PDMS Surface Research Articles

One‐Step Synthesis of Hierarchical Structure Polydimethylsiloxane Films with Micro‐/Nanosurfaces for Application in Triboelectric Nanogenerators

Surface optimization of tribomaterial is an effective approach for enhancing the output performance of triboelectric nanogenerators (TENGs). Developing advanced surface structures and their mass fabrication method is the essential technology component of TENG‐based electronics. Herein, a facile, scalable synthesis of hierarchical structure polydimethylsiloxane (HS‐PDMS) films with micro‐/nanosurface morphology and their application in TENGS are demonstrated. The micromorphology of the PDMS surface with random microcraters is achieved using water droplet‐based microtemplates and their rapid evaporation approach. Simultaneously, the nanomorphology on the microcrater surface with nanoripples is automatically formed under the action of surface tension of the PDMS film. The effect of water droplet mass fraction and the rate of heating, which play key roles in the surface morphology and the device's electrical output control, are investigated. This fabrication method of HS‐PDMS is simple and scalable, providing a promising solution of developing tribomaterial for practical TENG‐based electronic devices.

Assessment of human adipose-derived stem cell on surface-modified silicone implant to reduce capsular contracture formation.

Medical devices made from poly(dimethylsiloxane) (PDMS)‐based silicone implants have been broadly used owing to their inert properties, biocompatibility, and low toxicity. However, long‐term implantation is usually associated with complications, such as capsular contracture due to excessive local inflammatory response, subsequently requiring implant removal. Therefore, modification of the silicone surface to reduce a risk of capsular contracture has attracted increasing attention. Human adipose‐derived stem cells (hASCs) are known to provide potentially therapeutic applications for tissue engineering, regenerative medicine, and reconstructive surgery. Herein, hASCs coating on a PDMS (hASC‐PDMS) or itaconic acid (IA)‐conjugated PDMS (hASC‐IA‐PDMS) surface is examined to determine its biocompatibility for reducing capsular contracture on the PDMS surface. In vitro cell cytotoxicity evaluation showed that hASCs on IA‐PDMS exhibit higher cell viability than hASCs on PDMS. A lower release of proinflammatory cytokines is observed in hASC‐PDMS and hASC‐IA‐PDMS compared to the cells on plate. Multiple factors, including in vivo mRNA expression levels of cytokines related to fibrosis; number of inflammatory cells; number of macrophages and myofibroblasts; capsule thickness; and collagen density following implantation in rats for 60 days, indicate that incorporated coating hASCs on PDMSs most effectively reduces capsular contracture. This study demonstrates the potential of hASCs coating for the modification of PDMS surfaces in enhancing surface biocompatibility for reducing capsular contracture of PDMS‐based medical devices.

Ice adhesion of PDMS surfaces with balanced elastic and water-repellent properties

HypothesisIce adhesion to rigid materials is reduced with low energy surfaces of high receding contact angles. However, their adhesion strength values are above the threshold value to be considered as icephobic materials. Surface deformability is a promising route to further reduce ice adhesion. ExperimentsIn this work, we prepared elastomer surfaces with a wide range of elastic moduli and hydrophobicity degree and we measured their ice adhesion strength. Moreover, we also explored the deicing performance of oil-infused elastomeric surfaces. The ice adhesion was characterized by two detachment modes: tensile and shear. FindingsThe variety of elastomeric surfaces allowed us to simultaneously analyze the ice adhesion dependence with deformability and contact angle hysteresis. We found that the impact of these properties depends on the detachment mode, being deformability more important in shear mode and hydrophobicity more relevant in tensile mode. In addition, oil infusion further reduces ice adhesion due to the interfacial slippage. From an optimal balance between deformability and hydrophobicity, we were able to identify surfaces with super-low ice adhesion.

HYDROGEL ANTIBACTERIAL COATING FOR SILICONE MEDICAL DEVICES

Effective antibacterial coatings are in demand in medicine, especially for urological medical devices such as catheters and stents. We propose the production method of an antibacterial hydrogel coating on polydimethylsiloxane (PDMS, silicone), a popular surface for medical materials. The coating process consists of the following steps: PDMS surface activation (introduction of hydroxyl groups), silanisation (introduction of amine groups) and application of chitosan/alginate hydrogel with the addition of lysozyme as an antibacterial agent using the layer-by-layer method. We investigated the effect of polyion concentration on the coating mass, swelling ratio and stability. We analysed the adsorption of Micrococcus luteus, Escherichia coli and Proteus rettgeri on a PDMS surface using confocal laser scanning microscopy. The chitosan/alginate hydrogel coating with immobilised lysozyme protected the PDMS surface against adhesion for all three tested bacterial strains.

Single-Chain Nanoparticle-Based Coatings with Improved Bactericidal Activity and Antifouling Properties.

Dual-function antibacterial surfaces have exhibited promising potential in addressing implant-associated infections. However, both bactericidal and antifouling properties need to be further improved prior to practical uses. Herein, we report the preparation and properties of a linear block copolymer coating (LP-KF) and a single-chain nanoparticle coating (NP-KF) with poly(ethylene glycol) (PEG) and cationic polypeptide segments. NP-KF with cyclic PEG segments and densely charged polypeptide segments was expected to display improved bactericidal and antifouling properties. LP-KF was prepared by the combination of ring-opening polymerization of N-carboxyanhydride (NCA) monomers and subsequent deprotection. NP-KF was prepared by intramolecular cross-linking of LP-KF in diluted solutions. Both LP-KF- and NP-KF-coated PDMS surfaces were prepared by dipping with polydopamine-coated surfaces. They showed superior in vitro bactericidal activity against both Staphylococcus aureus and Escherichia coli with >99.9% killing efficacy, excellent protein adsorption resistance, antibacterial adhesion, and low cytotoxicity. The NP-KF coating showed higher bactericidal activity and antifouling properties than its linear counterpart. It also showed significant anti-infective property and histocompatibility in vivo, which makes it a good candidate for implants and biomedical device applications.

Fabricating patterned microstructures by embedded droplet printing on immiscible deformable surfaces

The deposition of droplets inside the deformable surfaces has attracted researchers for decades due to its application in direct-writing microporous polymer architectures, printing embedded flexible wires, patterning functional nanoparticles, etc. Herein, a patterned microstructure method based on droplets, named as the embedded droplet printing (EDP), is proposed. The experiments were conducted at a Weber number of 5.49–17.7 to explore the impact outcome, spreading laws and embedded morphology of the droplets. The rebound of droplets impacting the viscous surface was suppressed under appropriate conditions. The spreading factor of the droplet impacting on the high viscous surface followed the power law d*∝ t α. However, the exponent α was observed to be in the range 0.042–0.031, much smaller than the reported values (0.1–1), which could be explained by the Oh number. The diameters of droplets wrapped with viscous PDMS were only about 1/6th as compared to the spreading on the surface of PDMS precured for 30 min. In particular, a domain map was plotted in which patterns of solid bracts, plates and coffee rings were printed. Overall, EDP is a promising candidate to tailor the size, depth and morphology of droplets for preparing the patterned microstructures inside the soft materials.

A Study of the Effects of Plasma Surface Treatment on Lipid Bilayers Self-Spreading on a Polydimethylsiloxane Substrate under Different Treatment Times.

Plasma-treated poly(dimethylsiloxane) (PDMS)-supported lipid bilayers are used as functional tools for studying cell membrane properties and as platforms for biotechnology applications. Self-spreading is a versatile method for forming lipid bilayers. However, few studies have focused on the effect of plasma treatment on self-spreading lipid bilayer formation. In this paper, we performed lipid bilayer self-spreading on a PDMS surface with different treatment times. Surface characterization of PDMS treated with different treatment times is evaluated by AFM and SEM, and the effects of plasma treatment of the PDMS surface on lipid bilayer self-spreading behavior is investigated by confocal microscopy. The front-edge velocity of lipid bilayers increases with the plasma treatment time. By theoretical analyses with the extended-DLVO modeling, we find that the most likely cause of the velocity change is the hydration repulsion energy between the PDMS surface and lipid bilayers. Moreover, the growth behavior of membrane lobes on the underlying self-spreading lipid bilayer was affected by topography changes in the PDMS surface resulting from plasma treatment. Our findings suggest that the growth of self-spreading lipid bilayers can be controlled by changing the plasma treatment time.

Preparation of bactericidal PDMS surfaces by benzophenone photo-initiated grafting of polynorbornenes functionalized with quaternary phosphonium or pyridinium groups

Cationic ROMP-polymers bearing ammonium or phosphonium groups were photo-grafted onto a PDMS surface via the thiol-ene click reaction, in the presence of benzophenone. The PDMS surface was first thiolated using pentaerythritol tetrakis (3-mercaptopropionate) (PTTMP) under UV, in the presence of benzophenone. The obtained cationic surfaces exhibited charge densities above 1014 charge/cm2 and a higher degree of hydrophilicity compared to non-grafted surfaces. The live and dead tests performed by fluorescence microscopy revealed an effective contact bactericidal effect of these surfaces against Escherichia coli and Staphylococcus epidermidis.

Striped Poly(diacetylene) Monolayers Control Adsorption of Polyelectrolytes and Proteins on 2D Materials and Elastomers

Promoting, structuring, or limiting adsorption of polyelectrolytes at interfaces is of importance in applications ranging from biomedical devices to nanoscale electronics. Routes for surface functionalization, particularly those capable of controlling the molecular conformation of adsorbates, are typically highly customized for the substrate and must therefore be redeveloped for each application. Here, we show that polymerized diacetylene striped phases with embedded 1 nm resolution functional patterns control polyelectrolyte adsorption on a crystalline inorganic surface (graphite) and can be easily transferred to elastomeric materials (here, poly(dimethylsiloxane), PDMS) to control adsorption on amorphous surfaces as well. We demonstrate that transferred functional poly(diacetylene) layers on PDMS can be designed to promote adsorption of polyelectrolytes or to limit protein adsorption, two common goals in controlling PDMS surface chemistry for biomedical applications.

Long-lasting hydrophilic surface generated on poly(dimethyl siloxane) with photoreactive zwitterionic polymers

Poly(dimethylsiloxane) (PDMS) is known as one of the most established polymers for making elastomers. Therefore, it is commonly used for the fabrication of biomedical devices. Many PDMS surface modification processes have been proposed recently to increase PDMS reliability in medical fields. However, the modified surface's long-term stability is still limited. Hydrophobic recovery of PDMS is widely recognized as a factor that reduces the efficacy of PDMS surface modification. The photoreactive zwitterionic polymer effectively suppresses the hydrophobic recovery of PDMS, according to the current analysis. The photoreactive zwitterionic monomer, 2-[2-(Methacryloyloxy)ethyldimethylanmmonium] ethyl benzophenoxy phosphate (MBPP) was polymerized by conventional radical polymerization and coated on O2-plasma-treated PDMS specimens. The specimens were immersed in an aqueous solution of 2-methacryloyloxyethyl phosphorylcholine (MPC) and exposed under ultraviolet (UV) radiation for 3 h. Instead, of poly(MBPP) (PMBPP), benzophenone (BP) was also used as a conventional photoinitiator. The time-dependent change in the wettability and elemental composition of the specimen surface was monitored for nine weeks after photo-grafting of poly[2-methacryloyloxyethyl phosphorylcholine (MPC)] (PMPC). The advancing and receding contact angles (θA/θR) of the pristine PDMS specimen were 112°/71° and significantly decreased immediately after the grafting of PMPC regardless of types of photoinitiator. However, the hydrophobicity of the surface gradually recovered, and θA was changed from 12° to 81° for nine weeks of storage under air atmosphere when BP was used as a photoinitiator for graft polymerization of MPC. However, surface hydrophilicity (θA ≅ 20°) of the surface grafted with PMPC with PMBPP as an initiator was effectively preserved for nine weeks. This surface also showed excellent lubricity and non-fouling properties regardless of the storage periods. Therefore, zwitterionic photoreactive polymer, PMBPP, is then used as a macrophotoinitiator for the surface modification of PDMS.

Antimicrobial PDMS Surfaces Prepared through Fast and Oxygen-Tolerant SI-SARA-ATRP, Using Na2SO3 as a Reducing Agent.

Poly(dimethylsiloxane) (PDMS) is an attractive, versatile, and convenient material for use in biomedical devices that are in direct contact with the user. A crucial component in such a device is its surface in terms of antimicrobial properties preventing infection. Moreover, due to its inherent hydrophobicity, PDMS is rather prone to microbial colonization. Thus, developing an antimicrobial PDMS surface in a simple, large-scale, and applicable manner is an essential step in fully exploiting PDMS in the biomedical device industry. Current chemical modification methods for PDMS surfaces are limited; therefore, we present herein a new method for introducing an atom transfer radical polymerization (ATRP) initiator onto the PDMS surface via the base-catalyzed grafting of [(chloromethyl)phenylethyl]trimethoxysilane to the PDMS. The initiator surface was grafted with poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) brushes via a surface-initiated supplemental activator and reducing agent ATRP (SI-SARA-ATRP). The use of sodium sulfite as a novel reducing agent in SI-SARA-ATRP allowed for polymerization during complete exposure to air. Moreover, a fast and linear growth was observed for the polymer over time, leading to a 400 nm thick polymer layer in a 120 min reaction time. Furthermore, the grafted PDMAEMA was quaternized, using various alkylhalides, in order to study the effect on surface antimicrobial properties. It was shown that antimicrobial activity not only depended highly on the charge density but also on the amphiphilicity of the surface. The fast reaction rate, high oxygen tolerance, increased antimicrobial activity, and the overall robustness and simplicity of the presented method collectively move PDMS closer to its full-scale exploitation in biomedical devices.

Preparation of bactericidal surfaces with high quaternary ammonium content through photo-initiated polymerization of N-[2-(acryloyloxy)ethyl]-N,N-dimethyl-N-butylammonium iodide from native and thiolated PDMS surfaces

N-[2-(acryloyloxy)ethyl]-N,N-dimethyl-N-butylammonium iodide was successfully photo-polymerized from native and thiolated PDMS surface, in the presence of benzophenone. The directly and indirectly grafted surfaces exhibited quaternary ammonium densities of about 1015 and 1017 N+.cm−2, respectively, and very high hydrophilicity compared to non-grafted surfaces. The live and dead tests performed by fluorescence microscopy revealed an effective contact bactericidal effect of this surface against Escherichia coli and Staphylococcus epidermidis.

Self-disinfecting PDMS surfaces with high quaternary ammonium functionality by direct surface photoinitiated polymerization of vinylbenzyl dimethylbutylammonium chloride

Vinylbenzyl dimethylbutylammonium chloride was successfully grafted and photo-polymerized from a PDMS surface, in presence of benzophenone. The obtained surface exhibited quaternary ammonium density above 1017 charge/cm2 and very high hydrophilicity compared to ungrafted surfaces. Bacterial enumeration and fluorescence microscopy revealed an efficient contact killing of this surface against Escherichia coli, Staphylococcus aureus and Staphylococcus epidermidis.

Universal Plasma Jet for Droplet Manipulation on a PDMS Surface towards Wall-Less Scaffolds.

Droplet manipulation is important in the fields of engineering, biology, chemistry, and medicine. Many techniques, such as electrowetting and magnetic actuation, have been developed for droplet manipulation. However, the fabrication of the manipulation platform often takes a long time and requires well-trained skills. Here we proposed a novel method that can directly generate and manipulate droplets on a polymeric surface using a universal plasma jet. One of its greatest advantages is that the jet can tremendously reduce the time for the platform fabrication while it can still perform stable droplet manipulation with controllable droplet size and motion. There are two steps for the proposed method. First, the universal plasma jet is set in plasma mode for modifying the manipulation path for droplets. Second, the jet is switched to air-jet mode for droplet generation and manipulation. The jetted air separates and pushes droplets along the plasma-treated path for droplet generation and manipulation. According to the experimental results, the size of the droplet can be controlled by the treatment time in the first step, i.e., a shorter treatment time of plasma results in a smaller size of the droplet, and vice versa. The largest and the smallest sizes of the generated droplets in the results are about 6 µL and 0.1 µL, respectively. Infrared spectra of absorption on the PDMS surfaces with and without the plasma treatment are investigated by Fourier-transform infrared spectroscopy. Tests of generating and mixing two droplets on a PDMS surface are successfully achieved. The aging effect of plasma treatment for the proposed method is also discussed. The proposed method provides a simple, fast, and low-cost way to generate and manipulate droplets on a polymeric surface. The method is expected to be applied to droplet-based cell culture by manipulating droplets encapsulating living cells and towards wall-less scaffolds on a polymeric surface.

Influence of poly(N-isopropylacrylamide) (PIPAAm) graft density on properties of PIPAAm grafted poly(dimethylsiloxane) surfaces and their stability

A previous report shows that poly(N-isopropylacrylamide) (PIPAAm) gel grafted onto poly(dimethylsiloxane) (PDMS) (PI-PDMS) surfaces with large PIPAAm graft density (Lar-PI-PDMS), is prepared by using electron beam irradiation, demonstrating that applied mechanical stretching affects properties of the Lar-PI-PDMS surface. However, the influence of PIPAAm graft density on the properties of PI-PDMS surfaces and their stability are not understood. To provide insight into these points, the properties of PI-PDMS surfaces with low PIPAAm graft density (Low-PI-PDMS) surfaces with stretched (stretch ratio = 20%) and unstretched states were examined as stretchable temperature-responsive cell culture surface using contact angle measurement and cell attachment/detachment assays, compared to those with Lar-PI-PDMS, as previously reported. Long-term contact angle measurements (61 days) for unstretched Low-PI-PDMS and Lar-PI-PDMS surfaces indicated that the cross-linked structure of the grafted PIPAAm gel suppressed hydrophobic recovery of the basal PDMS surface. The cell attachment assay revealed that the stretched Low-PI-PDMS surface was less cell adhesive than that of the unstretched Low-PI-PDMS surface despite of a larger amount of adsorbed fibronectin (FN). The lower cell adhesiveness was possibly explained by denaturation of adsorbed FN, which was induced by the strong hydrophobic property of the stretched Low-PI-PDMS surface. The cell detachment assay revealed that dual stimuli, low temperature treatment and mechanical shrinking stress applied to the stretched Low-PI-PDMS surface promoted cell detachment compared to a single stimulus, low temperature treatment or mechanical shrinking stress. These results suggested that the PIPAAm gelgrafted PDMS surface was chemically stable and did not suffer from hydrophobic recovery. External mechanical stretching stress not only strongly dehydrated grafted PIPAAm chains, but also denatured the adsorbed FN when the grafted PIPAAm layer was extremely thin, as in Low-PI-PDMS surfaces. Thus, PI-PDMS may be utilized as a stretchable temperature-responsive cell culture surface without significant hydrophobic recovery.

Superhydrophobic and superoleophobic property enhancement on guard ring micro-patterned PDMS with simple flame treatment

Effects of new micro-structure design, a flame treatment process, and the addition of semifluorinated silane (SFS) on an improvement of superhydrophobicity and superoleophobicity of PDMS surfaces were investigated in this study. PDMS and PDMS-SFS surfaces with the special design of circular rings and eight stripe supporters (C-RESS) with a hexagonal guard ring (HGR) structure were found to be the most durable which maintained their superhydrophobicity after scratch tests. The flame treatment at 700 °C/15 s formed a unique nanoscale flower-like on the PDMS-SFS surface. A formation of re-entrant micro-structure on the C-RESS with the HGR structure exhibited superhydrophobicity and superoleophobicity with water and ethylene glycol contact angles of 160.5° ± 2.0° and 160.2° ± 6.6°, respectively. The addition of the SFS was found to increase surface roughness and decrease surface energy. In conclusion, the flame-treated C-RESS with the HGR structure on the PDMS-SFS surface is considered one of the promising antifouling approaches in several applications.

Robust water-repellant fabrics

Materials Science Materials that aggressively repel water are useful for protective fabrics and self-cleaning surfaces. However, it can be challenging to make coatings on woven fabrics that can resist rubbing and frequent washing cycles without using fluorinated molecules. Drawing inspiration from earthworms, which have wrinkled skins, Xu et al. created a similar surface texture on poly(ethylene terephthalate) fabric coating with poly(dimethylsiloxane) (PDMS). The PDMS is treated with an argon plasma, leading to the formation of cross links that have a graduated depth concentration profile. This causes the PDMS surface to wrinkle, which leads to water repellence. The fabrics could survive hundreds of washing or rubbing cycles, and damage to the PDMS could be repaired using heat or further argon plasma treatment. ACS Appl. Mat. Interfaces 13 , 6758 (2021).

Contact line relaxation of sessile drops on PDMS surfaces: A methodological perspective

HypothesisCharacterization of contact angle hysteresis on soft surfaces is sensitive to the measurement protocol and might present adventitious time-dependencies. Contact line dynamics on solid surfaces is altered by the surface chemistry, surface roughness and/or surface elasticity. We observed a “slow” spontaneous relaxation of static water sessile drops placed on elastic surfaces. This unexpected drop motion reveals unresolved equilibrium configurations that may affect the observed values of contact angle hysteresis. Drop relaxation on deformable surfaces is partially governed by a viscoelastic dissipation located at the contact line. ExperimentsIn this work, we studied the natural relaxation of water drops formed on several smooth PDMS surfaces with different elastic moduli. We monitored in time the contact angle and contact radius of each drop. For varying the initial contact angle, we used the growing-shrinking drop method. FindingsWe postulate that the so-called “braking effect”, produced by the surface deformability, affects the contact line velocity and in consequence, the contact angle measurements. We conclude that the wetting properties of elastic surfaces should be properly examined with reliable values of contact angle measured after drop relaxation.

Soft imprint lithography for liquid crystal alignment using a wrinkled UVO-treated PDMS transferring method

Soft imprint lithography for liquid crystal (LC) alignment using a poly(dimethylsiloxane) (PDMS) wrinkled structure formed by UV–ozone (UVO) treatment was investigated in this study. Oxidation of the PDMS surface formed a silica-like outer layer that caused elastic moduli mismatch with the inner PDMS layer, leading to the formation of a wrinkled structure. This wrinkled PDMS was used as a stamp for LC alignment via soft imprint lithography. The wrinkled structure was transferred to the polyimide alignment layer and then LC cells were fabricated using the wrinkled substrates. The pretilt angle and electro-optical properties of the LC cells fabricated in this manner had performance similar to that of LC cells fabricated using the conventional rubbing method.

Proliferation of mesenchymal stem cells by graphene-attached soft material structure

This paper aims to clarify the effect of a structure of graphene attached on soft material, as against conventional rigid materials, and the thereby-obtained graphene-based micro/nano-scale surface structures on proliferation of human mesenchymal stem cells (MSCs). The cell culture tests are conducted using graphene-attached poly(dimethylsiloxane) (PDMS) substrates having various surface structures and graphene-attached glass substrates as the scaffolds. The fluorescence microscopy observation results demonstrate that the existence of graphene film on PDMS significantly promotes the cellular proliferation and elongation in both cases using two kinds of media, and the rate of cell growth for the graphene-attached PDMS substrates is much larger compared to the graphene-attached glass substrates. The difference in the graphene-based surface structures on PDMS has little influence on the degree of cell growth. It is implied that the number of cells corresponds to the rate of graphene covering the surface of PDMS, suggesting that the cell growth strongly depends on the projected surface area of the graphene film attached on PDMS.