Properties of poly(dimethylsiloxane)/hydrogel multicomponent systems

The properties of surface‐ and bulk‐modified poly(dimethylsiloxane) (PDMS) were examined. Laser‐induced surface grafting of poly(2‐hydroxyethyl methacrylate) (PHEMA) on PDMS and a sequential method for preparation of interpenetrating polymer networks of PDMS/PHEMA were, respectively, used for surface and bulk modifications. The hydrogel content and water‐uptake capability of the modified samples were also investigated. The modified PDMS samples were examined by performing attenuated total reflection/Fourier transform infrared spectroscopy, dynamic mechanical thermal analysis, scanning electron microscopy, and water contact‐angle measurements. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2145–2156, 2003     

Hydrophilic surface modification of polydimetylsiloxane‐co‐2‐hydroxyethylmethacrylate (PDMS‐HEMA) by Silwet L‐77 (heptamethyltrisiloxane) surface treatment

Biomaterials and their host organism's quintessential place of interaction are the surfaces of materials, as transportation of liquids within microchannels requires hydrophilic surfaces. Modifying the hydrophobic surface of polydimethylsiloxane (PDMS) into a hydrophilic one which can be used in biomaterials remains a big challenge. Herein, PDMS‐hydroxyethylmethacrylate (HEMA) films were prepared by the condensation of PDMS using isophorone diisocyanate as a cross‐linker, followed by the incorporation of HEMA via radical copolymerization. The as‐prepared PDMS‐HEMA films were thereafter hydrophilized via physical treatment with heptamethyltrisiloxane. The surface properties of the obtained PDMS‐HEMA films were characterized in wettability, morphology, topography, swelling, mechanical properties, and protein adsorption. Compared to pristine PDMS‐HEMA as control, the surface wettability, roughness, and protein adsorption of the hydrophilized PDMS‐HEMA films were significantly improved while the films also exhibited excellent optical properties. However, the improvement of the swelling properties remains insignificant, indicating that the interior morphology was still based on the hydrophobic siloxane PDMS. The long‐term hydrophilicity was considered good as no significant hydrophobic recovery was noticeable in a period of 5 months after treatment.     

Surface modification of poly(2‐hydroxyethyl methacrylate) hydrogel for contact lens application

The present study examined poly(2‐hydroxyethyl methacrylate) (PHEMA)‐based hydrogels that have been extensively used in biomedical applications, including contact lens. In this research, we aimed to reduce adsorption of protein components from tears and bacterial deposition by surface modification of the hydrogel with different functional groups that included carboxylic acid, primary amine, and quaternary ammonium. The PHEMA was treated with a solution of sulfuric acid for partial hydrolysis of the HEMA ester groups to induce acid groups on the surface of the hydrogel. Carboxylic acid groups of the modified PHEMA were converted to primary amine and quaternary ammonium groups via carbodiimide chemistry. The surface physical and chemical properties of different samples were investigated by atomic force microscopy and X‐ray photoelectron spectroscopy, respectively. We conducted the bicinchoninic acid assay to evaluate protein deposition from artificial tear fluid on samples. Antibacterial properties of the modified hydrogels were investigated with a culture of Staphylococcus aureus, one of the major causes of eye infections. Our data showed that positively charged amine and ammonium groups efficiently resisted protein adsorption and bacterial deposition compared to alcohol and carboxylic acid groups.     

Preparation and characterization of poly(acrylic acid)‐iron rich smectite superabsorbent composites

A novel poly(acrylic acid)‐iron rich smectite (IRS) superabsorbent composite was synthesized by graft copolymerization reaction of acrylic acid (AA) in the presence of N,N‐methylenebisacrylamide (MBA) as a crosslinker. IRS was used to strengthen the hydrogel products in the polymerization process. Water absorbencies for these superabsorbent composites in water and saline solutions were investigated. IRS caused a reduced equilibrium swelling as low as 8–26%. However, grafted IRS particles resulted in improved gel strength as high as 66% compared to the IRS‐free sample. IRS modified superabsorbent hydrogel composites exhibited higher thermal stability compared to the IRS‐free sample. The pH dependent reversible swelling behavior of hydrogels was also investigated. It is found that the swelling process is pH dependent and reversible for synthesized superabsorbent. Superabsorbent hydrogel composites were characterized by thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). FTIR spectroscopy was confirmed grafting of acrylic chains onto the surface of IRS particles. From the standpoint of these results, these strengthened and thermostabilized hydrogels may be considered as good candidates for a controlled release study and agricultural applications. Copyright © 2007 John Wiley & Sons, Ltd.     

Improving the hydrophilic and antifouling properties of poly(vinyl chloride) membranes by atom transfer radical polymerization grafting of poly(ionic liquid) brushes

In this study, poly(1‐butyl‐3‐vinylimidazolium bromide) (PBVIm‐Br) was grafted onto the poly(vinyl chloride) (PVC) membrane surface via a 2‐step atom transfer radical polymerization (ATRP) reaction. Poly(2‐hydroxyethylmethacrylate) (PHEMA) was grafted onto the membrane surface by aqueous ATRP reaction; then, BVIm‐Br was introduced onto the surface of the PHEMA‐modified PVC membrane through traditional ATRP reaction. The analysis of surface chemistry confirmed the successful grafting of PHEMA and PBVIm‐Br on PVC membrane surface, and the grafting density (GD) of PBVIm‐Br gradually increased as the grafting time was prolonged. The modified membrane exhibited a positive charge and significantly enhanced surface hydrophilicity. The static water contact angle of the membrane surface decreased from 92.3° to 51.6° as the GD of the PBVIm‐Br brushes increased. Filtration experiments indicated that the water flux of the modified membrane increased with increasing GD, and their recovered fluxes were more than twice than the original. In addition, the total fouling ratio of the membranes decreased from 89% in M0 to 67% in M5, and most of the fouling was reversible as the GD of PBVIm‐Br brushes increased. These results indicated that the positive charged poly(ionic liquid) brushes featuring hydrophilic properties would have potential applications in membrane separation.     

Graft polymerization of 2-hydroxyethyl methacrylate via ATRP with poly(acrylonitrile-co-p-chloromethyl styrene) as a macroinitiator

Amphiphilic graft copolymers are excellent additives for the development of antifouling membranes by nonsolvent induced phase separation. We report a convenient approach to the synthesis of novel graft copolymers with hydrophobic polyacrylonitrile (PAN) backbones and hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) side chains. Atom transfer radical polymerization (ATRP) of 2-hydroxyethyl methacrylate was carried out with poly(acrylonitrile-co-p-chloromethyl styrene) (PAN-co-PCMS) as a macroinitiator in the presence of CuCl/2,2’-bipyridine at 50 °C in dimethyl sulfoxide. Kinetics of the graft polymerization was also evaluated. The synthesis of poly(acrylonitrile-co-p-chloromethyl styrene-g-2-hydroxyethyl methacrylate) (PAN-co-(PCMS-g-PHEMA)) can be relatively controlled when CMS (the ATRP sites) unit in the macroinitiator is around 5 mol%. Both the macroinitiators and graft copolymers were characterized by FTIR, NMR and GPC. The surface morphology and wettability of the copolymer films were studied by AFM and water contact angle measurement, respectively. We demonstrate that phase segregation between the PAN-co-PCMS backbones and the PHEMA side chains takes place and the surface hydrophilicity of the graft copolymers increases with the length of the PHEMA side chains. Because these amphiphilic graft copolymers can be synthesized in mass, they will be useful as latent additives for the fabrication of advanced PAN separation membranes.     

Three-dimensional interconnected microporous poly(dimethylsiloxane) microfluidic devices

This technical note presents a fabrication method and applications of three-dimensional (3D) interconnected microporous poly(dimethylsiloxane) (PDMS) microfluidic devices. Based on soft lithography, the microporous PDMS microfluidic devices were fabricated by molding a mixture of PDMS pre-polymer and sugar particles in a microstructured mold. After curing and demolding, the sugar particles were dissolved and washed away from the microstructured PDMS replica revealing 3D interconnected microporous structures. Other than introducing microporous structures into the PDMS replica, different sizes of sugar particles can be used to alter the surface wettability of the microporous PDMS replica. Oxygen plasma assisted bonding was used to enclose the microstructured microporous PDMS replica using a non-porous PDMS with inlet and outlet holes. A gas absorption reaction using carbon dioxide (CO(2)) gas acidified water was used to demonstrate the advantages and potential applications of the microporous PDMS microfluidic devices. We demonstrated that the acidification rate in the microporous PDMS microfluidic device was approximately 10 times faster than the non-porous PDMS microfluidic device under similar experimental conditions. The microporous PDMS microfluidic devices can also be used in cell culture applications where gas perfusion can improve cell survival and functions.     

Improvement of thermal conductivity of poly(dimethyl siloxane) using silica-coated multi-walled carbon nanotube

In order to enhance the thermal conductivity of MWCNT filled poly(dimethyl siloxane) (PDMS) composites, the MWCNT was coated with silica layer by three step reactions. The composites filled with raw and silica-coated MWCNTs were prepared and the properties were investigated in terms of the curing characteristics, mechanical properties, and thermal conductivity. Due to the poor compatibility between raw MWCNT and PDMS, raw MWCNT showed poor dispersion uniformity and wettability in PDMS. On the other hand, due to the chemical affinity between silica/MWCNT and PDMS throughout the hydrogen bonding, the silica-coated MWCNT filled PDMS showed improved mechanical properties in terms of tensile strength and 100% modulus, and good interfacial compatibility than raw MWCNT incorporated PDMS. Finally, the good wettability of silica/MWCNT in PDMS resulted in higher thermal conductivity caused from the facile phonon movement at the interface even with the smaller MWCNT contents.     

Influence of gel composition on the solubility parameter of poly(2‐hydroxyethyl methacrylate‐itaconic acid) hydrogels

The solubility parameters of pure poly(2‐hydroxyethyl methacrylate) (PHEMA) and poly(2‐hydroxyethyl methacrylate/itaconic acid) [P(HEMA/IA)] hydrogels were determined by 20 solvents with various solubility parameters in swelling experiments. The solubility parameter of pure PHEMA was 26.93 ± 0.46 (MPa)^1/2. The effect of mole percentages of itaconic acid (IA) in P(HEMA/IA) hydrogels on the solubility parameter was investigated. The measured values were compared to literature and solubility values theoretically determined by group contribution values of van Krevelen and Hoy. The incorporation of IA into the hydrogel system slightly increased the solubility parameter. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1995–2003, 2002     

Robust solvent‐free fabrication and characterization of (polydimethylsiloxane‐co‐2‐hydroxyethylmethacrylate)/poly (ethylene glycol) methacrylate (PDMS‐HEMA)/PEGMA hydrogels

Combining hydrophobic materials such as polydimethylsiloxane (PDMS), a natural hydrophobic material with typical hydrophilic monomers without using organic solvent remains a big challenge due to their extreme different properties; hence, fabricating a class of silicone hydrogels with two extremes without use of organic solvents could bring us a novel class of silicone hydrogels. Herein, a range of PDMS‐HEMA‐PEGMA hydrogels was prepared from hydroxyl‐terminated PDMS, 2‐hydroxyethylmethacrylate (HEMA), poly (ethylene glycol) methacrylate (PEGMA), and isophorone diisocyanate via condensation and radical copolymerization reactions. The infrared results confirmed the PDMS‐HEMA‐PEGMA network formation, while the hydrophilicity of the as‐prepared block copolymer was dependent on (PDMS‐HEMA)/PEGMA ratio. Increasing the PEGMA content resulted in increased equilibrium water content, phase separation, surface roughness, and tensile strength, while the tensile modulus, elongation at break, optical transmittance, water contact angle, and oxygen permeability (Dk) were decreasing. At PEGMA content of 28.3%, the relative protein adsorption ratio decreased to 20% and 36% for bovine serum albumin and lysozyme, respectively, compared with that of the control (PDMS‐HEMA), suggesting antiprotein adsorption ability. In overall, the results showed that the PDMS‐HEMA‐PEGMA hydrogels not only exhibited remarkable hydrophilicity and suppressed protein adsorption but also maintained higher optical transparency and oxygen permeability (Dk).     

Surface modification of polydimethylsiloxane with photo-grafted poly(ethylene glycol) for micropatterned protein adsorption and cell adhesion

In this study, we applied photo-induced graft polymerization to micropatterned surface modification of polydimethylsiloxane (PDMS) with poly(ethylene glycol). Two types of monomers, polyethylene glycol monoacrylate (PEGMA) and polyethylene glycol diacrylate (PEGDA), were tested for surface modification of PDMS. Changes in the surface hydrophilicity and surface element composition were characterized by contact angle measurement and electron spectroscopy for chemical analysis. The PEGMA-grafted PDMS surfaces gradually lost their hydrophilicity within two weeks. In contrast, the PEGDA-grafted PDMS surface maintained stable hydrophilic characteristics for more than two months. Micropatterned protein adsorption and micropatterned cell adhesion were successfully demonstrated using PEGDA-micropatterned PDMS surfaces, which were prepared by photo-induced graft polymerization using photomasks. The PEGDA-grafted PDMS exhibited useful characteristics for microfluidic devices (e.g. hydrophilicity, low protein adsorption, and low cell attachment). The technique presented in this study will be useful for surface modification of various research tools and devices.     

Antibacterial activity of pH‐sensitive genipin cross‐linked chitosan/poly(ethylene glycol)/silver nanocomposites

Novel nanocomposites consisting of genipin cross‐linked chitosan (GC), poly(ethylene glycol) (PEG), and silver nanoparticles were prepared for such biomedical applications as the wound‐healing materials. Various amounts of silver nanoparticles were dispersed in the GC/PEG hydrogel matrix without severe aggregation. The effects of composition and silver nanoparticles on the physico‐chemical properties of samples were evaluated by infrared analysis, contact angle measurements, and swelling tests. The GC/PEG/Ag nanocomposite showed a pH‐sensitive swelling behavior. The surface hydrophilicity of GC/PEG/Ag nanocomposites was improved with the increase of silver nanoparticle content. L929 cell attachment was improved in the presence of silver nanoparticles. The antimicrobial function was assessed for the GC/PEG/Ag nanocomposites containing the silver content over 100 ppm. The silver nanoparticles had the dual functions of reinforcing structural stability and enhancing antimicrobial activity of GC/PEG/Ag nanocomposites. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydrophobic interaction in poly(2-hydroxyethyl methacrylate) homogeneous hydrogel

Homogeneous poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel exhibits a narrow range of swelling at equilibrium in water (% H₂O, 41.09 ± 0.15 standard error of the mean of 24 samples), regardless of the dilution of the monomer solution and relatively low level of crosslinking. It is postulated that PHEMA hydrogel has, in addition to its covalently linked network structure, a secondary structure stabilized by hydrophobic bonding. The addition of microsolutes to the hydrogel seems to confirm this hypothesis. The hydrogel swells beyond its swelling equilibrium in water in presence of urea and its methyl derivatives. Swelling is also induced by organic solvents like alcohol and acetone, and by anions like iodide, acetate, trichloroacetate, and thiocyanate. Chlorides and sulfates produce a less swollen hydrogel than pure water, while bromides and cetylpyridinium chloride, in the concentrations tested, induce only a slight deswelling of the gel. When PHEMA gel prepared in organic solvent–water solutions is placed in water, the gel passes through an opaque state before becoming transparent again. This phenomenon is interpreted as being caused by the inability of water to solvate the hydrophilic ends of the unorganized polymer segments. Homogeneity returns to the gel after a rearrangement of the chains, directed by the interaction of the hydrophobic portions of the polymer segments, exposing to the solvent–water most of the hydrophilic sites in the network.     

Nonswellable hydrogels with robust micro/nano-structures and durable superoleophobic surfaces under seawater

Hydrogels, composed mainly of water trapped in three dimensional cross-linked polymer networks, have been widely utilized to construct underwater superoleophobic surfaces. However, the swelling nature and instability of hydrogels under complex marine environment will weaken their underwater superoleophobicity. Herein, we synthesize structured poly(2-hydroxyethylmethacrylate)(PHEMA) hydrogels by using sandpaper as templates. The robust non-swelling of PHEMA hydrogel ensures that micro/nano-structures on the surface of PHEMA hydrogels can be well maintained. Moreover, when roughness Ra of about 3~4 μm, the surface has superior oil-repellency. Additionally, even after immersing in seawater for one-month, their breaking strength and toughness can be well kept. The non-swellable hydrogels with long-term stable under seawater superoleophobicity will promote the development of robust superoleophobic materials in marine antifouling coatings,biomedical devices and oil/water separation.     

Interfacial properties of hydrophilized poly(lactic-co-glycolic acid) layers with various thicknesses

Biodegradable polyesters such as poly(lactic-co-glycolic acid) copolymers (PLGA) are preferred materials for drug carrier systems although their surface hydrophobicity greatly limits their use in controlled drug delivery. PLGA thin films on a solid support blended with PEG-containing compound (Pluronic) were used as model systems to study the interfacial interactions with aqueous media. Degree of surface hydrophilization was assessed by wettability, and X-ray photoelectron spectroscopy (XPS) measurements. Protein adsorption behavior was investigated by in situ spectroscopic ellipsometry. The degree of protein adsorption showed a good correlation with the hydrophilicity, and surface composition. Unexpectedly, the layer thickness was found to have a great impact on the interfacial characteristics of the polymer films in the investigated regime (20-200 nm). Thick layers presented higher hydrophilicity and great resistance to protein adsorption. That special behavior was explained as the result of the swelling of the polymer film combined with the partial dissolution of Pluronic from the layer. This finding might promote the rational design of surface modified biocompatible nanoparticles.     

Surface modification of poly(dimethylsiloxane) for retarding swelling in organic solvents

A method of alleviating swelling problems of poly(dimethysiloxane) (PDMS) molds in organic solvents is developed that allows repeated use of the molds without deleterious solvent effects. The method involves surface modification of PDMS surface with poly(urethaneacrylate) that results in a partially modified PDMS surface. This modification leads to a significant reduction in the rate of solvent absorption into PDMS such that the swelling can be controlled.     

Synthesis of poly(sorbitan methacrylate) hydrogel by free-radical polymerization

Hydrogels are materials with the ability to swell in water through the retention of significant fractions of water within their structures. Owing to their relatively high degree of biocompatibility, hydrogels have been utilized in a host of biomedical applications. In an attempt to determine the optimum conditions for hydrogel synthesis by the free-radical polymerization of sorbitan methacrylate (SMA), the hydrogel used in this study was well polymerized under the following conditions: 50% (w/v) SMA as monomer, 1% (w/w) alpha, alpha'-azo-bis(isobutyro-nitrile) as thermal initiator, and 1% (w/w) ethylene glycol dimethacrylate as cross-liking agent. Under these conditions, the moisture content of the polymerized SMA hydrogel was higher than in the other conditions. Moreover, the moisture content of the poly(SMA) hydrogel was also found to be higher than that of the poly(methyl methacrylate [MMA]) hydrogel. When the Fourier transform-infrared spectrum of poly(SMA) hydrogel was compared with that of poly(MMA) hydrogel, we noted a band at 1735-1730/cm, which did not appear in the Fourier transform-infrared spectrum of poly(MMA). The surface of the poly(SMA) hydrogel was visualized through scanning electron microscopy, and was uniform and clear in appearance.     

Water sorption properties of poly(ethyl acrylate-co-hydroxyethyl methacrylate) macroporous hydrogels

Synthetic porous hydrogels are becoming more and more important in the field of biomaterials. Different studies demonstrate that the porous structure promotes the colonisation of living cells and improves the biocompatibility of the implants. The macroporous structure allows not only the control of cellular ingrowth morphology but also the mechanical integration and the regulation of nutrient and hydraulic flow in the hydrogel. In this work poly(ethyl acrylate-co-hydroxyethyl methacrylate) (PEA/PHEMA) copolymers were polymerized using 2% of ethylene glycol dimethacrylate as cross-linking agent and azoiso-botyronitrile as initiator. Five samples were prepared with the EA/HEMA weight ratios of 75/25, 50/50, 25/75 and pure PEA and PHEMA polymers, obtaining different degrees of hydrophilicity. The macroporous structure was obtained by adding poly(acrylonitrile) fibres to the monomers. After polymerization the fibres were eliminated by dissolution in dimethyl formamide. The holes are cylinders of approximately 40μm diameter and are all, more or less, in the same direction, although they are not uniformly distributed. Water sorption isotherms and diffusion properties of the macroporous samples are compared with the samples without holes.     

Synthesis and characterization of thermoresponsive poly(ethylene glycol)‐based hydrogels and their magnetic nanocomposites

Temperature‐responsive hydrogels are one of the most widely studied types of stimuli‐responsive hydrogel systems. Their ability to transition between their swollen and collapsed states makes them attractive for controlled drug delivery, microfluidic devices, and biosensor applications. Recent work has shown that poly(ethylene glycol) (PEG) methacrylate polymers are temperature‐responsive and exhibit a wide range of lower critical solution temperatures based on the length of ethylene glycol units in the macromer chain. The addition of iron oxide nanoparticles into the hydrogel matrix can provide the ability to remotely heat the gels upon exposure to an alternating magnetic field (AMF). In this work, diethylene glycol (n = 2) methyl ether methacrylate and PEG (n = 4.5) methyl ether methacrylate copolymers were polymerized into hydrogels with 5 mol % PEG 600 (n = 13.6) dimethacrylate as the crosslinker along with 5 wt % iron oxide nanoparticles. Volumetric swelling studies were completed from 22 to 80 °C and confirmed the temperature‐responsive nature of the hydrogel systems. The ability of the gels to collapse in response to rapid temperature changes when exposed to an AMF was demonstrated showing their potential use in biomedical applications such as controlled drug delivery and hyperthermia therapy. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3229–3235, 2010     

Swelling kinetics of poly(aspartic acid)/poly(acrylic acid) semi‐interpenetrating polymer network hydrogels in urea solutions

Semi‐interpenetrating network (semi‐IPN) hydrogels, composed of poly(aspartic acid) (PAsp) and poly(acrylic acid) (PAAc) with various ratios of PAsp to AAc, were prepared. In this work, swelling kinetics was investigated through calculating some parameters. The swelling ratios were measured at room temperature, using urea solutions as liquids to be absorbed. Compared to in deionized water, the hydrogels showed larger swelling ratios in urea solutions, which might be attributed to the chemical composition of urea. The equilibrium swelling ratio could achieve 600 g/g, and the equilibrium urea/water contents were more than 0.99. The diffusion exponents were between 0.5 and 0.7, suggesting that the solvent transport into the hydrogel was dominated by both diffusion and relaxation controlled systems. Therefore, the PAsp/PAAc semi‐IPN hydrogels were appropriate to carry substances in a urea/water environment for pharmaceutical, agricultural, environmental, and biomedical applications. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 666–671, 2010