Kwang-Duk Ahn

Surface modification of biodegradable electrospun nanofiber scaffolds and their interaction with fibroblasts.

Biodegradable polymers, such as poly(glycolic acid) (PGA), poly(L-lactic acid) (PLLA) and poly(lactic-co-glycolic acid) (PLGA), were dissolved individually in the proper solvents and then subjected to electrospinning process to make nanofibrous scaffolds. Their surfaces were then chemically modified using oxygen plasma treatment and in situ grafting of hydrophilic acrylic acid (AA). The fiber thickness, pore size and porosity were estimated to 200–800 nm, 2–30 μm and 94–96%, respectively, and these properties were insignificant in the PGA, PLLA and PLGA nanofibrous scaffolds. The ultimate tensile strength of PGA was about 2.5 MPa on average and that of PLGA and PLLA was less than 2 MPa. The elongation-at-break was 100–130% for the three nanofibrous scaffolds. When the surface properties of AA-grafted scaffolds were examined, higher ratios of oxygen to carbon, lower contact angles and the presence of carboxylic (–COOH) groups were identified. The properties were significantly different from those of the unmodified nanofibrous scaffolds. Fibroblasts once seeded on the scaffolds were spreading over large surface area on the AA-grafted surface as compared to the unmodified PGA, PLLA and PLGA nanofibrous scaffolds. Cultured for up to 6 days, the fibroblast proliferation was found to be much better on the surface-modified nanofibrous scaffolds. The present study showed that, with the use of plasma treatment and AA grafting, the hydrophilic functional groups could be successfully adapted on the surface of electrospun nanofibrous scaffolds. Those surface-modified scaffolds made significant improvement on cell attachment and proliferation in vitro.

Synthesis and photopolymerization of multifunctional methacrylates derived from Bis-GMA for dental applications

Two multimethacrylates having three methacrylate groups (BPA-3M) and four methacrylate groups (BPA-4M) have been prepared by reacting hydroxyl groups of 2,2-bis[4-(2′-hydroxy-3′-methacryloyloxypropoxy)phenyl]propane (Bis-GMA) with methacryloyl chloride. BPA-3M and BPA-4M have much lower viscosities than the starting Bis-GMA, because they have only one or no hydroxyl group. Photopolymerizations of the multifunctional methacrylates were conducted by exposure to visible light using camphorquinone and 2-(N,N-dimethylamino)ethyl methacrylate as a photoinitiating system. High conversions >50% resulted from photopolymerization of BPA-3M, whereas Bis-GMA showed lower conversions under the same condition, implying better mechanical properties for the composite resins made from BPA-3M. BPA-4M showed much lower conversions in the photopolymerization condition. Water sorption of the photocured composite of BPA-3M containing 50 wt % of inorganic fillers was found to be 0.15%, which is only one-tenth of the commercial Bis-GMA composite.

Synthesis and polymerization of t-BOC protected maleimide monomers: N-(t-butyloxycarbonyloxy)maleimide and N-[p-(t-butyloxycarbonyloxy)phenyl]-maleimide

Abstract Two t-butyloxycarbonyl (t-BOC) protected maleimide monomers, N -(t-butyloxycarbonyloxy)maleimide (t-BOCOMI) and N -[ p -(t-butyloxycarbonyloxy)phenylmaleimide (t-BOCOPMI), have been synthesized and polymerized. The t-BOC group was used to protect the hydroxy groups of the starting N -substituted maleimides, N -hydroxymaleimide (HOMI) and N-(p -hydroxyphenyl)maleimide (HOPMI). t-BOCOMI and t-BOCOPMI were copolymerized with styrene derivatives (X-St) to obtain the t-BOC protected polymers P(t-BOCOMI/X-St) and P(t-BOCOPMI/X-St) in high conversions. P(t-BOCOMI/St) and P(t-BOCOPMI/St) were cleanly deprotected by heating the polymers to 110 and 175°C, respectively. The deprotection temperature of the copolymers of t-BOCOMI (ca. 100°C) is the lowest among the known t-BOC protected polymers. Deprotection of P(t-BOCOMI/X-St) and P(t-BOCOPMI/X-St) produced the corresponding polymers having HOMI and HOPMI structures, respectively, and resulted in large changes in physical properties. The deprotected polymers have very high glass transition temperatures (above 250°C) and good solubility in aqueous base solutions, whereas the t-BOC polymers are only soluble in organic solvents. P(t-BOCOMI/St) was found to have desirably low ultra-violet absorption and high sensitivity in resist formulations.

Preparation and surface properties of PEO-sulfonate grafted polyurethanes for enhanced blood compatibility

In order to improve the thromboresistance of the commercial polyurethane(PU), its surface modification was accomplished by three new different methods and their surface characteristics were investigated using ATR-FTIR, ESCA, SEM, and dynamic contact angle measurements. Sulfonations using propane sultone were performed directly onto PU or onto hydrophobic dodecanediol (DDO) grafted PU or onto hydrophilic poly(ethyleneoxide) (PEO) grafted PU. ESCA data coincided well with ATR-IR results, as more 0 at. % for PEO grafted PUs and the presence of S for the sulfonated PUs were revealed. At SEM observation the surfaces of PU-DDO and PU-PEO were relatively smooth, whereas all the sulfonated PU surfaces showed excellent smoothness and homogeneity. The hydrophilicity of the surfaces was considerably increased after PEO grafting or sulfonation. In addition, all the sulfonated PU surfaces, particularly PU-PEO-SO3, which has further hydrophilicity, exhibited complete wetting behavior due to the negatively charged SO3 groups.

Wafer distribution system for a clean room using a novel magnetic suspension technique

A linear transport system, which is capable of reducing the weight of a moving carrier by separating power-supplying devices, is developed by using a new magnetic levitation technique. This system is designed to distribute a wafer between semiconductor fabrication process modules in clean rooms, because it can eliminate particles and oil contamination that normally exist in conventional transporter systems due to rubbing of mechanical components. The transport system consists of a wafer carrier, two levitation tracks, two stabilization tracks, and a propelling system. Levitation is achieved by using opposing forces produced between electromagnet tracks and permanent magnets. Stabilization is achieved by using a simple feedback control. The continuous propelling force is obtained by sending specific current patterns to the propulsion coils. The dynamic model of the transport system is presented, and it is verified by experiment. The system performance is experimentally investigated.

Synthesis and polymerization of N-(tert-butyloxycarbonyl)maleimide and facile deprotection of polymer side-chain t-BOC groups

A new t-BOC protected monomer, N-(tert-butyloxycarbonyl)maleimide (t-BOCMI) 3 was synthesized by thermolysis in a high yield and a new kind of t-BOC protected polymer was prepared by radical copolymerization. t-BOCMI and styrene derivatives (X-St) were readily copolymerized to obtain alternating copolymers of P(t-BOCMI/X-St) 5 in high conversions. The t-BOCMI units of the t-BOC protected copolymers were converted to the maleimide (MI) units by heating at about 150°C or above, releasing 2-methylpropene and carbon dioxide. The facile deprotection of the side-chain t-BOC groups from the protected copolymers was attained to bring about a large polarity change. The deprotected copolymers P(MI/Y-St) 6 have very high glass transition temperatures (Tgs of about 250°C and good solubility in aqueous base solutions, whereas the t-BOC polymers P(t-BOCMI/X-St) are only soluble in organic solvents. The t-BOC protected maleimide polymers were found to have specific properties for application as chemically amplified resist materials such as alkaline solubility, high Tg and facile deprotection.

Polymerization of N-(tert-butyldimethylsilyloxy)maleimide and applications of the polymers as resist materials

A new silicon-containing maleimide monomer, N-(tert-butyldimethylsilyloxy)-maleimide (SiOMI) has been synthesized. SiOMI was radically copolymerized with styrene derivatives (XSt) to obtain alternating copolymers, P(SiOMI/XSt), in high conversions. The copolymers have high glass transition temperatures above 190°C, and the tert-butyldimethylsilyloxy groups are thermally stable up to 300°C. The SiOMI units in the copolymers were converted into N-hydroxymaleimide (HOMI) units by acidolytic deprotection of the tert-butyldimethylsilyloxy protecting groups. The facile deprotection of the side-chain tert-butyldimethylsilyloxy groups from the protected copolymers provided a significant change in solubility of the polymers due to the large polarity change. Submicron positive-tone images were obtained from the copolymers containing an onium salt as a photoacid generator by irradiation with electron beam and development with alkaline solutions. The polymer films also showed very high oxygen plasma etch resistance compared with novolac resins. The silicon-containing maleimide polymers were found to have required properties, such as good alkaline solubility after deprotection, superior adhesion, low optical density, high thermal stability with high Tg, and high plasma etch resistance for applications as deep ultraviolet and electron beam resist materials.

Photochemical acid generation from copolymers based on camphorsulfonyloxymaleimide and acidolytic deprotection

A new sulfonyloxymaleimide monomer, N-(10-camphorsulfonyloxy)maleimide (CSOMI), has been synthesized and copolymerized with styrene derivatives (XSt) to make photoacid-generating polymers P(CSOMI/XSt). The amount of photochemically generated 10-camphorsulfonic acid (CSA) from the polymer films was determined in percent conversion of CSOMI units by colorimetry using tetrabromophenol blue as an acid indicator. The capability of photochemical generation of the bulky CSA was enhanced either by adding hydroquinone as a hydrogen donor or by incorporation of p-hydroxystyrene (HOSt) into the polymer chains as a XSt. Upon exposure to 250-nm light with 300 mJ cm−2 the film of the phenolic copolymer P(CSOMI/HOSt) was found to undergo 19% conversion of CSOMI units and it further increased to 23% conversion when 5 wt% hydroquinone was included in the film. The thermal and photoacidolytic deprotection of the tert-butoxycarbonyl (t-BOC) protected copolymers having CSOMI units was investigated in detail. The complete deprotection of side-chain t-BOC groups was accomplished by the photogenerated CSA, thereby providing a significant change in solubility of the polymers. Thus the applicability of the polymers as a single-component photoresist was verified.

Photoacid generating polymers based on sulfonyloxymaleimides and application as single-component resists

Three sulfonyloxymaleimides (RsOMI), N-(tosyloxy)maleimide (TsOMI), N-(methane-sulfonyloxy)maleimide (MsOMI), and N-(trifluoromethanesulfonyloxy)maleimide (TfOMI), have been synthesized and used to make novel photoacid generating polymers. The sulfonyloxymaleimides easily copolymerized with styrene derivatives to give high molecular weight polymers having an alternating structure of both monomer units. Terpolymers based on RsOMI and p-(tert-butyloxycarbonyloxy)styrene (t-BOCSt) were prepared for enhancing resist properties such as adhesion to substrates, solubility in aqueous alkaline solutions, or transparency in the deep-UV region. The RsOMI copolymers were found to produce corresponding sulfonic acids (RsOH), TsOH, MsOH, and TfOH, by deep-UV irradiation in the film state. Thus, the polymers having both the RsOMI and t-BOCSt units show the capability of a single-component, chemically amplified resist system in the deep-UV region without addition of any photoacid generator. Positive- and negative-tone images were obtained by exposure of the polymer films to deep-UV and post-exposure bake followed by development with organic solvents or aqueous alkaline solutions.

Optimal synthesis of a spring-actuated cam mechanism using a cubic spline

Abstract This paper presents the optimal synthesis for a cam with non-constant angular velocity based on the dynamic model of a complete spring-actuated cam system. In order to satisfy asymmetric constraints and guarantee continuous contact at the cam-follower interface, the follower motion, which is dependent on the requirements during the closing period of a vacuum circuit breaker, is optimized using a cubic spline. The optimized cam is compared with a cam designed by assuming the cam angular velocity to be constant and with a polynomial cam. Simulation results reveal that the dynamic behaviours of the optimized cam are superior to those of the original cam and the polynomial cam.