Surface Topology of Redox- and Thermoresponsive Nanogel Droplets.

Hydrogels are usually depicted as a homogenous polymer block with a distinct surface. While defects in the polymer structure are looked into frequently, structural irregularities on the hydrogel surface are often neglected. In this work, thin hydrogel layers of ≈100 nm thickness (nanogels) are synthesized and characterized for their structural irregularities, as they represent the surface of macrogels. The nanogels contain a main-chain responsiveness (thermo responsive) and a responsiveness in the cross-linking points (redox responsive). By combining data from ellipsometry using box-model and two-segment-model analysis, as well as atomic force microscopy, a more defined model of the nanogel surface can be developed. Starting with a more densely cross-linked network at the silica wafer surface, the density of cross-linking gradually decreases toward the hydrogel-solvent interface. Thermo-responsive behavior of the main chain affects the entire network equally as all chain segments change solubility. Cross-linker-based redox-responsiveness, on the other hand, is only governed by the inner, more cross-linked layers of the network. Such dual responsive nanogels hence allow for developing a more detailed model of a hydrogel surface from free radical polymerization. It provides a better understanding of structural defects in hydrogels and how they are affected by responsive functionalities.

Sulfur-Rich Polyimide/TiO2 Hybrid Materials with a Tunable Refractive Index.

High refractive index (RI) polyimide/titania nanoparticle hybrid materials were synthesized and characterized in this study. The polyimide synthesis took place via the conventional polycondensation process following the preparation of poly(amic acid), and the nanoparticles were incorporated using an in situ sol-gel process. Thin films of the polyimide/titania nanoparticle hybrids were prepared by optimizing the coating conditions using a spin coater. Thermal imidization of the nanoparticle containing poly(amic acid) films on Si wafers was completed in a temperature-controlled drying oven under a N2 atmosphere. Fourier transform infrared spectroscopy revealed the successful formation of inorganic bonds as well as imide linkages, and transmission electron microscopy results show well-dispersed nanocrystalline TiO2 nanoparticles of around 5 nm in the polymer matrix. Thorough optimization of the reaction time and concentration of TiO2 precursors enabled to achieve a titania content as high as 30% (wt). The RI of the resultant hybrid materials was found to be tunable according to the titania content, while the RI increased linearly with increasing titania content. A homogeneous hybrid material with a very high RI of 1.84 at 589 nm was achieved in this work for 30% (wt) TiO2.

Monitoring Cell Adhesion on Polycaprolactone-Chitosan Films with Varying Blend Ratios by Quartz Crystal Microbalance with Dissipation.

A detailed understanding of the cell adhesion on polymeric surfaces is required to improve the performance of biomaterials. Quartz crystal microbalance with dissipation (QCM-D) as a surface-sensitive technique has the advantage of label-free and real-time monitoring of the cell-polymer interface, providing distinct signal patterns for cell-polymer interactions. In this study, QCM-D was used to monitor human fetal osteoblastic (hFOB) cell adhesion onto polycaprolactone (PCL) and chitosan (CH) homopolymer films as well as their blend films (75:25 and 25:75). Complementary cell culture assays were performed to verify the findings of QCM-D. The thin polymer films were successfully prepared by spin-coating, and relevant properties, i.e., surface morphology, ζ-potential, wettability, film swelling, and fibrinogen adsorption, were characterized. The adsorbed amount of fibrinogen decreased with an increasing percentage of chitosan in the films, which predominantly showed an inverse correlation with surface hydrophilicity. Similarly, the initial cell sedimentation after 1 h resulted in lesser cell deposition as the chitosan ratio increased in the film. Furthermore, the QCM-D signal patterns, which were measured on the homopolymer and blend films during the first 18 h of cell adhesion, also showed an influence of the different interfacial properties. Cells fully spread on pure PCL films and had elongated morphologies as monitored by fluorescence microscopy and scanning electron microscopy (SEM). Corresponding QCM-D signals showed the highest frequency drop and the highest dissipation. Blend films supported cell adhesion but with lower dissipation values than for the PCL film. This could be the result of a higher rigidity of the cell-blend interface because the cells do not pass to the next stages of spreading after secretion of their extracellular matrix (ECM) proteins. Variations in the QCM-D data, which were obtained at the blend films, could be attributed to differences in the morphology of the films. Pure chitosan films showed limited cell adhesion accompanied by low frequency drop and low dissipation.

Influence of the Atmosphere on the Wettability of Polymer Brushes.

Polymer brushes, i.e., end-tethered polymer chains on substrates, are sensitive to adaptation, e.g., swelling, adsorption, and reorientation of the surface molecules. This adaptation can originate from a contacting liquid or atmosphere for partially wetted substrates. The macroscopic contact angle of the aqueous drop can depend on both adaptation mechanisms. We analyze how the atmosphere around an aqueous droplet determines the resulting contact angle of the wetting droplet on polymer brush surfaces. Poly(N-isopropylacrylamide) (PNiPAAm)-based brushes are used due to their exceptional sensitivity to solvation and liquid mixture composition. We develop a method that reliably measures wetting properties when the drop and the surrounding atmosphere are not in equilibrium, e.g., when evaporation and condensation tend to contaminate the liquid of the drop and the atmosphere. For this purpose, we use a coaxial needle in the droplet, which continuously exchanges the wetting liquid, and in addition, we constantly exchange the almost saturated atmosphere. Depending on the wetting history, PNiPAAm can be prepared in two states, state A with a large water contact angle (∼65°) and state B with a small water contact angle (∼25°). With the coaxial needle, we can demonstrate that the water contact angle of a sample in state B significantly increases by ∼30° when a water-free atmosphere is almost saturated with ethanol, compared to an ethanol-free atmosphere at 50% relative humidity. For a sample in state A, the relative humidity has little influence on the water contact angle.

ß-Sheet Structure Formation within Binary Blends of Two Spider Silk Related Peptides.

Intrinsically disordered proteins (IDPs) play an important role in molecular biology and medicine because their induced folding can lead to so-called conformational diseases, where ß-amyloids play an important role. Still, the molecular folding process into the different substructures, such as parallel/antiparallel or extended ß-sheet/crossed ß-sheet is not fully understood. The recombinant spider silk protein eADF4(Cx) consisting of repeating modules C, which are composed of a crystalline (pep-c) and an amorphous peptide sequence (pep-a), can be used as a model system for IDP since it can assemble into similar structures. In this work, blend films of the pep-c and pep-a sequences were investigated to modulate the ß-sheet formation by varying the molar fraction of pep-c and pep-a. Dichroic Fourier-transform infrared spectroscopy (FTIR), circular dichroism, spectroscopic ellipsometry, atomic force microscopy, and IR nanospectroscopy were used to examine the secondary structure, the formation of parallel and antiparallel ß-sheets, their orientation, and the microscopic roughness and phase formation within peptide blend films upon methanol post-treatment. New insights into the formation of filament-like structures in these silk blend films were obtained. Filament-like structures could be locally assigned to ß-sheet-rich structures. Further, the antiparallel or parallel character and the orientation of the formed ß-sheets could be clearly determined. Finally, the ideal ratio of pep-a and pep-c sequences found in the fibroin 4 of the major ampullate silk of spiders could also be rationalized by comparing the blend and spider silk protein systems.

Tailored Polyelectrolyte Multilayer Systems by Variation of Polyelectrolyte Composition and EDC/NHS Cross-Linking: Physicochemical Characterization and In Vitro Evaluation.

The layer-by-layer (LbL) self-assembly technique is an effective method to immobilize components of the extracellular matrix (ECM) such as collagen and heparin onto, e.g., implant surfaces/medical devices with the aim of forming polyelectrolyte multilayers (PEMs). Increasing evidence even suggests that cross-linking influences the physicochemical character of PEM films since mechanical cues inherent to the substrate may be as important as its chemical nature to influence the cellular behavior. In this study, for the first-time different collagen/heparin films have been prepared and cross-linked with EDC/NHS chemistry. Quartz crystal microbalance, zeta potential analyzer, diffuse reflectance Fourier transform infrared spectroscopy, atomic force microscopy and ellipsometry were used to characterize film growth, stiffness, and topography of different film systems. The analysis of all data proves a nearly linear film growth for all PEM systems, the efficacy of cross-linking and the corresponding changes in the film rigidity after cross-linking and an appropriate surface topography. Furthermore, preliminary cell culture experiments illustrated those cellular processes correlate roughly with the quantity of newly created covalent amide bonds. This allows a precise adjustment of the physicochemical properties of the selected film architecture regarding the desired application and target cells. It could be shown that collagen improves the biocompatibility of heparin containing PEMs and due to their ECM-analogue nature both molecules are ideal candidates intended to be used for any biomedical application with a certain preference to improve the performance of bone implants or bone augmentation strategies.

Stretchable Thin Film Mechanical-Strain-Gated Switches and Logic Gate Functions Based on a Soft Tunneling Barrier.

Mechanical-strain-gated switches are cornerstone components of material-embedded circuits that perform logic operations without using conventional electronics. This technology requires a single material system to exhibit three distinct functionalities: strain-invariant conductivity and an increase or decrease of conductivity upon mechanical deformation. Herein, mechanical-strain-gated electric switches based on a thin-film architecture that features an insulator-to-conductor transition when mechanically stretched are demonstrated. The conductivity changes by nine orders of magnitude over a wide range of tunable working strains (as high as 130%). The approach relies on a nanometer-scale sandwiched bilayer Au thin film with an ultrathin poly(dimethylsiloxane) elastomeric barrier layer; applied strain alters the electron tunneling currents through the barrier. Mechanical-force-controlled electric logic circuits are achieved by realizing strain-controlled basic (AND and OR) and universal (NAND and NOR) logic gates in a single system. The proposed material system can be used to fabricate material-embedded logics of arbitrary complexity for a wide range of applications including soft robotics, wearable/implantable electronics, human-machine interfaces, and Internet of Things.

Enhancing sub-bandgap external quantum efficiency by photomultiplication for narrowband organic near-infrared photodetectors.

Detection of electromagnetic signals for applications such as health, product quality monitoring or astronomy requires highly responsive and wavelength selective devices. Photomultiplication-type organic photodetectors have been shown to achieve high quantum efficiencies mainly in the visible range. Much less research has been focused on realizing near-infrared narrowband devices. Here, we demonstrate fully vacuum-processed narrow- and broadband photomultiplication-type organic photodetectors. Devices are based on enhanced hole injection leading to a maximum external quantum efficiency of almost 2000% at -10 V for the broadband device. The photomultiplicative effect is also observed in the charge-transfer state absorption region. By making use of an optical cavity device architecture, we enhance the charge-transfer response and demonstrate a wavelength tunable narrowband photomultiplication-type organic photodetector with external quantum efficiencies superior to those of pin-devices. The presented concept can further improve the performance of photodetectors based on the absorption of charge-transfer states, which were so far limited by the low external quantum efficiency provided by these devices.

Surface Preconditioning Influences the Antifouling Capabilities of Zwitterionic and Nonionic Polymer Brushes.

Polymer brushes not only represent emerging surface platforms for numerous bioanalytical and biological applications but also create advanced surface-tethered systems to mimic real-life biological processes. In particular, zwitterionic and nonionic polymer brushes have been intensively studied because of their extraordinary resistance to nonspecific adsorption of biomolecules (antifouling characteristics) as well as the ability to be functionalized with bioactive molecules. However, the relation between antifouling behavior in real-world biological media and structural changes of polymer brushes induced by surface preconditioning in different environments remains unexplored. In this work, we use multiple methods to study the structural properties of numerous brushes under variable ionic concentrations and determine the impact of these changes on resistance to fouling from undiluted blood plasma. We describe different mechanisms of swelling, depending on both the polymer brush coating properties and the environmental conditions that affect changes in both hydration levels and thickness. Using both fluorescent and surface plasmon resonance methods, we found that the antifouling behavior of these brushes is strongly dependent on the aforementioned structural changes. Moreover, preconditioning of the brush coatings (incubation at a variable salt concentration or drying) prior to biomolecule interaction may significantly improve the antifouling performance. These results suggest a new simple approach to improve the antifouling behavior of polymer brushes. In addition, the results herein enhance the understanding for improved design of antifouling and bioresponsive brushes employed in biosensor and biomimetic applications.

A New Measuring System for the Determination of the Ice Adhesion Strength on Smooth Surfaces.

To gain knowledge about cause-effect relationships for the adhesion of ice on surfaces with different chemical groups, we wanted to study the effect of thin polymer layers on the ice adhesion strength. To minimize the effect of roughness, smooth substrates that have generally relatively low ice adhesion strengths were chosen. To be able to obtain highly reproducible values for the region of low ice adhesion and to measure small differences of ice adhesion at surfaces with different chemical compositions, a new measuring system for the determination of the ice adhesion strength which is based on a modified spin-coater was developed. We show its technical potential on the basis of first results on pure silicon wafers and selected hydrophilic polymer layers. Furthermore, we investigated the effect of the water quality on the ice adhesion strength. The obtained data are discussed in the context of physicochemical properties of the layers and of the chemical characteristics of the used polymers.

Amphiphilic Block Copolymer Micelles in Selective Solvents: The Effect of Solvent Selectivity on Micelle Formation.

We investigated the micellar behavior of a series of asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers in different P4VP-selective alcoholic solvents. The micellar behavior was further correlated with the spectroscopic ellipsometry results obtained on swelling of PS and P4VP polymer films in the corresponding solvent vapors. The time-resolved (in situ) dynamic light scattering (DLS) measurements, in combination with (ex situ) electron microscopy imaging, revealed information about the aggregation state of PS-b-P4VP BCP in different alcohols and the effect of heat treatment. The ellipsometry measurements allowed us to estimate the difference in solvent selectivity toward PS/P4VP pair. Both DLS and ellipsometric studies suggested that less polar alcohols (i.e., 1-propanol, 1-butanol, and 1-pentanol) are likely to be close to each other in terms of their selectivity toward PS/P4VP pair, whereas more polar ethanol and methanol show the highest and the lowest affinity toward P4VP, respectively.

Free Polyethylenimine Enhances Substrate-Mediated Gene Delivery on Titanium Substrates Modified With RGD-Functionalized Poly(acrylic acid) Brushes.

Substrate mediated gene delivery (SMD) is a method of immobilizing DNA complexes to a substrate via covalent attachment or nonspecific adsorption, which allows for increased transgene expression with less DNA compared to traditional bolus delivery. It may also increase cells receptivity to transfection via cell-material interactions. Substrate modifications with poly(acrylic) acid (PAA) brushes may improve SMD by enhancing substrate interactions with DNA complexes via tailored surface chemistry and increasing cellular adhesion via moieties covalently bound to the brushes. Previously, we described a simple method to graft PAA brushes to Ti and further demonstrated conjugation of cell adhesion peptides (i.e., RGD) to the PAA brushes to improve biocompatibility. The objective of this work was to investigate the ability of Ti substrates modified with PAA-RGD brushes (PAA-RGD) to immobilize complexes composed of branched polyethyleneimine and DNA plasmids (bPEI-DNA) and support SMD in NIH/3T3 fibroblasts. Transfection in NIH/3T3 cells cultured on bPEI-DNA complexes immobilized onto PAA-RGD substrates was measured and compared to transfection in cells cultured on control surfaces with immobilized complexes including Flat Ti, PAA brushes modified with a control peptide (RGE), and unmodified PAA. Transfection was two-fold higher in cells cultured on PAA-RGD compared to those cultured on all control substrates. While DNA immobilization measured with radiolabeled DNA indicated that all substrates (PAA-RGD, unmodified PAA, Flat Ti) contained nearly equivalent amounts of loaded DNA, ellipsometric measurements showed that more total mass (i.e., DNA and bPEI, both complexed and free) was immobilized to PAA and PAA-RGD compared to Flat Ti. The increase in adsorbed mass may be attributed to free bPEI, which has been shown to improve transfection. Further transfection investigations showed that removing free bPEI from the immobilized complexes decreased SMD transfection and negated any differences in transfection success between cells cultured on PAA-RGD and on control substrates, suggesting that free bPEI may be beneficial for SMD in cells cultured on bPEI-DNA complexes immobilized on PAA-RGD grafted to Ti. This work demonstrates that substrate modification with PAA-RGD is a feasible method to enhance SMD outcomes on Ti and may be used for future applications such as tissue engineering, gene therapy, and diagnostics.

Interactions of bioactive molecules with thin dendritic glycopolymer layers.

The authors report on highly swellable, stable layers of spherical dendritic glycopolymers, composed of hyperbranched poly(ethylene imine) (PEI) as core and two different maltose shells (A = dense shell and B = open shell). These glycopolymers are cross-linked and attached with poly(ethylene-alt-maleic anhydride) (PEMA) or citric acid on SiOx substrates. The swelling and adsorption of biomolecules were analyzed by spectroscopic ellipsometry and quartz crystal microbalance with dissipation. The swelling degree and complexation with the drug molecule adenosine triphosphate (ATP) were found to be up to 10 times higher for dendritic glycopolymer layers cross-linked with PEMA than for layers cross-linked with citric acid. ATP complexation by electrostatic interaction with the PEI cores was confirmed by x-ray photoelectron spectroscopy analysis. Complexation led to partial collapsing, stiffening, and increase of polymer layer viscosity of the PEMA cross-linked layers. From modeling of ellipsometric data, it was deduced that ATP complexation preferably takes place at the polymer layer-solution interface. The size effect of the adsorbates was investigated by comparing ATP complexation with the adsorption of larger vitamin B12 and human serum albumin (HSA) protein. PEI-Mal A cross-linked with PEMA was found to be resistant toward B12 and HSA adsorption due to the diffusion barrier of the polymer layer. Thus, the authors present potentially biocompatible polymer surfaces for drug loading and their surface supported release.

Biofunctionalization of Titanium Substrates Using Nanoscale Polymer Brushes with Cell Adhesion Peptides.

The grafting of polymer brushes to substrates is a promising method to modify surface properties such as wettability and the affinity toward proteins and cells for applications in microelectronics, biomedical devices, and sensors. Poly(acrylic) acid (PAA) brushes are of high interest because of their stimuli-responsive behavior and the presence of carboxy (COOH) groups, which allow for immobilization of bioactive molecules. The "grafting-to" approach results in homogeneous and well-defined polymer brushes, but, although grafting-to has been demonstrated with PAA brushes on silicon (Si) substrates, it has not been performed on biocompatible materials such as titanium (Ti). Here, we have described a facile method to modify biocompatible Ti substrates with PAA brushes to amplify their substrate functionality. The grafting-to PAA "pseudo" brushes were successfully grafted to Ti substrates and retained their pH-dependent swelling behavior. An RGD peptide was covalently bound to COOH groups of the PAA brushes (PAA-RGD) as a model bioactive group. While NIH/3T3 cell adhesion was significantly decreased on PAA-functionalized Ti substrates, PAA-RGD on Ti had cell adhesion comparable to that of flat Ti at 24 and 48 h, with significantly more cells adhered to PAA-RGD compared to PAA on Ti at 48 h.

Bioinspired thermoresponsive nanoscaled coatings: Tailor-made polymer brushes with bioconjugated arginine-glycine-aspartic acid-peptides.

The development of bioengineered surface coatings with stimuli-responsive properties is beneficial for a number of biomedical applications. Environmentally responsive and switchable polymer brush systems have a great potential to create such smart biointerfaces. This study focuses on the bioconjugation of cell-instructive peptides, containing the arginine-glycine-aspartic acid tripeptide sequence (RGD motif), onto well-defined polymer brush films. Herein, the highly tailored end-grafted homo polymer brushes are either composed of the polyelectrolyte poly(acrylic) acid (PAA), providing the reactive carboxyl functionalities, or of the temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm). Of particular interest is the preparation of grafted-to binary brushes using both polymers and their subsequent conversion to RGD-biofunctionalized PNIPAAm-PAA binary brushes by a carbodiimide conjugation method. The bioconjugation process of two linear RGD-peptides Gly-Arg-Gly-Asp-Ser and Gly-Arg-Gly-Asp-Ser-Pro-Lys and one cyclic RGD-peptide cyclo(Arg-Gly-Asp-D-Tyr-Lys) is comparatively investigated by complementary analysis methods. Both techniques, in situ attenuated total reflectance Fourier transform infrared spectroscopy measurements and the in situ spectroscopic ellipsometric analysis, describe changes of the brush surface properties due to biofunctionalization. Besides, the bound RGD-peptide amount is quantitatively evaluated by ellipsometry in comparison to high performance liquid chromatography analysis data. Additionally, molecular dynamic simulations of the RGD-peptides themselves allow a better understanding of the bioconjugation process depending on the peptide properties. The significant influence on the bioconjugation result can be derived, on the one hand, of the polymer brush composition, especially from the PNIPAAm content, and, on the other hand, of the peptide dimension and its reactivity.

Salt Sensitivity of the Thermoresponsive Behavior of PNIPAAm Brushes.

We report investigations on the salt sensitivity of the thermoresponsive behavior of PNIPAAm brushes applying the quartz crystal microbalance coupled with spectroscopic ellipsometry technique. This approach enables a detailed study of the optical and mechanical behavior of the polymer coatings. Additional conclusions can be drawn from the difference between both techniques due to a difference in the contrast mechanism of both methods. A linear shift of the phase-transition temperature to lower temperatures with the addition of sodium chloride was found, similar to the behavior of free polymer chains in solution. The thermal hysteresis was found to be decreased by the addition of sodium chloride to the solution, hinting to the interaction of the ions with the amide groups of the polymer, whereby the formation of hydrogen bonds is hindered. The results of this study are of relevance to the application of PNIPAAm brushes in biological fluids and demonstrate the additional potential of the ion sensitivity besides the better known thermosensitivity.

In Situ Monitoring of Linear RGD-Peptide Bioconjugation with Nanoscale Polymer Brushes.

Bioinspired materials mimicking the native extracellular matrix environment are promising for biotechnological applications. Particularly, modular biosurface engineering based on the functionalization of stimuli-responsive polymer brushes with peptide sequences can be used for the development of smart surfaces with biomimetic cues. The key aspect of this study is the in situ monitoring and analytical verification of the biofunctionalization process on the basis of three complementary analytical techniques. In situ spectroscopic ellipsometry was used to quantify the amount of chemisorbed GRGDS at both the homopolymer poly(acrylic acid) (PAA) brush and the binary poly(N-isopropylacrylamide) (PNIPAAm)-PAA brushes, which was finally confirmed by an acidic hydrolysis combined with a subsequent reverse-phase high-performance liquid chromatography analysis. In situ attenuated total reflection-Fourier transform infrared spectroscopy provided a step-by-step detection of the biofunctionalization process so that an optimized protocol for the bioconjugation of GRGDS could be identified. The optimized protocol was used to create a temperature-responsive binary brush with a high amount of chemisorbed GRGDS, which is a promising candidate for the temperature-sensitive control of GRGDS presentation in further cell-instructive studies.

Adsorption of enzymes to stimuli-responsive polymer brushes: Influence of brush conformation on adsorbed amount and biocatalytic activity.

Polyelectrolyte brushes can be utilized to immobilize enzymes on macroscopic surfaces. This report investigates the influence of the pH value of the surrounding medium on the amount and the activity of enzymes adsorbed to poly(2-vinylpyridine) and poly(acrylic acid) brushes, as well as the creation of thermoresponsive biocatalytically active coatings via the adsorption of enzymes onto a mixed brush consisting of a polyelectrolyte and temperature-sensitive poly(N-isopropylacryl amide). Spectroscopic ellipsometry and attenuated total reflection-Fourier transform infrared spectroscopy are used to monitor the adsorption process. Additionally, infrared spectra are evaluated in terms of the secondary structure of the enzymes. Glucose oxidase is used as a model enzyme, where the enzymatic activity is measured after different adsorption conditions. Poly(acrylic acid) brushes generally adsorb larger amounts of enzyme, while less glucose oxidase is found on poly(2-vinylpyridine), which however exhibits higher specific activity. This difference in activity could be attributed to a difference in secondary structure of the adsorbed enzyme. For glucose oxidase adsorbed to mixed brushes, switching of enzymatic activity between an active state at 20°C and a less active state at 40°C as compared to the free enzyme in solution is observed. However, this switching is strongly depending on pH in mixed brushes of poly(acrylic acid) and poly(N-isopropylacryl amide) due to interactions between the polymers.

Nanostructured Biointerfaces: Nanoarchitectonics of Thermoresponsive Polymer Brushes Impact Protein Adsorption and Cell Adhesion.

Controlling the reversibility, quantity, and extent of biomolecule interaction at interfaces has a significant relevance for biomedical and biotechnological applications, because protein adsorption is always the first step when a solid surface gets in contact with a biological fluid. Polymer brushes, composed of end-tethered linear polymers with sufficient grafting density, are very promising to control and alter interactions with biological systems because of their unique structure and distinct collaborative response to environmental changes. We studied protein adsorption and cell adhesion at polymer brush substrates which consisted of poly(N-isopropylacrylamide) (PNIPAAm), having a lower critical solution temperature (LCST), to control bioadsorptive processes by changing the environmental temperature. Preparing the PNIPAAm brushes by the "grafting-to"-method two differently synthesized PNIPAAm polymers were used, at which one possessed an additional hydrophobic terminal headgroup. It is known that hydrophobic moieties can influence protein adsorption significantly. The films were comprehensively analyzed by in situ spectroscopic ellipsometry, contact angle measurements, streaming potential, and atomic force microscopy. Our study was mainly focused on the investigation of the fibrinogen (FGN) adsorption responsiveness both on homo polymer PNIPAAm brushes with and without the hydrophobic terminal functionalization, and further on binary brushes made of the polyelectrolyte poly(acrylic acid) (PAA) and one of the prior described two PNIPAAm species. The results show that the terminal hydrophobic modification of PNIPAAm has a considerable impact on wettability, LCST, and morphology of the homo and the binary brush systems, which consequently led to an alteration of FGN adsorption. By using binary PNIPAAm-PAA brushes with different composition it was possible to induce stimuli dependent FGN adsorption with a considerable amplified switching effect by introducing a hydrophobic terminal residue to PNIPAAm. Cell adhesion studies with human mesenchymal stem cells reflected the results of the FGN adsorption.

In Situ Infrared Ellipsometry for Protein Adsorption Studies on Ultrathin Smart Polymer Brushes in Aqueous Environment.

The protein-adsorbing and -repelling properties of various smart nanometer-thin polymer brushes containing poly(N-isopropylacrylamide) and poly(acrylic acid) with high potential for biosensing and biomedical applications are studied by in situ infrared-spectroscopic ellipsometry (IRSE). IRSE is a highly sensitive nondestructive technique that allows protein adsorption on polymer brushes to be investigated in an aqueous environment as external stimuli, such as temperature and pH, are varied. These changes are relevant to conditions for regulation of protein adsorption and desorption for biotechnology, biocatalysis, and bioanalytical applications. Here brushes are used as model surfaces for controlling protein adsorption of human serum albumin and human fibrinogen. The important finding of this work is that IRSE in the in situ experiments in protein solutions can distinguish between contributions of polymer brushes and proteins. The vibrational bands of the polymers provide insights into the hydration state of the brushes, whereas the protein-specific amide bands are related to changes of the protein secondary structure.