Dewetting of polystyrene thin films on poly(ethylene glycol)-modified surfaces as a simple approach for patterning proteins

Physically Unclonable Surfaces via Dewetting of Polymer Thin Films

From anti-counterfeiting to biotechnology applications, there is a strong demand for encoded surfaces with multiple security layers that are prepared by stochastic processes and are adaptable to deterministic fabrication approaches. Here, we present dewetting instabilities in nanoscopic (thickness <100 nm) polymer films as a form of physically unclonable function (PUF). The inherent randomness involved in the dewetting process presents a highly suitable platform for fabricating unclonable surfaces. The thermal annealing-induced dewetting of poly(2-vinyl pyridine) (P2VP) on polystyrene-grafted substrates enables fabrication of randomly positioned functional features that are separated at a microscopic length scale, a requirement set by optical authentication systems. At a first level, PUFs can be simply and readily verified via reflection of visible light. Area-specific electrostatic interactions between P2VP and citrate-stabilized gold nanoparticles allow for fabrication of plasmonic PUFs. The strong surface-enhanced Raman scattering by plasmonic nanoparticles together with incorporation of taggants facilitates a molecular vibration-based security layer. The patterning of P2VP films presents opportunities for fabricating hybrid security labels, which can be resolved through both stochastic and deterministic pathways. The adaptability to a broad range of nanoscale materials, simplicity, versatility, compatibility with conventional fabrication approaches, and high levels of stability offer key opportunities in encoding applications.

A Novel Method to Measure the Effective Change of the Interfacial Energy due to Kinetic Self-Assembly of Amyloid Fibrils

Adsorbates growing a self-assembled layer on a solid-liquid interface can significantly change the effective interfacial energy at the solid surface. However, measuring the changes in the effective surface energy while these adsorbates accumulate is challenging, as static contact angle measurements can be affected by the motion and accumulation of these adsorbates at the droplet's boundary (coffee stain effects). In this report, we utilize a novel method that takes advantage of spin-induced dewetting to measure the change in the effective surface energy as the self-assembly progresses. We use a previously well-studied model system of self-assembled fibrils of amyloid-β (Aβ) peptides on the mica substrate to demonstrate the feasibility of this method. Using variations of terminal spin speeds and acceleration rates, we measure the terminal spin speed at which a wetting-dewetting transition (WDT) occurs on a surface that hosts self-assembled Aβ_12-28 fibrils. By comparing this speed with the WDT speed on the bare mica substrate, we can quantify the spreading coefficient and thus the effective change of the substrate's interfacial energy due to the adsorption of mobile peptides at various stages of the self-assembly. These measurements show that the surface becomes more hydrophilic as the self-assembly progresses and thus can explain previous observations that the self-assembly of this particular peptide system is self-limiting and stops before full surface coverage.

Potential Artifacts in Sample Preparation Methods Used for Imaging Amyloid Oligomers and Protofibrils due to Surface-Mediated Fibril Formation

Accurate imaging of nanometer-sized structures and morphologies is essential to characterizing amyloid species formed at various stages of amyloid aggregation. In this article, we examine the effect of different drying procedures on the final morphology of surface-mediated fibrils formed during the incubation period, which may then be mistaken as oligomers or protofibrils intentionally formed in solution for a particular study. Atomic force microscopy results show that some artifacts, such as globules, flakelike structures, and even micrometer-long fibrils, can be produced under various drying conditions. We also demonstrate that one can prevent drying artifacts by using an appropriate spin-coating procedure to dry amyloid samples. This procedure can bypass the wetting/dewetting transition of the liquid layer during the drying process and preserve the structure of interest on the substrate without generating drying artifacts.

Dewetting based fabrication of fibrous micro-scaffolds as potential injectable cell carriers

Although regenerative medicine utilizing tissue scaffolds has made enormous strides in recent years, many constraints still hamper their effectiveness. A limitation of many scaffolds is that they form surface patches, which are not particularly effective for some types of "wounds" that are deep within tissues, e.g., stroke and myocardial infarction. In this study, we reported the generation of fibrous micro-scaffolds feasible for delivering cells by injection into the tissue parenchyma. The micro-scaffolds (widths<100μm) were made by dewetting of poly(lactic-co-glycolic acid) thin films containing parallel strips, and cells were seeded to form cell/polymer micro-constructs during or post the micro-scaffold fabrication process. Five types of cells including rat induced vascular progenitor cells were assessed for the formation of the micro-constructs. Critical factors in forming fibrous micro-scaffolds via dewetting of polymer thin films were found to be properties of polymers and supporting substrates, temperature, and proteins in the culture medium. Also, the ability of cells to attach to the micro-scaffolds was essential in forming cell/polymer micro-constructs. Both in vitro and in vivo assessments of injecting these micro-scaffolding constructs showed, as compared to free cells, enhanced cell retention at the injected site, which could lead to improved tissue engineering and regeneration.

Applicability of the extended Derjaguin-Landau-Verwey-Overbeek theory on the adsorption of bovine serum albumin on solid surfaces

Protein adsorption is the prerequisite for bacterial attachment and cellular adhesion, which are critical for many biomedical applications. To understand protein adsorption onto substrates, predictive models are generally informative prior to experimental studies. In this study, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was employed to determine whether or not it could interpret the protein adsorption behaviors. The experimental results of fluorescein isothiocyanate labeled bovine serum albumin (BSA) adsorbed on six different surfaces: glass, octadecyltrichlorosilane modified glass, 2-[methoxypoly(ethyleneoxy)propyl]trimethoxy-silane (PEG)-modified glass, polystyrene, poly(dimethylsiloxane), and poly(methyl methacrylate) were utilized. The XDLVO interaction energy curves, especially from the contribution of acid-base interactions, obtained using the surface properties of substrates and BSA molecules qualitatively predict/interpret the protein adsorption behaviors on these surfaces. Some derivation of the experimental results from the prediction was noticed for the glass and the PEG-modified glass. When including a hydration layer to the PEG-modified glass surface, the nonfouling result of such surface by proteins was also elucidated by the XDLVO theory.

Effects of polythiophene surface structure on adsorption and conformation of bovine serum albumin: a multivariate and multitechnique study

Protein interactions with surfaces of promising conducting polymers are critical for development of bioapplications. Surfaces of spin-cast and postbaked poly(3-alkylthiophenes), regiorandom P3BT, and regioregular RP3HT are examined prior to and after adsorption of model protein, bovine serum albumin, with time-of-flight secondary ion mass spectrometry, atomic force microscopy, and X-ray photoelectron spectroscopy. The multivariate method of principal component analysis applied to ToF-SIMS data maximizes information on subtle differences in surface chemistry: PCA reveals alkyl side chains and conjugated backbones, exposed for RP3HT and P3BT, respectively. Phase imaging AFM shows semicrystalline microstructure of RP3HT and amorphous morphology of P3BT films. A cellular-like pattern of proteins adsorbed on RP3HT develops with coverage to more uniform overlayer, observed always on P3BT. The amount of adsorbed protein, determined by XPS as a function of BSA concentration (up to 10 mg/mL), is ∼21% lower for RP3HT than P3BT (up to 1.1 mg/m(2)). Although PCA differentiates protein from polythiophene, relative protein surface composition evaluated from ToF-SIMS saturates rather than increases with amount of adsorbed BSA from XPS. This reflects ToF-SIMS sensitivity to outermost layer of proteins, enabling multivariate analysis of protein conformation or orientation. PCA distinguishes between amino acids characteristic for external regions of BSA adsorbed to P3BT and RP3HT. These amino acids are identified for P3BT and RP3HT as hydrophilic and hydrophobic, respectively, by relative hydrophobicity of amino acid side chains. Alternative identification with BSA domains fails, pointing to substrate-induced changes in conformation and degree of denaturation rather than orientation of adsorbed protein.

Chemical modification of polymer brushes via nitroxide photoclick trapping

The preparation of polymer brushes (PBs) bearing α-hydroxyalkylphenylketone (2-hydroxy-2-methyl-1-phenylpropan-1-one) moieties as photoreactive polymer backbone substituents is presented. Photoreactive polymer brushes with defined thicknesses (up to 60 nm) and high grafting densities are readily prepared by surface initiated nitroxide mediated radical polymerization (SINMP). The photoactive moieties can be transformed via Norrish-type I photoreaction to surface-bound acyl radicals. Photolysis in the presence of a persistent nitroxide leads to chemically modified PBs bearing acylalkoxyamine moieties as side chains resulting from trapping of the photogenerated acyl radicals with nitroxides. Application of functionalized nitroxides to the photochemical PB postmodification provides functionalized PBs bearing cyano, polyethylene glycol (PEG), perfluoroalkyl, and biotin moieties. As shown for one case, photochemical postfunctionalization of the PB through a mask using a biotin-conjugated nitroxide as the trapping reagent leads to the corresponding site-selective chemically modified PB, which is successfully used for site-specific streptavidin immobilization. Surface analysis of PBs was performed by contact angle (CA) measurements, X-ray photoelectron spectroscopy (XPS), attenuated total reflection (ATR), fourier transform infrared (FTIR) spectroscopy, and fluorescence microscopy.

Physico-chemical characterization of bovine serum albumin in solution and as deposited on surfaces

The interactions between proteins and solid surfaces are important for the formation of biocompatible materials. In this study, the physicochemical properties of Bovin serum albumin (BSA) in solution and on a solid surface were studied. The zeta potential and number of uncompensated charges on BSA surfaces were determined from electrophoretic mobility measurements. The dynamic viscosity was also measured to determine BSA conformations in solution, and the data were converted to the effective length L(ef) of the BSA molecule. The length of a BSA molecule was measured to be 8.3 nm in the compact state (N form at pH 4-9) and 26.7 nm in the extended state (F-form). This study demonstrates that the relationship between the hydrodynamic radius, dynamic viscosity and electrophoretic mobility can provide information about the shape and conformation of biopolymer in solution. The contact angle measurements and deposition of fluorescent latex particles were used to characterise BSA monolayers on a mica surface, which were produced by controlled adsorption under diffusion transport. The results suggest that the distribution of charge across a BSA molecule is heterogeneous as evidenced by the presence of positive and negative patches. The maximum contact angle was observed under conditions in which both BSA and mica were oppositely charged. A higher positive zeta potential of BSA was observed to correlate with a higher contact angle. However, at a higher negative zeta potential, BSA exhibited a lower binding affinity. The charge distribution across BSA monolayers was also studied via the colloidal deposition method using negatively charged fluorescent latex particles. Unexpectedly, the fluorescent latex particles adsorbed onto BSA monolayers, even when the effective zeta potential of BSA was negative. This phenomenon may originate from the heterogeneous charge distribution across BSA molecules.

Engineering 3D ordered molecular thin films by nanoscale control

This perspective aims to report on experimental preparation and investigation tools for engineering molecular thin films with a three-dimensional (3D) nanoscale control that is of relevant interest for different emerging applications as well as for the development of calibration standards. Such thin films may be obtained by man-made methods, self-assembly or spatio-temporal self-organization and/or by the combination of these last approaches with external tools. Understanding the main features and the physical-chemistry underlying the related ordering phenomena is in due course and a theoretical framework is under development. In this respect it is of fundamental importance to achieve the ability to get 3D structural images with a nanoscale detail. This issue is at the early stage and novel techniques like electron tomography and scanning transmission X-ray microscopy are very promising.

Plasma nanotextured PMMA surfaces for protein arrays: increased protein binding and enhanced detection sensitivity

Poly(methyl methacrylate) (PMMA) substrates were nanotextured through treatment in oxygen plasma to create substrates with increased surface area for protein microarray applications. Conditions of plasma treatment were found for maximum uniform protein adsorption on these nanotextured PMMA surfaces. Similar results were obtained using both a high-density plasma (HDP) and a low-density reactive ion etcher (RIE), suggesting independence from the plasma reactor type. The protein binding was evaluated by studying the adsorption of two model proteins, namely, biotinylated bovine serum albumin (b-BSA) and rabbit gamma-globulins (RgG). The immobilization of these proteins onto the surfaces was quantitatively determined through reaction with fluorescently labeled binding molecules. It was found that the adsorption of both proteins was increased up to 6-fold with plasma treatment compared to untreated surfaces and up to 4-fold compared to epoxy-coated glass slides. The sensitivity of detection was improved by 2 orders of magnitude. Moreover, highly homogeneous protein spots were created on optimized plasma-nanotextured surfaces through deposition with an automated microarray spotter, revealing the potential of plasma-nanotextured surfaces as protein microarray substrates.

Water-swelling-induced morphological instability of a supported polymethyl methacrylate thin film

The instability of supported poly(methyl methacrylate) (PMMA) thin films in water has been investigated. It is found that PMMA films partially detach from the solid substrate, resulting in the formation of bubbles under water. The process is reversible. Surface morphology analysis shows that the radius of curvature of the bubbles is dependent on the thickness of the PMMA films and is independent of the treatment of the films, such as the annealing temperature and the annealing time. Theoretical analysis based on a two-layer model (the swollen layer and the interior layer) shows that the partial swelling of PMMA in water is the physical origin of bubble formation.

The role of surface energy of technical polymers in serum protein adsorption and MG-63 cells adhesion

Polymeric materials are widely used as supports for cell culturing in medical implants and as scaffolds for tissue regeneration. However, novel applications in the biosensor field require materials to be compatible with cell growth and at the same time be suitable for technological processing. Technological polymers are key materials in the fabrication of disposable parts and other sensing elements. As such, it is essential to characterize the surface properties of technological polymers, especially after processing and sterilization. It is also important to understand how technological polymers affect cell behavior when in contact with polymer materials. Therefore, the aim of this research was to study how surface energy and surface roughness affect the biocompatibility of three polymeric materials widely used in research and industry: poly(methyl methacrylate), polystyrene, and poly(dimethylsiloxane). Glass was used as the control material.

FROM THE CLINICAL EDITOR

Polymeric materials are widely used as supports for cell culturing in medical implants and as scaffolds for tissue regeneration. The aim of this research is to study how surface energy and surface roughness affect the biocompatibility of three polymeric materials widely used in research and industry: poly(methylmethacrylate) (PMMA), polystyrene (PS), and poly(dimethylsiloxane) (PDMS).

Protein nanopatterning on self-organized poly(styrene-b-isoprene) thin film templates

Templated surfaces can be used to create patterns of proteins for applications in cell biology, biosensors, and tissue engineering. A diblock copolymer template, which contains a pair of hydrophobic blocks, has been developed. The template is created from well-ordered, nonequilibrium surface structures of poly(styrene-b-isoprene) (PS-b-PI) diblock copolymers, which are achieved in ultrathin films having a thickness of less than one domain period. Adsorption and nanopatterning of bovine serum albumin (BSA) on these thin films were studied. After incubation of the copolymer templates in BSA solutions (500 microg/mL) for a period of 1 h, BSA molecules formed either a striped or a dense, ringlike structure, closely resembling the underlying polymer templates. In this "hard-soft" PS-b-PI system, BSA molecules were preferentially adsorbed on the hard PS domains, rather than on the soft PI domains. Secondary ion mass spectroscopy (SIMS) and contact angle analysis revealed that, with more PI localized at the free surface, fewer BSA molecules were adsorbed. SIMS analysis confirmed that BSA molecules were adsorbed selectively on the PS blocks. This is the first example of two hydrophobic blocks of a diblock copolymer being used as a protein patterning template. Previously reported diblock copolymer templates used hydrophilic and hydrophobic pairs. A potentially useful characteristic of this template is that it is effective at high protein solution concentrations (up to 1 mg/mL) and for long incubation times (up to 2 h), which broadens its range of applicability in various uses.

Dewetting of polymer films by ion implantation

We report dewetting of thermodynamically stable, thick (approximately 100 nm) polystyrene films by titanium ion implantation. The dynamic dewetting patterns in time evolution are recorded. The dewetting mechanism is determined to be heterogeneous nucleation, where the defects and Ti nanoparticles formed by ion implantation serve as the nuclei. In addition, we observe abundant rims with regular polygonal shapes in dewetting patterns. This is attributed to fingering instability, which results from the balance between the driving force arisen from thermally induced surface tension gradient and the resistive forces from the combination of friction force, Laplace pressure and long-range van der Waals interactions. Finally, a model based on mass conservation is used to qualitatively describe the transition from circular to polygonal shaped rims at a critical diameter for holes.