Flat Polymer Surface Research Articles

Laser Direct Writing via Two-Photon Polymerization of 3D Hierarchical Structures with Cells-Antiadhesive Properties.

We report the design and fabrication by laser direct writing via two photons polymerization of innovative hierarchical structures with cell-repellency capability. The structures were designed in the shape of “mushrooms”, consisting of an underside (mushroom’s leg) acting as a support structure and a top side (mushroom’s hat) decorated with micro- and nanostructures. A ripple-like pattern was created on top of the mushrooms, over length scales ranging from several µm (microstructured mushroom-like pillars, MMP) to tens of nm (nanostructured mushroom-like pillars, NMP). The MMP and NMP structures were hydrophobic, with contact angles of (127 ± 2)° and (128 ± 4)°, respectively, whereas flat polymer surfaces were hydrophilic, with a contact angle of (43 ± 1)°. The cell attachment on NMP structures was reduced by 55% as compared to the controls, whereas for the MMP, a reduction of only 21% was observed. Moreover, the MMP structures preserved the native spindle-like with phyllopodia cellular shape, whereas the cells from NMP structures showed a round shape and absence of phyllopodia. Overall, the NMP structures were more effective in impeding the cellular attachment and affected the cell shape to a greater extent than the MMP structures. The influence of the wettability on cell adhesion and shape was less important, the cellular behavior being mainly governed by structures’ topography.

Selective Laser Pyrolytic Micropatterning of Stretched Elastomeric Polymer Surfaces

The patterning of surface structures is a critical step for interfacial engineering applications that are based on soft lithography, casting, or imprinting techniques in general. However, photolithography and mold/die fabrication processes are expensive and time-consuming. To solve this problem, we developed a novel laser-based process that exploits the high stretchability of a polymer. In this study, continuous-wave laser-induced pyrolysis was conducted on a pre-stretched Ecoflex polymer substrate to obtain a controllable hydrophobic surface with functional structures at the micro/nanoscale, which were constructed via the facile removal of pyrolytic byproducts. Under the optimum strain rate and laser parameters, periodic wrinkle generation and a proportional channel pitch reduction were observed, which provided hierarchical micro/nanostructures and enlarged the air gaps between the target droplet and the hydrophobic structures. The processed surface exhibited improved hydrophobic properties (WCA ~ 135°) compared with a flat Ecoflex polymer surface (WCA ~ 105°). Thus, a facile manufacturing method for stretchable polymer substrates with hydrophobic patterned surfaces is proposed.

3D printing-assisted fabrication of microgrid patterns for flexible antiadhesive polymer surfaces

Flexible antiadhesive polymer surfaces were fabricated using an array of microgrid patterns generated by three-dimensional (3D) printing. A disadvantage of the fused deposition modeling-type 3D printing (rough surface) was utilized to create casting molds with microgrid patterns. The shape of the microscale pattern was controlled by rotating the bottom epoxy mold where polylactic acid filaments were directly printed. By changing the direction of the bottom epoxy mold and printing resolution, a total of 12 different antiadhesive surfaces were formed and tested to characterize the adhesive forces. The surface cast from the mold with a microgrid pattern with an internal angle of 60° and a printing resolution of ~800 μm exhibited greater antiadhesive property compared to a flat polymer surface. The average adhesive force was reduced by 92.6% (from 162 to 11.9 mN) when the Kapton tape was immediately attached and detached and 99.5% (from 735 to 3.7 mN) when the Kapton tape was detached after 18 d. This study shows the use of 3D printing for a simple cost-effective controllable microfabrication of microgrid patterns for significant reductions in the adhesive forces of polymer surfaces.

Influence of synthetic wastewater on entrapped air on the isotactic and atactic polypropylene microplastic surfaces.

The municipal wastewater collection system is recognized as an initial point of interaction between microplastics (MPs) and the urban wastewater matrix. The raw wastewater contains a wide variety of organic and inorganic substances including chemicals and heavy metals. However, the fate of MPs in urban sewer systems is not yet well understood. In this work two types of virgin polypropylene (PP) samples, isotactic (iPP) and atactic (aPP), were exposed to two synthetic wastewater solutions in order to study their effects on the physical properties of the hydrophobic polymer surfaces. Particular attention was paid to the pollution adhesion at the air-liquid-solid interfaces of the surface air pockets entrapped on the polymer surfaces. The first wastewater solution consists of mixed fat, oil and grease (FOG) - surfactant and another which is an exclusively contained wastewater surfactant. The interaction experiment over a period of 10min between the polymer's air pocket and solutions indicated that the size of the bubble in the mixed FOG-surfactant solution increased more pronouncedly for iPP (%152) in contrast to aPP (%31) and was also compared with the greater surface roughness of the polymers. The size variation of the spherical cap on the immersed polymer surfaces were measured between 17µm and 85µm using image processing techniques while the data was analyzed by the Young-Laplace equation. The corresponding technical surface roughness of the polymers, the surface tension of the liquids and their air/water contact angle on the flat polymer surfaces were also measured. The results of this study indicated that surface air pockets influence the adsorption capacity of MPs and thus their buoyancy and contamination potential.

A Versatile Sacrificial Layer for Transfer Printing of Wide Bandgap Materials for Implantable and Stretchable Bioelectronics

AbstractImproving and optimizing the processes for transfer printing have the potential to further enhance capabilities in heterogeneous integration of various sensing materials on unconventional substrates for implantable and stretchable electronic devices in biosensing, diagnostics, and therapeutic applications. An advanced transfer printing method based on sacrificial layer engineering for silicon carbide materials in stretchable electronic devices is presented here. In contrast to the typical processes where defined anchor structures are required for the transfer step, the use of a sacrificial layer offers enhances versatility in releasing complex microstructures from rigid donor substrates to flexible receiver platforms. The sacrificial layer also minimizes twisting and wrinkling issues that may occur in free‐standing microstructures, thereby facilitating printing onto flat polymer surfaces (e.g., polydimethylsiloxane). The experimental results demonstrate that transferred SiC microstructures exhibit good stretchability, stable electrical properties, excellent biocompatibility, as well as promising sensing‐functions associated with a high level of structural perfection, without any cracks or tears. This transfer printing method can be applied to other classes of wide bandgap semiconductors, particularly group III‐nitrides and diamond films epitaxially grown on Si substrates, thereby serving as the foundation for the development and possible commercialization of implantable and stretchable bioelectronic devices that exploit wide bandgap materials.

Influence of roughness and capillary size on the zeta potential values obtained by streaming potential measurements

Streaming potential measurements are performed to determine the zeta potential of flat surfaces, particles, or fibers. Although the zeta potential is a well‐defined property of solid surfaces in a liquid, there are indications that the absolute values of the zeta potential calculated using the Helmholtz‐Smoluchowski equation are affected by surface roughness and—in case of particle or fiber assemblies—their packing density. The study at hand investigates these influences using flat polymer surfaces with different roughness and topography and assemblies of basalt spheres. It was found that increasing roughness of the flat surface and larger size or smaller number of particles in particle assemblies result in flatter slopes of the streaming potential versus pressure and thus lower apparent absolute values of the zeta potential. The interpretation of streaming potential measurements should therefore not focus on absolute zeta potential values but on trends in pH‐ and concentration‐dependent measurements.

Realization of Superhydrophobic Surfaces Based on Three-Dimensional Printing Technology

A superhydrophobic surface was successfully realized using fused deposition modeling-type three-dimensional (3D) printing technology. The low printing resolution (400 μm) and various printing angles from 0° to 90° were employed to print the mold for casting of polymer surfaces. The polymer surface cast from the mold exhibited waveform microstructures that had a tilting angle almost identical to the printing angle. The maximum average water contact angle (WCA) of fabricated polymer surfaces was 160°, which is much higher than that of flat (bare) polymer surfaces (up to 52.3% increase in the WCA). In particular, water droplets immediately rolled off along 8°-tilted surfaces, cast from the mold printed with printing angle of 70°. This demonstrated the superhydrophobic property. The result of this study shows the feasibility of a facile, rapid, inexpensive, and effective microfabrication of superhydrophobic surfaces using the current 3D printing technology.

In Situ Incorporation of Pores and Nanoparticles into Polymer Surfaces Using Melt Crystallization.

Engineering the surface properties of a material without affecting its bulk properties is desirable for many applications, but it is often not readily achievable because it requires a complex series of processing steps. This study presents for the first time a simple and straightforward method that can convert regular flat polymer surfaces into various porous composite surfaces. The controlled dissolution of the polymer surface by a dispersion and subsequent melt crystallization allows for the successful embedding of dispersed inorganic or organic particles in the pore walls. The porous surface morphology is not significantly affected by the type of dispersed particle, but the contact and sliding angles and surface wettability are. Both superhydrophobic and oil/water separation surfaces can be conveniently fabricated from flat polymer surfaces. This novel and versatile technique could provide engineering freedom for the future development of various functional materials.

N2supercritical jet to modify the characteristics of polymer material surfaces: Influence of the process parameters on the surface topography

Foreign body reaction (FBR) is a critical issue to be addressed when polymeric implants are considered in the medical field to treat pathologies involving hernia repair, revascularization strategies in arterial disease, and aneurysm or heart valve replacement. The natural porosity of textile materials tend to induce exaggerated tissue ingrowth which may prevent the implants from remaining flexible. One hypothesized way to limit the FBR process is to increase the material surface roughness. Supercritical N2jet particle projection is a novel technique to provide enough velocity to particles to induce plastic deformation on the impacted surface. This work investigates the influence of supercritical N2jet projection parameters like jet static pressure, standoff distance (SoD), particle size and density on the roughness that can be obtained on polyester flat polymer surfaces. Results bring out that particles generate craters on the polymeric impacted surface, with size increases correlated with the jet pressure and the particle diameter. With a pressure of 1,000 bars, 300‐mm SoD, and a particle mass flowrate of 0.6 g/s, it is possible to obtain a uniform 5 μm roughness on the impacted surface. This value is in the range of the 10 μm fiber size which is commonly used in medical textile implants. Basically, result brings out that whatever the particle size or particle flow rate, the SoD has the most significant effect on the roughness. POLYM. ENG. SCI., 59:616–624, 2019. © 2018 Society of Plastics Engineers

Argon cluster cleaning of Ga+ FIB‐milled sections of organic and hybrid materials

Secondary ion mass spectrometry studies have been made of the removal of the degraded layer formed on polymeric materials when cleaning focused ion beam (FIB)‐sectioned samples comprising both organic and inorganic materials with a 30‐keV Ga+ FIB. The degraded layer requires a higher‐than‐expected Ar gas cluster ion beam (GCIB) dose for its removal, and it is shown that this arises from a significant reduction in the layer sputtering yield compared with that for the undamaged polymer. Stopping and Range of Ions in Matter calculations for many FIB angles of incidence on flat polymer surfaces show the depth of the damage and of the implantation of the Ga+ ions, and these are compared with the measured depth profiles for Ga+‐implanted flat polymer surfaces at several angles of incidence using an Ar+ GCIB. The Stopping and Range of Ions in Matter depth and the measured dose give the sputtering yield volume for this damaged and Ga+‐implanted layer. These, and literature yield values for Ga+ damaged layers, are combined on a plot showing how the changing sputtering yield is related to the implanted Ga density for several polymer materials. This plot contains data from both the model flat poly(styrene) surfaces and FIB‐milled sections showing that these 2 surfaces have the same yield reduction. The results show that the damaged and Ga+‐implanted layer's sputtering rate, after FIB sectioning, is 50 to 100 times lower than for undamaged polymers and that it is this reduction in sputtering rate, rather than any development of microtopography, that causes the high Ar+ GCIB dose required for cleaning these organic surfaces.

UV degradation model for polymers and polymer matrix composites

We are proposing in this work a new model of ultraviolet (UV) damage for polymers and Polymer Matrix Composites (PMCs). Flat and sinusoidal polymer surfaces were numerically simulated for their UV damage as a function of UV intensity, surface topography, and exposure time. Experimentally determined UV degradation rates for a unidirectional glass/epoxy composite were used to predict numerically the local rates of material degradation on sinusoidal epoxy surfaces subjected to UV. This allowed us to show that UV damage on uneven polymer surfaces reduces their surface roughness making them planar and that the degradation rates are the largest at the tips of the local heights of the surfaces. This was subsequently verified experimentally by exposing neat epoxy specimens to UV in air at 80 °C for 1000 h and by precisely monitoring their surface topography as a function of time. It was found that the surface roughness of the epoxy was reduced by about 12.5% and that UV affected the local peaks on the surfaces of the specimens more than the valleys.

High-quality graphene flakes exfoliated on a flat hydrophobic polymer

We show that graphene supported on a hydrophobic and flat polymer surface results in flakes with extremely low doping and strain as assessed by their Raman spectroscopic characteristics. We exemplify this technique by micromechanical exfoliation of graphene on flat poly(methylmethacrylate) layers and demonstrate Raman peak intensity ratios I(2D)/I(G) approaching 10, similar to pristine freestanding graphene. We verify that these features are not an artifact of optical interference effects occurring at the substrate: they are similarly observed when varying the substrate thickness and are maintained when the environment of the graphene flake is completely changed, by encapsulating preselected flakes between hexagonal boron nitride layers. The exfoliation of clean, pristine graphene layers directly on flat polymer substrates enables high performance, supported, and non-encapsulated graphene devices for flexible and transparent optoelectronic studies. We additionally show that the access to a clean and supported graphene source leads to high-quality van der Waals heterostructures and devices with reproducible carrier mobilities exceeding 50 000 cm2 V−1 s−1 at room temperature.

Hierarchical micro- and nanofabrication by pattern-directed contact instabilities of thin viscoelastic films

We propose that strategic use of an array of periodic patterns on a debonding contactor during elastic contact lithography can transform a flat polymer surface into a biomimetic super-hydrophobic surface having high-aspect ratio hierarchical patterns with tuneable adhesive properties.

The plasma footprint of an atmospheric pressure plasma jet on a flat polymer substrate and its relation to surface treatment

The aim of this work is to show the correlation between the plasma propagation in the footprint of an atmospheric pressure plasma jet on a flat polymer surface and the plasma treatment impact on the polymer properties. An argon plasma jet working in open air is used as plasma source, while PET thin films are used a substrates for plasma treatment. Light emission photographs are taken with an ICCD camera to have a close look at the generated structures in the plasma jet footprint on the surface. Water contact angle (WCA) measurement and X-ray photoelectron spectroscopy (XPS) analysis are also performed to obtain information about the impact of the plasma treatment on the PET surface characteristics. A variation in ICCD camera gate duration (1 mu s, 100 mu s, 50 ms) results in the photographs of the different plasma structures occurring during the plasma propagation on the flat PET surface. Contact angle measurements provide results on improvement of the PET hydrophilic character, while XPS analysis shows the distribution of atomic elements on the treated substrate surface. Light emission images help explaining the obtained WCA and XPS results.

Nanoscale patterning of biopolymers for functional biosurfaces and controlled drug release.

We compare the rates of drug release from nanopatterned and flat biodegradable polymer surfaces, and observe significantly lower release rates from the nanopatterned surfaces. Specifically, we nanopattern poly(l-lactic acid) (PLLA), a biodegradable polymer frequently used for fabricating drug-eluting coronary stents, through microtransfer molding and solvent casting and investigate the nanopattern's impact on the release of sirolimus, an immunosuppressant agent, coated on the PLLA surface using high performance liquid chromatography/mass spectrometry. We find that PLLA surfaces nanopatterned with 750 nm-pitch nanocup or nanocone arrays exhibit drug release rates significantly lower (25-30%) than that of the flat surface, which is counter-intuitive given the nanopattern-induced increase in their surface areas. Based on diffusion and meniscus curvature minimization analyses, we attribute the decreased drug release rate to the incomplete wetting of the nanopatterned surface. These results provide new insights on how the surface nanopatterning of biomaterials can functionalize the surface and tailor the release kinetics of therapeutic agents coated on it for controlled drug elution.

Falling film break-up and thermal performance of thin polymer film heat exchangers

Polymers as alternate construction materials in apparatus design feature special advantages as column internals or in heat transfer operation when handling corrosive media (resistance to corrosion and fouling, costs). However, high overall heat transfer coefficients can only be realized when applying thin polymer films (25 μm) in combination with a spacer grid. The hydrodynamic characterization of such heat exchangers is necessary in respect to its specific design criteria as well as general understanding of falling film heat transfer enhancement. The falling film break-up propensity of flat polymer surfaces and polymer-spacer combination is investigated and heat transfer with and without phase change is studied and modeled as well. New correlations are proposed for the heat transfer enhancement induced by the spacer grid of the Polymer Film Heat Exchanger (PFHX).

Versatile synthesis of ZnO nanowires for quantitative optical sensing of molecular biorecognition

A zinc oxide nanowires (ZnO NWs) forest has been grown by a versatile hydrothermal method on solid supports of very different nature, such as flat crystalline silicon, glass fiber and polymer surface. ZnO NWs shown a characteristic photoluminescence (PL) spectrum that has been used for optical transduction of molecular interactions. In this study, ZnO NWs were chemically modified in order to bind a proper bioprobe for selective protein–protein biorecognition. Techniques such as scanning electron microscopy (SEM), water contact angle (WCA), fluorescence microscopy and Fourier transform infrared (FTIR) spectroscopy were used for characterization of nanostructures bioconjugation, demonstrating that ZnO NWs can be easily and efficiently functionalized. Quantitative and label-free sensing of protein–protein interaction was obtained by monitoring PL emission of ZnO NWs under laser irradiation.

Electrohydrodynamic Micro-/Nanostructuring Processes Based on Prepatterned Polymer and Prepatterned Template

As widely studied, the electrically induced micro-/nanostructuring process generally uses a prepatterned electrode to generate a spatially modulated electric field on an initially flat polymer surface to produce a nonuniform polymer deformation, leading to a structure with low aspect ratio, positive to the template pattern. However, a variant to this process that is proposed in this paper, which uses a flat electrode over an initially prepatterned polymer, can also generate the spatially modulated electric field. This paper presents a comparative study on the applicability of these two processes for microstructuring with high aspect ratio based on observations from our electrohydrodynamic simulation and experiment and shows that the proposed approach can provide a stronger electric modulation for the same experimental settings, therefore being more suitable for micro/nanostructuring with high aspect ratio.

Lotus-Leaf-Like Topography Predominates over Adsorbed ECM Proteins in Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) Surface/Cell Interactions

It is well-known that extracellular matrix (ECM) proteins mediate cell/surface interactions. However, introduction of a specific surface topography may disturb the correlation between ECM proteins adsorption and cells adhesion on a given surface. In present study, lotus-leaf-like topography was introduced on the surface of a biodegradable material, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx). Protein adsorption and cell interactions with this lotus-leaf-like surface (designated PHBHHx-L) were investigated. Water contact angle data indicated that the hydrophobicity of PHBHHx was enhanced by the introduction of lotus-leaf-like topography. The adsorption of extracellular matrix proteins (fibronectin and vitronectin) on PHBHHx-L was measured by enzyme linked immunosorbent assay (ELISA). Compared with flat PHBHHx, adsorption on the PHBHHx-L surface increased by ~260% for fibronectin and ~40% for vitronectin. In contrast, fibroblast and endothelial cell adhesion and proliferation were reduced on the PHBHHx-L compared to the flat polymer surface. These results suggest that the inhibition of cell adhesion and proliferation caused by the lotus-leaf-like topography dominates over the effect of the adsorbed adhesive proteins in promoting adhesion and proliferation. It can be concluded that the lotus-leaf-like topography plays a dominant role in cell/PHBHHx-L interactions. The present findings indicate the complexity of the interplay among surface topography, adsorbed proteins, and cell-surface interactions.

Abstract 250: Surface Microscale Topograhical Features Promotes Endothelial Cell Proliferation and Migration on a Nanocomposite Vascular Graft Material

Objectives Micro and nanoscale surface topographical features have been shown to have an influence on cellular behaviour. Endothelisation of small diameter vascular grafts has long been thought to have beneficial and protective effects as well as ensuring patency of the graft. Our objective is to explore the potential of using surface topographical features on a biocompatible nanocomposite polymeric material to produce a ‘self-endothelising’ vascular graft. Methods Photolithography was used to fabricate microscale topographical grooves onto a mask. These features were then heat-casted onto a nanocomposite polymeric material made of polyhedral oligomeric silsesquioxane (POSS) and poly (carbonate urea) urethane (PCU). Fidelity of the microscale grooves were confirmed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Human umbilical vein endothelial cells (HUVECs) were seeded onto POSS-PCU sheets with different microscale grooves and cellular behaviour was monitored daily. Cell proliferation and metabolic activity was measured using a colorimetric assay. Adhesion of the cells were confirmed using fluorescence microscopy. Results Our results show that grooves can be reproduced with high fidelity using masks produced by photolithography onto POSS-PCU nanocomposite polymer. Ridges of width 12.5 μm with channels widths of 25μm and subsequent depths of 300 - 700 nm were measured and confirmed using SEM and AFM These ordered grooves enhance cell spreading and adhesion compared with flat polymer surfaces. Furthermore, cell proliferation is further improved on these microscale features compared to featureless surfaces. Conclusion Microscale grooves on the surface of a nanocomposite polymeric material promotes increased endothelial cell adhesion, migration and proliferation. These ordered topographical features could be used to produce a new generation of vascular grafts with ‘self-endothelising’ potential.