based implantable polymer. To engineer these polymers we screened the affinity of different proteoglycans, to use them as CCL7-binding hosts. We found heparin to be the strongest binding host for CCL7 with a 0.323 nM dissociation constant. Our experimental approach indicates conjugation of heparin to a polymer backbone (using either bovine serum albumin or poly (ethylene glycol) as the base polymer) can be used as a delivery system capable of providing sustained concentrations of CCL7 in a therapeutically useful range up to a month in vitro. With this approach we are able to detect, after polymer implantation, significant increase in CCL7 in the urethral tissue directly surrounding the polymer implants with only trace amounts of human CCL7 present in the blood of the animals. Whole animal serial sectioning shows evidence of retention of locally injected human mesenchymal stem cells (hMSCs) only in animals with sustained CCL7 delivery, 2 weeks after affinity-polymers were implanted.

Designer protein delivery: From natural to engineered affinity-controlled release systems

Exploiting binding affinities between molecules is an established practice in many fields, including biochemical separations, diagnostics, and drug development; however, using these affinities to control biomolecule release is a more recent strategy. Affinity-controlled release takes advantage of the reversible nature of noncovalent interactions between a therapeutic protein and a binding partner to slow the diffusive release of the protein from a vehicle. This process, in contrast to degradation-controlled sustained-release formulations such as poly(lactic-co-glycolic acid) microspheres, is controlled through the strength of the binding interaction, the binding kinetics, and the concentration of binding partners. In the context of affinity-controlled release--and specifically the discovery or design of binding partners--we review advances in in vitro selection and directed evolution of proteins, peptides, and oligonucleotides (aptamers), aided by computational design.

Local retention of antibodies in vivo with an injectable film embedded with a fluorogen-activating protein

Herein we report an injectable film by which antibodies can be localized in vivo. The system builds upon a bifunctional polypeptide consisting of a fluorogen-activating protein (FAP) and a β-fibrillizing peptide (βFP). The FAP domain generates fluorescence that reflects IgG binding sites conferred by Protein A/G (pAG) conjugated with the fluorogen malachite green (MG). A film is generated by mixing these proteins with molar excess of EAK16-II, a βFP that forms β-sheet fibrils at high salt concentrations. The IgG-binding, fluorogenic film can be injected in vivo through conventional needled syringes. Confocal microscopic images and dose-response titration experiments showed that loading of IgG into the film was mediated by pAG(MG) bound to the FAP. Release of IgG in vitro was significantly delayed by the bioaffinity mechanism; 26% of the IgG were released from films embedded with pAG(MG) after five days, compared to close to 90% in films without pAG(MG). Computational simulations indicated that the release rate of IgG is governed by positive cooperativity due to pAG(MG). When injected into the subcutaneous space of mouse footpads, film-embedded IgG were retained locally, with distribution through the lymphatics impeded. The ability to track IgG binding sites and distribution simultaneously will aid the optimization of local antibody delivery systems.

Using carbohydrate-based biomaterials as scaffolds to control human stem cell fate

This review describes the current state and applications of several important and extensively studied natural polysaccharide and glycoprotein scaffolds that can control the stem cell fate.

Tuning cellular responses to BMP-2 with material surfaces

Bone morphogenetic protein 2 (BMP-2) has been known for decades as a strong osteoinductive factor and for clinical applications is combined solely with collagen as carrier material. The growing concerns regarding side effects and the importance of BMP-2 in several developmental and physiological processes have raised the need to improve the design of materials by controlling BMP-2 presentation. Inspired by the natural cell environment, new material surfaces have been engineered and tailored to provide both physical and chemical cues that regulate BMP-2 activity. Here we describe surfaces designed to present BMP-2 to cells in a spatially and temporally controlled manner. This is achieved by trapping BMP-2 using physicochemical interactions, either covalently grafted or combined with other extracellular matrix components. In the near future, we anticipate that material science and biology will integrate and further develop tools for in vitro studies and potentially bring some of them toward in vivo applications.

Extracellular matrix-inspired growth factor delivery systems for bone regeneration

Growth factors are very promising molecules to enhance bone regeneration. However, their translation to clinical use has been seriously limited, facing issues related to safety and cost-effectiveness. These problems derive from the vastly supra-physiological doses of growth factor used without optimized delivery systems. Therefore, these issues have motivated the development of new delivery systems allowing better control of the spatiotemporal release and signaling of growth factors. Because the extracellular matrix (ECM) naturally plays a fundamental role in coordinating growth factor activity in vivo, a number of novel delivery systems have been inspired by the growth factor regulatory function of the ECM. After introducing the role of growth factors during the bone regeneration process, this review exposes different issues that growth factor-based therapies have encountered in the clinic and highlights recent delivery approaches based on the natural interaction between growth factor and the ECM.

Coacervate delivery of HB-EGF accelerates healing of type 2 diabetic wounds

Chronic wounds such as diabetic ulcers pose a significant challenge as a number of underlying deficiencies prevent natural healing. In pursuit of a regenerative wound therapy, we developed a heparin-based coacervate delivery system that provides controlled release of heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) within the wound bed. In this study, we used a polygenic type 2 diabetic mouse model to evaluate the capacity of HB-EGF coacervate to overcome the deficiencies of diabetic wound healing. In full-thickness excisional wounds on NONcNZO10 diabetic mice, HB-EGF coacervate enhanced the proliferation and migration of epidermal keratinocytes, leading to accelerated epithelialization. Furthermore, increased collagen deposition within the wound bed led to faster wound contraction and greater wound vascularization. Additionally, in vitro assays demonstrated that HB-EGF released from the coacervate successfully increased migration of diabetic human keratinocytes. The multifunctional role of HB-EGF in the healing process and its enhanced efficacy when delivered by the coacervate make it a promising therapy for diabetic wounds.

Quantification of aldehyde terminated heparin by SEC-MALLS-UV for the surface functionalization of polycaprolactone biomaterials

A straight forward strategy of heparin surface grafting employs a terminal reactive-aldehyde group introduced through nitrous acid depolymerization. An advanced method that allows simultaneously monitoring of both heparin molar mass and monomer/aldehyde ratio by size exclusion chromatography, multi-angle laser light scattering and UV-absorbance (SEC-MALLS-UV) has been developed to improve upon heparin surface grafting. Advancements over older methods allow quantitative characterization by direct (aldehyde absorbance) and indirect (Schiff-based absorbance) evaluation of terminal functional aldehydes. The indirect quantitation of functional aldehydes through labeling with aniline (and the formation of a Schiff-base) allows independent quantitation of both polymer mass and terminal functional groups with the applicable UV mass extinction coefficients determined. The protocol was subsequently used to synthesize an optimized heparin-aldehyde that had minimal polydispersity (PDI<2) and high reaction yields (yield >60% by mass). The 8 kDa weight averaged molar mass heparin-aldehyde was then grafted on polycaprolactone (PCL), a common implant material. This optimized heparin-aldehyde retained its antithrombin activity, assessed in freshly drawn blood or surface immobilized on PCL films. Anticoagulant activity was equal to or better than the 24 kDa unmodified heparin it was fragmented from.

A new avenue to the synthesis of GAG-mimicking polymers highly promoting neural differentiation of embryonic stem cells

A new strategy for the fabrication of glycosaminoglycan (GAG) analogs with high bioactivities was proposed by copolymerizing the sulfonated unit and the glyco unit, ‘splitted’ from the sulfated saccharide building blocks of GAGs.

Robust, highly elastic and bioactive heparin-mimetic hydrogels

We construct robust, highly elastic, and bioactive graphene oxide doped heparin-mimetic hydrogels for use in drug delivery and other potential biomedical applications.

Extracorporeal membrane oxygenation-hemostatic complications

The use of extracorporeal membrane oxygenation (ECMO) support for cardiac and respiratory failure has increased in recent years. Improvements in ECMO oxygenator and pump technologies have aided this increase in utilization. Additionally, reports of successful outcomes in supporting patients with respiratory failure during the 2009 H1N1 pandemic and reports of ECMO during cardiopulmonary resuscitation have led to increased uptake of ECMO. Patients requiring ECMO are a heterogenous group of critically ill patients with cardiac and respiratory failure. Bleeding and thrombotic complications remain a leading cause of morbidity and mortality in patients on ECMO. In this review, we describe the mechanisms and management of hemostatic, thrombotic and hemolytic complications during ECMO support.

Polymeric nanoarchitectures on Ti-based implants for antibacterial applications

Because of the excellent mechanical properties and good biocompatibility, titanium-based metals are widely used in hard tissue repair, especially load-bearing orthopedic applications. However, bacterial infection and complication during and after surgery often causes failure of the metallic implants. To endow titanium-based implants with antibacterial properties, surface modification is one of the effective strategies. Possessing the unique organic structure composed of molecular and functional groups resembling those of natural organisms, functionalized polymeric nanoarchitectures enhance not only the antibacterial performance but also other biological functions that are difficult to accomplish on many conventional bioinert metallic implants. In this review, recent advance in functionalized polymeric nanoarchitectures and the associated antimicrobial mechanisms are reviewed.

Mussel-inspired one-step adherent coating rich in amine groups for covalent immobilization of heparin: hemocompatibility, growth behaviors of vascular cells, and tissue response

Heparin, an important polysaccharide, has been widely used for coatings of cardiovascular devices because of its multiple biological functions including anticoagulation and inhibition of intimal hyperplasia. In this study, surface heparinization of a commonly used 316L stainless steel (SS) was explored for preparation of a multifunctional vascular stent. Dip-coating of the stents in an aqueous solution of dopamine and hexamethylendiamine (HD) (PDAM/HD) was presented as a facile method to form an adhesive coating rich in primary amine groups, which was used for covalent heparin immobilization via active ester chemistry. A heparin grafting density of about 900 ng/cm(2) was achieved with this method. The retained bioactivity of the immobilized heparin was confirmed by a remarkable prolongation of the activated partial thromboplastin time (APTT) for about 15 s, suppression of platelet adhesion, and prevention of the denaturation of adsorbed fibrinogen. The Hep-PDAM/HD also presented a favorable microenvironment for selectively enhancing endothelial cell (EC) adhesion, proliferation, migration and release of nitric oxide (NO), and at the same time inhibiting smooth muscle cell (SMC) adhesion and proliferation. Upon subcutaneous implantation, the Hep-PDAM/HD exhibited mitigated tissue response, with thinner fibrous capsule and less granulation formation compared to the control 316L SS. This number of unique functions qualifies the heparinized coating as an attractive alternative for the design of a new generation of stents.

Immobilization of heparin/poly-(L)-lysine nanoparticles on dopamine-coated surface to create a heparin density gradient for selective direction of platelet and vascular cells behavior

Restenosis, thrombosis formation and delayed endothelium regeneration continue to be problematic for coronary artery stent therapy. To improve the hemocompatibility of the cardiovascular implants and selectively direct vascular cell behavior, a novel kind of heparin/poly-l-lysine (Hep/PLL) nanoparticle was developed and immobilized on a dopamine-coated surface. The stability and structural characteristics of the nanoparticles changed with the Hep:PLL concentration ratio. A Hep density gradient was created on a surface by immobilizing nanoparticles with various Hep:PLL ratios on a dopamine-coated surface. Antithrombin III binding quantity was significantly enhanced, and in plasma the APTT and TT times as coagulation tests were prolonged, depending on the Hep density. A low Hep density is sufficient to prevent platelet adhesion and activation. The sensitivity of vascular cells to the Hep density is very different: high Hep density inhibits the growth of all vascular cells, while low Hep density could selectively inhibit smooth muscle cell hyperplasia but promote endothelial progenitor cells and endothelial cell proliferation. These observations provide important guidance for modification of surface heparinization. We suggest that this method will provide a potential means to construct a suitable platform on a stent surface for selective direction of vascular cell behavior with low side effects.