Controlled release of low-molecular weight, polymer-free corticosteroid coatings suppresses fibrotic encapsulation of implanted medical devices.

Inflammation-driven foreign body reactions, and the frequently associated encapsulation by fibrogenic fibroblasts, reduce the functionality and longevity of implanted medical devices and materials. Anti-inflammatory drugs, such as dexamethasone, can suppress the foreign body reaction for a few days post-surgery, but lasting drug delivery strategies for long-term implanted materials remain an unmet need. We here establish a thin-coating strategy with novel low molecular weight corticosteroid dimers to suppress foreign body reactions and fibrotic encapsulation of subcutaneous silicone implants. The dimer coatings are >75% dexamethasone by mass and directly processable into conformal coatings using conventional solvent-based techniques, such as casting or spray coating without added polymers or binding agents. In vitro, surface erosion of the coating, and subsequent hydrolysis, provide controlled release of free dexamethasone. In a rat subcutaneous implantation model, the resulting slow and sustained release profile of dexamethasone is effective at reducing the number and activation of pro-fibrotic macrophages both acutely and at chronic time points. Consequently, fibroblast activation, collagen deposition and fibrotic encapsulation are suppressed at least 45 days post-implantation. Thus, our approach to protect implants from host rejection is advantageous over polymeric drug delivery systems, which typically have low drug loading capacity (<30%), initial burst release profiles, and unpredictable release kinetics.

Polymer-free corticosteroid dimer implants for controlled and sustained drug delivery.

Polymeric drug carriers are widely used for providing temporal and/or spatial control of drug delivery, with corticosteroids being one class of drugs that have benefitted from their use for the treatment of inflammatory-mediated conditions. However, these polymer-based systems often have limited drug-loading capacity, suboptimal release kinetics, and/or promote adverse inflammatory responses. This manuscript investigates and describes a strategy for achieving controlled delivery of corticosteroids, based on a discovery that low molecular weight corticosteroid dimers can be processed into drug delivery implant materials using a broad range of established fabrication methods, without the use of polymers or excipients. These implants undergo surface erosion, achieving tightly controlled and reproducible drug release kinetics in vitro. As an example, when used as ocular implants in rats, a dexamethasone dimer implant is shown to effectively inhibit inflammation induced by lipopolysaccharide. In a rabbit model, dexamethasone dimer intravitreal implants demonstrate predictable pharmacokinetics and significantly extend drug release duration and efficacy (>6 months) compared to a leading commercial polymeric dexamethasone-releasing implant.

Sequence-Controlled Polyurethane Block Copolymer Displays Differentiated Immunoglobulin-G Adsorption That Influences Human Monocyte Adhesion and Activity.

The ability to specify an adsorbed protein layer through the polymer chemistry design of immunomodulatory biomaterials is important when considering a desired immune response, such as reducing pro-inflammatory activity. Limited work has been undertaken to elucidate the role of monomer sequence in this process, when copolymeric systems are involved. In this study, we demonstrate the advantage of an alternating radical copolymerization strategy as opposed to a random statistical copolymerization to order monomers in the synthesis of degradable polar-hydrophobic-ionic polyurethanes (D-PHI), biomaterials originally designed to reduce inflammatory monocyte activation. A monomer system consisting of a vinyl-terminated polyurethane cross-linker, maleic acid (MA), and ethyl vinyl ether (EVE), not only generated a diverse chemical environment of polar, hydrophobic, and ionic functional groups, but also formed a charge transfer complex (CTC) reactive to alternating polymerizations. Conversion of MA and EVE occurred in a constant proportion regardless of monomer availability, a phenomenon not observed in conventional D-PHI formulations. For feeds with unequal molar quantities of MA and EVE, the final conversion was limited and proportional to the limiting reagent, leading to an overall higher polyurethane cross-linker content. The presence of a reactive CTC was also found to limit the monomer conversion. Compared to a D-PHI with random monomer arrangement using methacrylic acid (MAA) and methyl methacrylate (MMA), a reduction in Fab region exposure from adsorbed immunoglobulin G and a reduction in average adherent monocyte activity were found in the sequence-controlled version. These results represent the first example of using an alternating copolymerization approach to generate regularly defined polymer chemistries in radical chain-growth biomaterials for achieving immunomodulation, and highlight the importance of considering sequence control as a design strategy for future immunomodulatory biomaterial development.

Synthesis and characterization of electrospun nanofibrous tissue engineering scaffolds generated from in situ polymerization of ionomeric polyurethane composites.

Tissue scaffolds need to be engineered to be cell compatible, have timely biodegradable character, be functional with respect to providing niche cell support for tissue repair and regeneration, readily accommodate multiple cell types, and have mechanical properties that enable the simulation of the native tissue. In this study, electrospun degradable polar hydrophobic ionic polyurethane (D-PHI) scaffolds were generated in order to yield an extracellular matrix-like structure for tissue engineering applications. D-PHI oligomers were synthesized, blended with a degradable linear polycarbonate polyurethane (PCNU), and electrospun with simultaneous in situ UV cross-linking in order to generate aligned nanofibrous scaffolds in the form of elastomeric composite materials. The D-PHI/PCNU scaffold fibre morphology, cross-linking efficiency, surface nature, mechanical properties, in vivo degradation and integration, as well as in vitro cell compatibility were characterized. The results showed that D-PHI/PCNU scaffolds had a high cross-linking efficiency, stronger polar nature, and lower stiffness relative to PCNU scaffolds. In vivo, the D-PHI/PCNU scaffold degraded relatively slowly, thereby enabling new tissue time to form and yielding very good integration with the latter tissue. Based on a study with A10 vascular smooth muscle cells, the D-PHI/PCNU scaffold was able to support high cell viability, adhesion, and expression of typical smooth muscle cell markers after a 7-day culture period, which was comparable to PCNU scaffolds. These characterization results demonstrate that the unique properties of a D-PHI/PCNU scaffold, combined with the benefits of electrospinning, could allow for the generation of a tissue engineered scaffold that mimics important aspects of the native extracellular matrix and could be used for functional tissue regeneration. STATEMENT OF SIGNIFICANCE: Tissue engineered scaffolds should recapitulate native extracellular matrix features. This study investigates the processing of a classical polycarbonate polyurethane (PCNU) with a cross-linked and degradable ionomeric polyurethane (D-PHI), polymerized via in situ rapid light curing to yield a 3-dimensional co-electrospun nanofibre matrix with chemical diversity and low modulus character. This research advances the use of D-PHI for tissue engineering applications by providing a facile means of changing physical and chemical properties in classical PCNUs without the need to adjust spinning viscosities of the base polymer. Further, the in vivo and cell culture findings set the stage for introducing unique elastic materials which inherently support wound healing, repair, and regeneration in tissues, for applications that require the recapitulation of native extracellular matrix physical features.

Differential Regulation of Extracellular Matrix Components Using Different Vitamin C Derivatives in Mono- and Coculture Systems.

Vascular tissue engineering strategies using cell-seeded scaffolds require uniformly distributed vascular cells and sufficient extracellular matrix (ECM) production. However, acquiring sufficient ECM deposition on synthetic biomaterial scaffolds during the in vitro culture period prior to tissue implantation still remains challenging for vascular constructs. Two forms of vitamin C derivatives, ascorbic acid (AA) and sodium ascorbate (SA), are commonly supplemented in cell culture to promote ECM accumulation. However, the literature often refers to AA and SA interchangeably, and their differential effects on cell growth and ECM molecule (glycosaminoglycan, collagen, elastin) accumulation have never been reported when used in monoculture or coculture systems developed with synthetic three-dimensional (3D) scaffolds. In this study, it was found that 200 µM AA stimulated an increase in cell number, whereas SA (50, 100, and 200 µM) supported more calponin expression (immunostaining) and higher ECM accumulation from vascular smooth muscle cells (VSMCs) after 1 week in the degradable polar hydrophobic ionic polyurethane scaffold. The influence of AA and SA on ECM deposition was also studied in VSMC-monocyte cocultures to replicate some aspects of a wound healing environment in vitro and compared to their effects in respective VSMC monocultures after 4 weeks. Although 100 µM SA promoted ECM deposition in coculture, the condition of 100 µM AA + 100 µM SA was more effective toward enhancing ECM accumulation in VSMC monoculture after 4 weeks. The results demonstrated that AA and SA are not interchangeable, and the different effects of AA and/or SA on ECM deposition were both culture system (co- vs monoculture) and culture period (1 vs 4 week) dependent. This study provides further insight into practical vascular tissue engineering strategies when using 3D synthetic biomaterial-based constructs.

Mono vs multilayer fibronectin coatings on polar/hydrophobic/ionic polyurethanes: Altering surface interactions with human monocytes.

Monocyte interactions with materials that are biofunctionalized with fibronectin (Fn) are of interest because of the documented literature which associates this protein with white blood cell function at implant sites. A degradable-polar hydrophobic ionic polyurethane (D-PHI), has been reported to promote an anti-inflammatory response from human monocytes. The aim of the current work was to study the influence of intrinsic D-PHI material chemistry on Fn adsorption (mono and multi-layer structures), and to investigate the influence of such chemistry on the structural state of the Fn, as well as the latter's influence on the activity of human monocytes on the protein coated substrates. Significant differences in Fn adsorption, surface hydrophobicity and the availability of defined peptide sequences (N terminal, C terminal or Cell Binding Domain) for the Fn in mono vs multilayer structures were observed as a function of the changes in intrinsic material chemistry. A D-PHI-formulated polyurethane substrate with subtle changes in anionic and hydrophobic domain content relative to the polar non-ionic urethane/carbonate groups within the polymer matrix promoted the lowest activation of monocytes, in the presence of multi-layer Fn constructs. These results highlight the importance of chemical heterogeneity as a design parameter for biomaterial surfaces, and establishes a desired strategy for controlling human monocyte activity at the surface of devices, when these are coated with multi-layer Fn structures. The latter is an important step towards functionalizing the materials with multi-layer protein drug carriers as interventional therapeutic agents. STATEMENT OF SIGNIFICANCE: The control of the behavior of monocytes, especially migration and activation, is of crucial interest to modulate the inflammatory response at the site of implanted biomaterial. Several studies report the influence of adsorbed serum proteins on the behavior of monocytes on biomaterials. However, few studies show the influence of surface chemical group distribution on the controlled adsorption and the subsequent induced conformation- of mono versus multi-layer assembled structures generated from specific proteins implicated in wound repair. The current research considered the role of Fn adsorption and conformation in thin films while interacting with the intrinsic chemistry of segmented block polyurethanes; and the influence of the former on modulation and activation of human monocytes.

Generating favorable growth factor and protease release profiles to enable extracellular matrix accumulation within an in vitro tissue engineering environment.

Tissue engineering (particularly for the case of load-bearing cardiovascular and connective tissues) requires the ability to promote the production and accumulation of extracellular matrix (ECM) components (e.g., collagen, glycosaminoglycan and elastin). Although different approaches have been attempted in order to enhance ECM accumulation in tissue engineered constructs, studies of underlying signalling mechanisms that influence ECM deposition and degradation during tissue remodelling and regeneration in multi-cellular culture systems have been limited. The current study investigated vascular smooth muscle cell (VSMC)-monocyte co-culture systems using different VSMC:monocyte ratios, within a degradable polyurethane scaffold, to assess their influence on ECM generation and degradation processes, and to elucidate relevant signalling molecules involved in this in vitro vascular tissue engineering system. It was found that a desired release profile of growth factors (e.g. insulin growth factor-1 (IGF-1)) and hydrolytic proteases (e.g. matrix-metalloproteinases 2, 9, 13 and 14 (MMP2, MMP9, MMP13 and MMP14)), could be achieved in co-culture systems, yielding an accumulation of ECM (specifically for 2:1 and 4:1 VSMC:monocyte culture systems). This study has significant implications for the tissue engineering field (including vascular tissue engineering), not only because it identified important cytokines and proteases that control ECM accumulation/degradation within synthetic tissue engineering scaffolds, but also because the established culture systems could be applied to improve the development of different types of tissue constructs. STATEMENT OF SIGNIFICANCE: Sufficient extracellular matrix accumulation within cardiovascular and connective tissue engineered constructs is a prerequisite for their appropriate function in vivo. This study established co-culture systems with tissue specific cells (vascular smooth muscle cells (VSMCs)) and defined ratios of immune cells (monocytes) to investigate extracellular matrix (ECM) generation and degradation processes, revealing important mechanisms underlying ECM turnover during vascular tissue regeneration/remodelling. A specific growth factor (IGF-1), as well as hydrolytic proteases (e.g. MMP2, MMP9, MMP13 and MMP14), were identified as playing important roles in these processes. ECM accumulation was found to be dependent on achieving a desired release profile of these ECM-promoting and ECM-degrading factors within the multi-cellular microenvironment. The findings enhance our understanding of ECM deposition and degradation during in vitro tissue engineering and would be applicable to the repair or regeneration of a variety of tissues.

Characterization of a degradable polar hydrophobic ionic polyurethane with circulating angiogenic cells in vitro.

This study investigated the interaction of human circulating angiogenic cells (CACs) with a degradable polar hydrophobic ionic polyurethane (D-PHI) which has been previously shown to exhibit anti-inflammatory character and favorable interactions with human endothelial cells (ECs). Given the implication of the CACs in microvessel development it was of intrinsic interest to expand our knowledge of D-PHI biocompatibility with this relevant primary cell involved in angiogenesis. The findings will be compared to a well-established benchmark substrate for CACs, fibronectin-coated tissue culture polystyrene (TCPS). Immunoblotting analysis showed that CACs were a heterogeneous population of cells composed mostly of monocytic cells expressing the CD14 marker. Assessment of the cytokine release profile, using ELISA, showed that D-PHI supported a higher concentration of interleukin-10 (IL-10) when compared to the concentration of tumor necrosis factor alpha, which is indicative of an anti-inflammatory phenotype, and was different from the response with TCPS. It was found that the CACs were attached to D-PHI and remained viable and functional (nitric oxide production) during the seven days of culture. However, there did not appear to be any significant proliferation on D-PHI, contrary to the CAC growth on fibronectin-coated TCPS. It was concluded that D-PHI displayed some of the qualities suitable to enable the retention of CACs onto this substrate, as well as maintaining an anti-inflammatory phenotype, characteristics which have been reported to be important for angiogenesis in vivo.

Biomaterials in co-culture systems: towards optimizing tissue integration and cell signaling within scaffolds.

Most natural tissues consist of multi-cellular systems made up of two or more cell types. However, some of these tissues may not regenerate themselves following tissue injury or disease without some form of intervention, such as from the use of tissue engineered constructs. Recent studies have increasingly used co-cultures in tissue engineering applications as these systems better model the natural tissues, both physically and biologically. This review aims to identify the challenges of using co-culture systems and to highlight different approaches with respect to the use of biomaterials in the use of such systems. The application of co-culture systems to stimulate a desired biological response and examples of studies within particular tissue engineering disciplines are summarized. A description of different analytical co-culture systems is also discussed and the role of biomaterials in the future of co-culture research are elaborated on. Understanding the complex cell-cell and cell-biomaterial interactions involved in co-culture systems will ultimately lead the field towards biomaterial concepts and designs with specific biochemical, electrical, and mechanical characteristics that are tailored towards the needs of distinct co-culture systems.

Protein binding mediation of biomaterial-dependent monocyte activation on a degradable polar hydrophobic ionic polyurethane.

Protein adsorption is an important phenomenon influencing the cellular response to biomaterials. Previous studies comparing monocyte activation on a degradable polar hydrophobic ionic polyurethane (D-PHI) indicated a reduced pro-inflammatory monocyte response relative to tissue culture polystyrene (TCPS) and poly(lactide-co-glycolide) (PLGA) substrates. The present study investigated the influence of protein binding in order to gain further insight into the observed differential monocyte activation. Several proteins, identified in different relative amounts within the bound protein layers on D-PHI vs. PLGA and TCPS, were evaluated for their effect on monocyte activation. It was found that, in general, both non-coated and protein pre-adsorbed D-PHI supported a reduced pro-inflammatory response relative to PLGA, as indicated by lower levels of tumor necrosis factor-α (TNF-α) release. An initial increase in TNF-α release occurred when α(2)-macroglobulin (A2M) was pre-adsorbed to D-PHI, which was shown to involve the α(2)-macroglobulin receptor and was active on D-PHI but not on the two other biomaterials. This response was not observed during competitive protein binding in the presence of fetal bovine serum (FBS), suggesting that a more complex arrangement of the bound proteins and their interactions with one another, as well as with the surface chemistry of the individual biomaterials, resulted in the low-activating character of D-PHI when interacting with human monocytes.

Electrospun elastin-like polypeptide enriched polyurethanes and their interactions with vascular smooth muscle cells.

In vascular tissue, elastin is an essential extracellular matrix protein that plays an important biomechanical and biological signalling role. Native elastin is insoluble and is difficult to extract from tissues, which results in its relatively rare use for the fabrication of vascular tissue engineering scaffolds. Recombinant elastin-like polypeptide-4 (ELP4), which mimics the structure and function of native tropoelastin, represents a practical alternative to the native elastic fibre for vascular applications. In this study, electrospinning was utilized to fabricate fibrous scaffolds which were subsequently surface modified with ELP4 and used as substrates for smooth muscle cell culture. ELP4 surface modified materials demonstrated enhanced smooth muscle cell (SMC) adhesion and maintenance of cell numbers over a 1-week period relative to controls. SMCs seeded on the ELP4 surface modified materials were also shown to exhibit the cell morphology and biological markers of a contractile phenotype including a spindle-like morphology, actin filament organization and smooth muscle myosin heavy chain expression. Competitive inhibition experiments demonstrated that the elastin-laminin cell surface receptor and its affinity for the VGVAPG peptide sequence on ELP4 molecules are likely involved in the initial SMC contact with the ELP4 modified materials. Elastin-like polypeptides show promise as surface modifiers for candidate scaffolds for engineering contractile vascular tissues.

Differences in protein binding and cytokine release from monocytes on commercially sourced tissue culture polystyrene.

Tissue culture polystyrene (TCPS) is a ubiquitous substrate used by many researchers in the biomedical and biological sciences. Different parameters involved in the production of TCPS, including the treatment time and the use of reactive gases and chemical agents, can have a significant influence on the ultimate surface properties achieved. The assumption that they will all yield a consistent and controlled product has not proven to be true. To provide a better insight into the bioactivity differences in TCPS supplied by different manufacturers, TCPS from three different companies (Sarstedt, Wisent Corp., and Becton Dickinson (BD)) were analyzed for their surface properties, protein adsorption characteristics, and interactions with human monocytes. Marked differences were observed in terms of surface wettability and surface chemistry. Furthermore, Wisent TCPS adsorbed more than twice the amount of serum proteins compared with BD and Sarstedt TCPS. Sarstedt showed significantly more cell retention (more DNA) compared with both BD and Wisent TCPS brands over a 7 day culture period. Cytokine release from monocytes adherent on the three different TCPS also differed significantly, suggesting that the differences in the surface properties were sufficient to differentially mediate monocyte activation. These results have important implications for TCPS research use, in terms of appreciating the interpretation of the data when TCPS is used as a control substrate as well as when it is used where a pre-conditioned state would influence the outcome of the study.

The effect of degradable polymer surfaces on co-cultures of monocytes and smooth muscle cells.

Strategies to optimize biomaterial chemistry for applications in vascular tissue engineering attempt to promote endothelial and smooth muscle cell recruitment into porous material constructs. The primary objective is to facilitate relevant tissue formation in a wound healing versus pro-inflammatory manner. The present work investigated the interactive co-cellular response of human monocytes and human vascular smooth muscle cells (VSMCs) with a novel degradable, polar/hydrophobic/ionic (D-PHI) polyurethane and compared it to a commercially available biomaterial, poly-lactic-glycolic acid (PLGA) as well as tissue culture polystyrene (TCPS). D-PHI triggered a smaller pro-inflammatory response (acid phosphatase, esterase, tumor necrosis factor-α) at later time points (>14 d) than PLGA suggesting that monocytes may be transitioning to a more wound-healing phenotype on the D-PHI surface. When D-PHI was coated with collagen, monocyte cell attachment did not differ with the native D-PHI; however, PLGA showed significant differences between collagen coated versus uncoated surfaces. There were more VSMCs and monocytes attached in co-culture to D-PHI when compared to PLGA. Co-cultures on D-PHI released more IL-10 (anti-inflammatory) than monocytes cultured alone, while the VSMCs retained the expression of its marker protein calponin. Together the above data suggest that co-culturing monocytes with VSMCs may aid in stimulating the attachment of VSMCs to D-PHI while eliciting the desired functional phenotypes for both monocytes (i.e. low inflammation based on IL-10 values) and VSMCs (expressing calponin, a marker of contractility). Moreover, the results of this study demonstrated that D-PHI performed equally or better to PLGA in terms of the assayed biological parameters.

Surface immobilization of elastin-like polypeptides using fluorinated surface modifying additives.

Elastin-like polypeptide (ELP) surface modification represents a valuable approach for the development of biomaterials in a wide range of medical applications. In this study, ELP surface modification has been achieved through the use of elastin cross-linking peptide (ECP) bioactive fluorinated surface modifiers (ECP-BFSMs). The synthesis of low molecular weight fluorinated additives was described and their subsequent blending with a base polycarbonate urethane (PCNU) was shown to successfully enrich the surface to allow for ELP surface cross-linking via lysine moieties on the peptide segments of the ECP-BFSMs. The kinetics for the surface migration of fluorescent ECP-BFSMs was studied over a 2-week period by two-photon confocal microscopy. A decrease in advancing contact angle from 87.9° to 75.3° was observed for ECP-BFSM modified PCNU and was associated with the presence of ECP peptides on the surface. X-ray photoelectron spectroscopy demonstrated an increase in surface atomic percent of fluorine (from 0.2 to 7.2%) and nitrogen (from 1.0 to 3.0%) associated with the surface localization of fluoro groups and amide groups associated with the peptides in the ECP-BFSMs. A further increase in surface atomic percent of nitrogen (from 3.0 to 8.3%) was observed after ELP surface cross-linking. These ELP-modified surfaces were shown to promote increased smooth muscle cell adhesion, spreading and retention over a 7-day culture period relative to their non-ELP4 analogs. This novel surface modifying additive approach may be used for various biomimetic applications since it generates a stable ECM-like surface retained onto a relatively inert fluorinated background.