Gordon Minru Xiong

Surface modification of polycaprolactone substrates using collagen-conjugated poly(methacrylic acid) brushes for the regulation of cell proliferation and endothelialisation

The incorporation and presentation of cell recognition ligands on the surfaces of biodegradable blood-vessel implants to promote endothelialisation is considered to be a promising approach to prevent platelet aggregation and hence thrombogenesis. In this study, cell-adhesive collagen was covalently immobilised onto polycaprolactone (PCL) substrates via surface-initiated atom transfer radical polymerization (ATRP) to improve cell–material interactions. Functional polymer brushes of poly(methacrylic acid) (P(MAA)) containing dense and reactive carboxyl groups (–COOH) were formed on the PCL substrates in a controllable manner. The amount of collagen, which was conjugated to the pendant carboxyl groups via carbodiimide chemistry, increased with the concentration of –COOH groups on the grafted P(MAA) brushes. The affinity and growth of endothelial cells (ECs) were found to be significantly improved on the collagen-immobilised PCL substrates, and this improvement is positively correlated with the amount of covalently conjugated collagen. Thus, surface-initiated ATRP provides an alternative methodology for the surface functionalisation of biodegradable polyester scaffolds to enable the formation of a confluent layer of ECs. An optimally endothelialised material surface will play a major role in the minimisation of thrombogenicity and inflammation, and hence can be potentially used for vascular graft applications.

Immobilization of Gelatin onto Poly(Glycidyl Methacrylate)-Grafted Polycaprolactone Substrates for Improved Cell–Material Interactions

To enhance the cytocompatibility of polycaprolactone (PCL), cell-adhesive gelatin is covalently immobilized onto the PCL film surface via two surface-modified approaches: a conventional chemical immobilization process and a surface-initiated atom transfer radical polymerization (ATRP) process. Kinetics studies reveal that the polymer chain growth from the PCL film using the ATRP process is formed in a controlled manner, and that the amount of immobilized gelatin increases with an increasing concentration of epoxide groups on the grafted P(GMA) brushes. In vitro cell adhesion and proliferation studies demonstrate that cell affinity and growth are significantly improved by the immobilization of gelatin on PCL film surfaces, and that this improvement is positively correlated to the amount of covalently immobilized gelatin. With the versatility of the ATRP process and tunable grafting efficacy of gelatin, this study offers a suitable methodology for the functionalization of biodegradable polyesters scaffolds to improve cell–material interactions.

Polymer-enriched 3D graphene foams for biomedical applications

Graphene foams (GFs) are versatile nanoplatforms for biomedical applications because of their excellent physical, chemical, and mechanical properties. However, the brittleness and inflexibility of pristine GF (pGF) are some of the important factors restricting their widespread application. Here, a chemical-vapor-deposition-assisted method was used to synthesize 3D GFs, which were subsequently spin-coated with polymer to produce polymer-enriched 3D GFs with high conductivity and flexibility. Compared to pGF, both poly(vinylidene fluoride)-enriched GF (PVDF/GF) and polycaprolactone-enriched GF (PCL/GF) scaffolds showed improved flexibility and handleability. Despite the presence of the polymers, the polymer-enriched 3D GF scaffolds retained high levels of electrical conductivity because of the presence of microcracks that allowed for the flow of electrons through the material. In addition, polymer enrichment of GF led to an enhancement in the formation of calcium phosphate (Ca-P) compounds when the scaffolds were exposed to simulated body fluid. Between the two polymers tested, PCL enrichment of GF resulted in a higher in vitro mineralization nucleation rate because the oxygen-containing functional group of PCL had a higher affinity for Ca-P deposition and formation compared to the polar carbon-fluorine (C-F) bond in PVDF. Taken together, our current findings are a stepping stone toward future applications of polymer-enriched 3D GFs in the treatment of bone defects as well as other biomedical applications.

Endothelial cell thrombogenicity is reduced by ATRP-mediated grafting of gelatin onto PCL surfaces

Reducing the thrombogenicity of a tissue-engineered vascular graft prior to implantation is important for improving graft patency. As functionalization of synthetic materials with cell-adhesive proteins is routinely utilized as a means to promote endothelial cell (EC) growth, we conducted detailed investigation on the proliferation and thrombogenicity of ECs on such functionalized surfaces. We observed that polycaprolactone (PCL) surfaces functionalized with poly(glycidyl methacrylate) [(P(GMA)] brushes via atom transfer radical polymerization (ATRP) alone resulted in the enhancement of an activated EC profile characterized by low production of nitric oxide (NO), platelet activation and elevated expression levels of von Willebrand factor (vWF) and matrix metalloproteinase-2 (MMP-2). When gelatin was conjugated onto the PCL-g-P(GMA) surfaces, not only were EC proliferation and endothelial coverage significantly improved, but an anti-thrombogenic profile was also observed. We demonstrated that PCL can be successfully functionalized by a controllable surface-initiated polymerization method and importantly, the thrombogenic profile of the endothelial cells can be influenced by material surface chemistry (e.g. the presence of polymer graft chains). Our findings emphasize the importance of a careful consideration of materials for vascular graft applications, as well as differential endothelial cell physiology on surfaces with different material chemistry.

Surface modification of PVDF using non-mammalian sources of collagen for enhancement of endothelial cell functionality

Although polyvinylidene fluoride (PVDF) is non-toxic and stable in vivo, its hydrophobic surface has limited its bio-applications due to poor cell-material interaction and thrombus formation when used in blood contacting devices. In this study, surface modification of PVDF using naturally derived non-mammalian collagen was accomplished via direct surface-initiated atom transfer radical polymerisation (SI-ATRP) to enhance its cytocompatibility and hemocompatibility. Results showed that Type I collagen was successfully extracted from fish scales and bullfrog skin. The covalent immobilisation of fish scale-derived collagen (FSCOL) and bullfrog skin-derived collagen (BFCOL) onto the PVDF surface improves the attachment and proliferation of human umbilical vein endothelial cells (HUVECs). Furthermore, both FSCOL and BFCOL had comparable anti-thrombogenic profiles to that of commercially available bovine collagen (BVCOL). Also, cell surface expression of the leukocyte adhesion molecule was lower on HUVECs cultured on non-mammalian collagen surfaces than on BVCOL, which is an indication of lower pro-inflammatory response. Overall, results from this study demonstrated that non-mammalian sources of collagen could be used to confer bioactivity to PVDF, with comparable cell-material interactions and hemocompatibility to BVCOL. Additionally, higher expression levels of Type IV collagen in HUVECs cultured on FSCOL and BFCOL were observed as compared to BVCOL, which is an indication that the non-mammalian sources of collagen led to a better pro-angiogenic properties, thus making them suitable for blood contacting applications.

Imparting electroactivity to polycaprolactone fibers with heparin-doped polypyrrole: Modulation of hemocompatibility and inflammatory responses.

Hemocompatibility, anti-inflammation and anti-thrombogenicity of acellular synthetic vascular grafts remains a challenge in biomaterials design. Using electrospun polycaprolactone (PCL) fibers as a template, a coating of polypyrrole (PPy) was successfully polymerized onto the fiber surface. The fibers coated with heparin-doped PPy (PPy-HEP) demonstrated better electroactivity, lower surface resistivity (9-10-fold) and better anti-coagulation response (non-observable plasma recalcification after 30min vs. recalcification at 8-9min) as compared to fibers coated with pristine PPy. Red blood cell compatibility, measured by% hemolysis, was greatly improved on PPy-HEP-coated PCL in comparison to uncoated PCL (3.9±2.1% vs. 22.1±4.1%). PPy-HEP-coated PCL fibers also exhibited higher stiffness values (6.8±0.9MPa vs. 4.2±0.8MPa) as compared to PCL fibers, but similar tensile strengths. It was also observed that the application of a low alternating current led to a 4-fold reduction of platelet activation (as quantitated by CD62p expression) for the PPy-HEP-coated fibers as compared to non-stimulated conditions. In parallel, a reduction in the leukocyte adhesion to both pristine PPy-coated and PPy-HEP-coated fibers was observable with AC stimulation. Overall, a new strategy involving the use of hemocompatible conducting polymers and electrical stimulation to control thrombogenicity and inflammatory responses for synthetic vascular graft designs was demonstrated.

PCL microspheres tailored with carboxylated poly(glycidyl methacrylate)–REDV conjugates as conducive microcarriers for endothelial cell expansion

Microcarrier cell culture systems provide one of the most promising techniques for cell amplification due to their high surface area-to-volume ratio. In this study, biodegradable polycaprolactone (PCL) microspheres tethered with carboxylated poly(glycidyl methacrylate)-REDV conjugates were developed by a combination of surface-initiated atom transfer radical polymerization (ATRP) and azide-alkyne click chemistry as conducive microcarriers for rapid cell expansion of human umbilical vein endothelial cells (HUVECs). Azido-terminated poly(glycidyl methacrylate) (PGMA-N3) brushes were grafted onto the PCL microspheres by surface-initiated ATRP. Subsequent carboxylation of PGMA-N3 brushes was accomplished by azide-alkyne click reaction with hexynoic acid. REDV peptides were covalently conjugated to the pendent carboxyl groups on the side chain of carboxylated PGMA-COOH brushes via carbodiimide chemistry to enhance the cytocompatibility of the three-dimension (3D) PCL scaffolding system. Success in each functionalization step was ascertained by the measurement of attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and wet laser particle size analysis. In vitro cell-loading assay of HUVECs demonstrated a significant improvement of cell adhesion and proliferation on the REDV-immobilized PCL microsphere surfaces, and a confluent layer of HUVECs was formed after 7 days of incubation. The highly biocompatible and transportable nature of functionalized PCL microcarriers offers significant potential as a cell expansion platform.

Materials technology in drug eluting balloons: Current and future perspectives.

The coating material technology is important for the delivery of anti-proliferative drugs from the surface of drug-eluting balloons (DEBs), which are emerging as alternatives to drug-eluting stents (DES) in the field of interventional cardiology. Currently, several shortcomings limit their competition with DES, including low drug transfer efficiency to the arterial tissues and undesirable particulate generation from the coating matrix. In this review, we provide a survey of the materials used in existing DEBs, and discussed the mechanisms of actions of both the drugs and coating materials. The type of drug and the influence of the coating material characteristics on the drug uptake, distribution and retention in arterial tissues are described. We also summarize the novel coating excipients under development and provide our perspective on the possible use of nano-scale carriers to address the shortcomings of current coating technology. The scope of this review includes only materials that have been approved for biomedical applications or are generally recognized as safe (GRAS) for drug delivery.

Multifunctional REDV-conjugated zwitterionic polycarboxybetaine–polycaprolactone hybrid surfaces for enhanced antibacterial activity, anti-thrombogenicity and endothelial cell proliferation

Ideal synthetic polymeric vascular scaffolds should provide an excellent physiological environment to facilitate cell adhesion and growth, and appropriate physicochemical properties to prevent thrombogenicity and secondary infection. In the current study, a multifunctional polycaprolactone (PCL) surface for simultaneously enhancing the adhesion and proliferation of endothelial cells (ECs), as well as inhibiting pathogenic microbial adhesion and preserving hemocompatibility was demonstrated. The achievement of such a multifunctional surface was accomplished by the conjugation of Arg-Glu-Asp-Val (REDV) short peptides to zwitterionic polycarboxybetaine brush-grafted PCL films via surface-initiated atom transfer radical polymerization (ATRP). An in vitro antibacterial test demonstrated a high antibacterial efficiency against Gram-negative E. coli on the as-synthesized REDV-conjugated zwitterionic polycarboxybetaine hybrid surfaces. In addition, the platelet adhesion assay results showed that the zwitterionic polycarboxybetaine-REDV conjugates led to the amelioration of surface hemocompatibility, and this enhancement was not negated by the conjugation of REDV. Celluar studies further revealed that the EC attachment and proliferation were substantially improved by zwitterionic polycarboxybetaine-REDV conjugation as compared to other PCL surfaces. The current multifunctional PCL hybrid surface is potentially useful in tissue engineered constructs for vascular graft applications as it allows for better initial attachment and proliferation of ECs and improved hemocompatibility, whilst simultaneously reducing graft-associated infections.

A current-mode stimulator circuit with two-step charge balancing background calibration

Current-mode CMOS stimulation systems have offered unprecedented opportunities for accurate and high through put in-vitro and in-vivo physiological studies. As these circuits are in long term contact with living organisms, they must be flexible, safe and power efficient. Any mismatch in biphasic current pulses will result in charge imbalance, leading to tissue/cell damage. Therefore, it is the most important to maintain the balance of the charge injected and retracted by the anode and the cathode, respectively. This work first adjusts the body biasing voltage of the anode to match with the cathode current. It is robust, process-variation-aware and can reduce the imbalanced current to less than 1%. Second, any residue charge at the stimulation site is retracted only when it reaches a critical value. This process is performed in the background and thus does not disturb the front-end operation. Overall, it can achieve less than 0.4 nA DC error current and thus is a suitable candidate for long term stimulation applications.