Topological Dirac Semimetal Na3Bi Films in the Ultrathin Limit via Alternating Layer Molecular Beam Epitaxy

Ultrathin films of Na3Bi on insulating substrates are desired for opening a bulk band gap and generating the quantum spin Hall effect from a topological Dirac semimetal, though continuous films in the few nanometer regime have been difficult to realize. Here, we utilize alternating layer molecular beam epitaxy (MBE) to achieve uniform and continuous single crystal films of Na3Bi(0001) on insulating Al2O3(0001) substrates and demonstrate electrical transport on films with 3.8 nm thickness (4 unit cells). The high material quality is confirmed through in situ reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). In addition, these films are employed as seed layers for subsequent growth by codeposition, leading to atomic layer-by-layer growth as indicated by RHEED intensity oscillations. These material advances facilitate the pursuit of quantum phenomena in thin films of Dirac semimetals.Comment: 11 pages, 5 figure

Magnetically actuated glaucoma drainage device for regulating intraocular pressure after implantation

The key risk factor for glaucoma is increased intraocular pressure (IOP). Glaucoma drainage devices implanted in the eye can reduce IOP and thus stop disease progression. However, most devices currently used in clinical practice are passive and do not allow for postsurgical IOP control, which may result in serious complications such as hypotony (i.e., excessively low IOP). To enable noninvasive IOP control, we demonstrate a novel, miniature glaucoma implant that will enable the repeated adjustment of the hydrodynamic resistance after implantation. This is achieved by integrating a magnetic microvalve containing a micropencil-shaped plug that is moved using an external magnet, thereby opening or closing fluidic channels. The microplug is made from biocompatible poly(styrene-block-isobutylene-block-styrene) (SIBS) containing iron microparticles. The complete implant consists of an SIBS drainage tube and a housing element containing the microvalve and fabricated with hot embossing using femtosecond laser-machined glass molds. Using in vitro and ex vivo microfluidic experiments, we demonstrate that when the microvalve is closed, it can provide sufficient hydrodynamic resistance to overcome hypotony. Valve function is repeatable and stable over time. Due to its small size, our implant is a promising, safe, easy-to-implant, minimally invasive glaucoma surgery device. [Figure not available: see fulltext.]</p

The use of polyisobutylene-based polymers in ophthalmology

A novel polyolefin called poly(styrene-block-isobutylene-block-styrene) (“SIBS”) originated from Joseph P. Kennedy's laboratory at the University of Akron (Akron, Ohio, United States) and was developed as a biomaterial for long-term implant applications by the author. SIBS has no cleavable groups on its backbone or sidechains, is comprised predominantly of alternating secondary and quaternary carbons on its backbone, which prevents embrittlement and cracking under flexion, and undergoes multiple purification steps which renders it extremely biocompatible and well-suited for long-term applications in the eye. This article explores two ophthalmic devices; 1) the PRESERFLO® MicroShunt (Santen Pharmaceutical Co. Ltd., Osaka, Japan) made from SIBS that lowers intraocular pressure to thwart progression of vision loss from glaucoma, and 2) a novel intraocular lens (IOL) made from crosslinked polyisobutylene, which is under-development by Xi'an Eyedeal Medical Technology Co., Ltd. (Xi'an, China) that does not glisten nor cloud over time, as do most conventional IOLs

Physicochemical Factors That Affect Copolymeric Gels Of Poly(2-Hydroxyethyl Methacrylate)/methacrylic Acid As Materials For Urinary Tract Prostheses (hydrogel, Swelling, Synthesis)

The ionizable constituent, methacrylic acid, MAA, in copolymers of 2-hydroxyethyl methacrylate, HEMA, and MAA leads to complex physicochemical behavior of gels in environments where the pH, ionic strength, and other aspects of the chemical solvent composition are changeable. Specifically, small changes of pH near a critical value can produce massive changes of physical properties, such as the swollen volume or water content of these gels. Urine exhibits a changeable composition, and importantly, the critical pH of pHEMA/MAA copolymers is within the common range of urine compositions. The physicochemical changes of these copolymers were exploited in the design of a prosthetic material for use in the urinary tract.Included in this dissertation are protocols for purification of HEMA monomer, analytical procedures for analyzing impurities in HEMA monomer, protocols for polymerizing bulk pHEMA under pressure, a protocol for grafting pHEMA to a polyether polyurethane, Tecoflex HR, and a protocol for bonding pHEMA to hydrogel grafts. A formulation, and a protocol for fabricating a urine compatible biomaterial are proposed, and numerous laboratory experiments are presented that demonstrate the responsive nature of the gel in environments mimicing urological and physiological situations. In vitro tests using physiological urine are presented that clearly demonstrate the shrink/swell behavior of the gel. Results of short term In vivo tests (6 months), showing no calcium encrustation of these materials, are also presented.The availability of a responsive material of this nature, for use in the changing environment of the urinary tract, lays the foundation for an entirely new class of materials that may have numerous applications in other non-regulated environments, such as those found in the intestinal tract, or bile ducts, or in non-physiological environments such as pipes or ships, where a responsive surface may provide an economical way to effect descaling

Ab externo implantation of the MicroShunt, a poly (styrene-block-isobutylene-block-styrene) surgical device for the treatment of primary open-angle glaucoma: a review

Trabeculectomy remains the ‘gold standard’ intraocular pressure (IOP)-lowering procedure for moderate-to-severe glaucoma; however, this approach is associated with the need for substantial post-operative management. Micro-invasive glaucoma surgery (MIGS) procedures aim to reduce the need for intra- and post-operative management and provide a less invasive means of lowering IOP. Generally, MIGS procedures are associated with only modest reductions in IOP and are targeted at patients with mild-to-moderate glaucoma, highlighting an unmet need for a less invasive treatment of advanced and refractory glaucoma. The PRESERFLO® MicroShunt (formerly known as InnFocus MicroShunt) is an 8.5 mm-long (outer diameter 350 μm; internal lumen diameter 70 μm) glaucoma drainage device made from a highly biocompatible, bioinert material called poly (styrene- block -isobutylene- block -styrene), or SIBS. The lumen size is sufficiently small that at normal aqueous flow hypotony is avoided, but large enough to avoid being blocked by sloughed cells or pigment. The MicroShunt achieves the desired pressure range in the eye by draining aqueous humor from the anterior chamber to a bleb formed under the conjunctiva and Tenon’s capsule. The device is implanted ab externo with intraoperative Mitomycin C via a minimally invasive (relative to incisional surgery) surgical procedure, enabling precise control of placement without the need for gonioscopy, suture tension control, or suture lysis. The implantation procedure can be performed in combination with cataract surgery or as a standalone procedure. The MicroShunt received Conformité Européenne (CE) marking in 2012 and is intended for the reduction of IOP in eyes of patients with primary open-angle glaucoma in which IOP remains uncontrolled while on maximum tolerated medical therapy and/or in which glaucoma progression warrants surgery. Three clinical studies assessing the long-term safety and efficacy of the MicroShunt have been completed; a Phase 3 multicenter, randomized clinical study comparing the MicroShunt to primary trabeculectomy is underway. In preliminary studies, the MicroShunt effectively reduced IOP and use of glaucoma medications up to 3 years after implantation, with an acceptable safety profile. This article summarizes current literature on the unique properties of the MicroShunt, the preliminary efficacy and safety findings, and discusses its potential use as an alternative to trabeculectomy for glaucoma surgery

The Interaction of Urea with the Generic Class of Poly(2-hydroxyethyl Methacrylate) Hydrogels

Work reported here shows that, contrary to reports in the literature, hydrogels made from pure poly(2-hydroxyethyl methacrylate), pHEMA, at crosslinker content greater than 0. 15 mol% do not swell above the usual equilibrium values of 39-42% water content in aqueous urea solution. However, hydrogels containing small (impurity) amounts of methacrylic acid (MAA) do swell dramatically (approximately 90%) in dilute urea solution, but not directly due to the urea. the urea decomposes to produce ammonium ions, thus raising the pH of the solution. Ionization of MAA occurs above pH 6, causing electrostatic interactions within the gel. the grossly swollen state of these gels represents an internal equilibrium among forces due to rubber elasticity, polymer-polymer/solvent affinity, and electrostatic interactions

Physiochemical Factors That Affect Ionic Hydrogels (PHEMA/MAA) as Materials for Use in the Urinary Tract

Prosthetic hydrogel ureters, made from poly(2-hydroxyethyl methacrylate), pHEMA, have demonstrated resistance to calcification during long term implantation in the urinary tract of dogs (1). Recent data suggests that impurity quantities of an ionizable comonomer, methacrylic acid, MAA, incorporated into the structure of the gel, may play a critical role in the calcification resistance of these materials. The ionic constituent of MAA in pHEMA leads to complex physicochemical behavior of gels in environments where the pH, ionic strength, and other aspects of the electrochemical solvent composition may vary. Specifically, small changes of pH or electrolyte near a critical value can produce massive changes of physical properties, such as the swollen volume or water content of these gels. Urine exhibits a changeable composition, and importantly, the critical pH and electrolyte content of pHEMA/MAA copolymers is within the common range of urine compositions. The physiochemical changes of these copolymers were exploited in the design of a prosthetic material for use in the urinary tract

A phospholipid-modified polystyrene-polyisobutylene-polystyrene (SIBS) triblock polymer for enhanced hemocompatibility and potential use in artificial heart valves

Poly(styrene-block-isobutylene-block-styrene) (\u27SIBS\u27) is selected for a novel trileaflet heart valve due to its high resistance to oxidation, hydrolysis, and enzyme attack. SIBS is modified using six different phospholipids and its mechanical properties characterized by tensile stress, peel strength, shear strength, contact angle, and surface energy, and then for hemocompatibility by studying the adhesion of fluorescently labeled platelets in a parallel plate chamber under physiological flow conditions. Phospholipid modification decreases SIBS tensile stress (at 45% strain) by 30% and reduces platelet adhesion by a factor of 10, thereby improving its hemocompatibility and its potential use as a synthetic heart valve. © SAGE Publications 2009

A novel small animal model for biocompatibility assessment of polymeric materials for use in prosthetic heart valves

A composite polymeric material, poly(styreneblock-isobutylene-block- styrene) (SIBS) with an embedded reinforcement polyethylene terephthalate (PET) fabric, is undergoing investigation for potential use in a novel heart valve. The purpose of this study was to develop and implement a small animal model to assess the biocompatibility of composite samples in a cardiovascular tissue and blood-contacting environment. Composite samples were manufactured using dip coating and solvent casting with two coating thicknesses (25 and 50 lm). A novel rat abdominal aorta model was developed to test the dip-coated samples in a similar pulsatile flow condition to its intended use, and both dip-coated and solvent-cast samples were tested using a rat subcutaneous model. Tissue response, defined by degree of cellular infiltration and encapsulation, was minimized when a smooth coating of SIBS shielded the PET fabric from exposure to blood and tissue, and the degree of tissue response was directly correlated with the degree of surface roughness. Surface modification using phospholipid further reduces the tissue response. We have demonstrated the applicability of using a novel rat abdominal aorta model for biocompatibility assessment of polymeric materials that will be used in cardiovascular implants. For the purpose of this study, our results indicate that solvent casting with a 25-lm SIBS coating thickness will provide optimal biocompatibility for the SIBS valve. © 2009 Wiley Periodicals, Inc