Dynamics of swollen gel layers anchored to solid surfaces

Space-resolved dynamic light scattering within a millimeter-sized drop: From Brownian diffusion to the swelling of hydrogel beads

We present a dynamic light scattering setup to probe, with time and space resolution, the microscopic dynamics of soft matter systems confined within millimeter-sized spherical drops. By using an ad hoc optical layout, we tackle the challenges raised by refraction effects due to the unconventional shape of the samples. We first validate the setup by investigating the dynamics of a suspension of Brownian particles. The dynamics measured at different positions in the drop, and hence different scattering angles, are found to be in excellent agreement with those obtained for the same sample in a conventional light scattering setup. We then demonstrate the setup capabilities by investigating a bead made of a polymer hydrogel undergoing swelling. The gel microscopic dynamics exhibit a space dependence that strongly varies with time elapsed since the beginning of swelling. Initially, the dynamics in the periphery of the bead are much faster than in the core, indicative of nonuniform swelling. As the swelling proceeds, the dynamics slow down and become more spatially homogeneous. By comparing the experimental results to numerical and analytical calculations for the dynamics of a homogeneous, purely elastic sphere undergoing swelling, we establish that the mean square displacement of the gel strands deviates from the affine motion inferred from the macroscopic deformation, evolving from fast diffusivelike dynamics at the onset of swelling to slower, yet supradiffusive, rearrangements at later stages.

Thermoresponsive and Photocrosslinkable Poly(2-alkyl-2-oxazoline) Toolbox - Customizable Ultralow-Fouling Hydrogel Coatings for Blood Plasma Environments

This study focuses on developing surface coatings with excellent antifouling properties, crucial for applications in the medical, biological, and technical fields, for materials and devices in direct contact with living tissues and bodily fluids such as blood. This approach combines thermoresponsive poly(2-alkyl-2-oxazoline)s, known for their inherent protein-repellent characteristics, with established antifouling motifs based on betaines. The polymer framework is constructed from various monomer types, including a novel benzophenone-modified 2-oxazoline for photocrosslinking and an azide-functionalized 2-oxazoline, allowing subsequent modification with alkyne-substituted antifouling motifs through copper(I)-catalyzed azide-alkyne cycloaddition. From these polymers surface-attached networks are created on benzophenone-modified gold substrates via photocrosslinking, resulting in hydrogel coatings with several micrometers thickness when swollen with aqueous media. Given that poly(2-alkyl-2-oxazoline)s can exhibit a lower critical solution temperature in water, their temperature-dependent solubility is compared to the swelling behavior of the surface-attached hydrogels upon thermal stimulation. The antifouling performance of these hydrogel coatings in contact with human blood plasma is further evaluated by surface plasmon resonance and optical waveguide spectroscopy. All surfaces demonstrate extremely low retention of blood plasma components, even with undiluted plasma. Notably, hydrogel layers with sulfobetaine moieties allow efficient penetration by plasma components, which can then be easily removed by rinsing with buffer.

Confined Dynamics in Spherical Polymer Brushes

We investigate the dynamics of polymers grafted to spherical nanoparticles in solution using hybrid molecular dynamics simulations with a coarse-grained solvent modeled via the multiparticle collision dynamics algorithm. The mean-square displacements of monomers near the surface of the nanoparticle exhibit a plateau on intermediate time scales, indicating confined dynamics reminiscent of those reported in neutron spin-echo experiments. The confined dynamics vanish beyond a specific radial distance from the nanoparticle surface that depends on the polymer grafting density. We show that this dynamical confinement transition follows theoretical predictions for the critical distance associated with the structural transition from confined to semidilute brush regimes. These findings suggest the existence of a hitherto unreported dynamic length scale connected with theoretically predicted static fluctuations in spherical polymer brushes and provide new insights into recent experimental observations.

Photomotion of Hydrogels with Covalently Attached Azo Dye Moieties—Thermoresponsive and Non-Thermoresponsive Gels

The unique photomotion of azo materials under irradiation has been in the focus of research for decades and has been expanded to different classes of solids such as polymeric glasses, liquid crystalline materials, and elastomers. In this communication, azo dye-containing gels are obtained by photocrosslinking of non-thermoresponsive and lower critical solution temperature type thermoresponsive copolymers. These are analysed with light microscopy regarding their actuation behaviour under laser irradiation. The influences of the cloud-point temperature and of the laser power are investigated in a series of comparative experiments. The thermoresponsive hydrogels show more intense photoactuation when the cloud-point temperature of the non-crosslinked polymer is above, but closer to, room temperature, while higher laser powers lead to stronger motion, indicating a photothermal mechanism. In non-thermoresponsive gels, considerably weaker photoactuation occurs, signifying a secondary mechanism that is a direct consequence of the optical field-azo dye interaction.

Rapid Actuation of Thermo-Responsive Polymer Networks: Investigation of the Transition Kinetics

The swelling and collapsing of thermo-responsive poly(N-isopropylacrylamide)-based polymer (pNIPAAm) networks are investigated in order to reveal the dependency on their kinetics and maximum possible actuation speed. The pNIPAAm-based network was attached as thin hydrogel film to lithographically prepared gold nanoparticle arrays to exploit their localized surface plasmon resonance (LSPR) for rapid local heating. The same substrate also served for LSPR-based monitoring of the reversible collapsing and swelling of the pNIPAAm network through its pronounced refractive index changes. The obtained data reveal signatures of multiple phases during the volume transition, which are driven by the diffusion of water molecules into and out of the network structure and by polymer chain re-arrangement. For the micrometer-thick hydrogel film in the swollen state, the layer can respond as fast as several milliseconds depending on the strength of the heating optical pulse and on the tuning of the ambient temperature with respect to the lower critical solution temperature of the polymer. Distinct differences in the time constants of swelling and collapse are observed and attributed to the dependence of the cooperative diffusion coefficient of polymer chains on polymer volume fraction. The reported results may provide guidelines for novel miniature actuator designs and micromachines that take advantages of the non-reciprocal temperature-induced volume transitions in thermo-responsive hydrogel materials.

Thermal Response and Thermochromism of Methyl Red-Based Copolymer Systems - Coupled Responsiveness in Critical Solution Behaviour and Optical Absorption Properties

Until now, only limited experimental knowledge and sparse theoretical treatment about the mechanisms of thermochromism of azo dyes in solution has been available. Especially the coupling of thermoresponsiveness of polymers...

"CHicable" and "Clickable" Copolymers for Network Formation and Surface Modification

In this study, we present the generation of novel, multifunctional polymer networks through a combination of C,H-insertion cross-linking (CHic) and click chemistry. To this, copolymers consisting of hydrophilic N,N-dimethylacrylamide as matrix component and repeat units containing azide moieties, as well as benzophenone or anthraquinone groups, are generated. The benzophenone or anthraquinone groups allow photo-cross-linking, surface attachment or covalent immobilization of adjacent (bio)molecules through CHic reactions. The azide moieties either can react with available alkynes through conventional click reactions or can be activated to form nitrenes, which can also undergo CHic reactions. By choosing appropriate reaction conditions, the same polymer can be used to follow very different reaction paths, opening up a plethora of choices for the generation of functional polymer networks. In the exemplary presented case ("CHic-Click"), irradiation of the copolymers with UV-A light (λ_irr = 365 nm) leads to cross-linking (network formation) and surface attachment simultaneously. The azide units remain intact during this cross-linking step, and alkyne-modified (bio)molecules can be bound through click reactions. Biofunctionalization of the polymer network with alkynylated streptavidin, followed by application of biotin-conjugated antibody and a model analyte, highlights the potential of these surface architectures as a toolbox which can be adapted for diverse bioanalytical applications.

Bioinspired All-Polyester Diblock Copolymers Made from Poly(pentadecalactone) and Poly(2-(2-hydroxyethoxy)benzoate): Synthesis and Polymer Film Properties

The bioinspired diblock copolymers poly(pentadecalactone)-block-poly(2-(2-hydroxyethoxy)-benzoate) (PPDL-block-P2HEB) were synthesized from pentadecalactone and dihydro-5H-1,4-benzodioxepin-5-one (2,3-DHB). No transesterification between the blocks was observed. In a sequential approach, PPDL obtained by ring-opening polymerization (ROP) was used to initiate 2,3-DHB. Here, the molar mass Mn of the P2HEB block was limited. In a modular approach, end-functionalized PPDL and P2HEB were obtained separately by ROP with functional initiators, and connected by 1,3-dipolar Huisgen reaction ("click-chemistry"). Block copolymer compositions from 85:15 mass percent to 28:72 mass percent (PPDL:P2HEB) were synthesized, with Mn of from about 30,000-50,000 g mol-1. The structure of the block copolymer was confirmed by proton NMR, FTIR spectroscopy, and gel permeation chromatography. Morphological studies by atomic force microscopy (AFM) further confirmed the block copolymer structure, while quantitative nanomechanical AFM measurements revealed that the DMT moduli of the block copolymers ranged between 17.2 ± 1.8 MPa and 62.3 ± 5.7 MPa, i.e. between the values of the parent P2HEB and PPDL homopolymers (7.6 ± 1.4 MPa and 801 ± 42 MPa, respectively). Differential scanning calorimetry showed that the thermal properties of the homopolymers were retained by each of the copolymer blocks (melting temperature 90 °C, glass transition temperature 36 °C).

Bioinspired All-Polyester Diblock Copolymers Made from Poly(pentadecalactone) and Poly(3-hydroxycinnamate): Synthesis and Polymer Film Properties

A bioinspired diblock copolymer was synthesized from pentadecalactone and 3-hydroxy cinnamic acid. Poly(pentadecalactone) (PPDL) with a molar mass of up to 43,000 g mol-1 was obtained by ring-opening polymerization initiated propargyl alcohol. Poly(3-hydroxy cinnamate) (P3HCA) was obtained by polycondensation and end-functionalized with 3-azido propanol. The two functionalized homopolymers were connected via 1,3-dipolar Huisgen addition to yield the block copolymer PPDL-triazole-P3HCA. The structure the block copolymer was confirmed by proton NMR, FTIR spectroscopy and GPC. By analyzing the morphology of polymer films made from the homopolymers, from a 1:1 homopolymer blend, and from the PPDL-triazole-P3HCA block copolymer, clearly distinct micro- and nanostructures were revealed. Quantitative nanomechanical measurements revealed that the block copolymer PPDL-triazole-P3HCA had a DMT modulus of 22.3 ± 2.7 MPa, which was lower than that of the PPDL homopolymer (801 ± 42 MPa), yet significantly higher than that of the P3HCA homopolymer (1.77 ± 0.63 MPa). Thermal analytics showed that the melting point of PPDL-triazole-P3HCA was similar to PPDL (89-90 °C), while it had a glass transition was similar to P3HCA (123-124 °C). Thus, the semicrystalline, potentially degradable all-polyester block copolymer PPDL-triazole-P3HCA combines the thermal properties of either homopolymer, and has an intermediate elastic modulus.

Stimulus-Responsive Polyzwitterionic Surfaces Made from Itaconic Acid: Self-Triggered Antimicrobial Activity, Protein Repellency, and Cell Compatibility

A functional monomer carrying a carboxylate and a protected primary ammonium group is synthesized from itaconic acid. When copolymerized with dimethyl acrylamide and 4-methacryloyloxybenzophenone, cross-linkable polyzwitterions are obtained. These are converted to surface-attached polyzwitterion networks by simultaneous UV-triggered C,H insertion reactions. The resulting polyzwitterion-coated substrates were studied by surface plasmon resonance spectroscopy measurements, ζ potential and various biological assays. They were (expectedly) protein repellent, yet at the same time (and unexpectedly) cell-adhesive and antimicrobially active. This was attributed to stimulus-responsiveness of the polyzwitterion (confirmed by the ζ potential measurements), which enables charge adjustment at different pH values. When protonated, the polyzwitterions become amphiphilic polycations and, in this state, kill bacteria upon contact like their parent structures (polymer-based synthetic mimics of antimicrobial peptides, SMAMPs).

Structure and dynamics of hagfish mucin in different saline environments

The defense mechanism of hagfish against predators is based on its ability to form slime within a few milliseconds. Hagfish slime consists of two main components, namely mucin-like glycoproteins and long protein threads, which together entrap vast amounts of water and thus form a highly dilute hydrogel. Here, we investigate the mucin part of this hydrogel, in particular the role of the saline marine environment on the viscoelasticity and structure. By means of dynamic light scattering (DLS), shear and extensional rheology we probe the diffusion dynamics, the flow behavior, and the longest filament breaking time of hagfish mucin solutions. Using DLS we find a concentration-independent diffusion coefficient - characteristic for polyelectrolytes - up to the entanglement regime of 0.2 mg ml-1, which is about ten times higher than the natural concentration of hagfish mucin in hagfish slime. We also observe a slow relaxation process associated with clustering, probably due to electrostatic interactions. Shear rheology further revealed that hagfish mucin possesses pronounced viscoelastic properties at high concentrations (3 mg ml-1), showing that mucin alone achieves mechanical properties similar to those of natural hagfish slime (mucins and protein threads). The main effects of added seawater salts, and predominantly CaCl2 is to reduce the intensity of the slow relaxation process, which suggests that calcium ions lead to an ionotropic gelation of hagfish mucins.

Wafer-Scale Fabrication of Conducting Polymer Hydrogels for Microelectrodes and Flexible Bioelectronics

Future-oriented directions in neural interface technologies point towards the development of multimodal devices that combine different functionalities such as neural stimulation, neurotransmitter sensing, and drug release within one platform. Conducting polymer hydrogels (CPHs) are suggested as materials for the coating of standard metal electrodes to add functionalities such as local delivery of therapeutic drugs. However, to make such coatings truly useful for multimodal devices, it is necessary to develop process technologies that allow the micropatterning of CPHs onto selected electrode sites. In this study, a wafer-scale fabrication procedure is presented, which is used to coat the CPH, based on the hydrogel P(DMAA-co-5%MABP-co-2,5%SSNa) and the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT), onto flexible neural probes. The resulting material has favorable properties for the generation of recording electrodes and in addition offers a convenient platform for biofunctionalization. By controlling the PEDOT content within the hydrogel matrix, charge injection limits of up to 3.7 mC cm^{- 2} are obtained. Long-term stability is tested by immersing coated samples in phosphate-buffered saline solution at 37 °C for 1 year. Non-cytotoxicity of the coatings is confirmed with a direct cell culture test using a fluorescent neuroblastoma cell line.

Self-Regenerating Antimicrobial Polymer Surfaces via Multilayer-Design - Sequential and Triggered Layer Shedding under Physiological Conditions

Regeneration of materials properties through surface regeneration could extend the lifetime of devices and is still an emerging field of research. (Self-)regenerating antimicrobial polymer surfaces could help to fight biofilm formation and related bacterial infections. In this paper, four different polymer multilayer designs for the regeneration of antimicrobial surfaces by layer shedding are presented. The multilayer architectures consist of 100-200 nm thick, discrete polymer layers. They are made from poly(guanidinium oxanorbornene) networks as the antimicrobial component, and different interlayers made from degradable poly(adipic anhydrides), depolymerizable poly(ethyl glyoxylate), or water-soluble poly(acrylamide). Layer shedding is designed to occur after hydrolysis, dissolution or depolymerization under simulated physiological conditions. The multilayer fabrication and disassembly is monitored by fluorescence microscopy, ellipsometry FT-IR spectroscopy and atomic force microscopy. By testing the antimicrobial activity of the restored surfaces, their functional integrity after layer shedding is confirmed.

Surface Structuring Combined with Chemical Surface Functionalization: An Effective Tool to Manipulate Cell Adhesion

In this study, we investigate how a surface structure underneath a surface-attached polymer coating affects the bioactivity of the resulting material. To that end, structured surfaces were fabricated using colloidal lithography (lateral dimensions: 200 nm to 1 µm, height ~15 to 50 nm). The surface structures were further functionalized either with antimicrobial, cell-adhesive polycations or with protein-repellent polyzwitterions. The materials thus obtained were compared to non-functionalized structured surfaces and unstructured polymer monolayers. Their physical properties were studied by contact-angle measurements and atomic force microscopy (AFM). Protein adhesion was studied by surface plasmon resonance spectroscopy, and the antimicrobial activity against Escherichia coli bacteria was tested. The growth of human mucosal gingiva keratinocytes on the materials was analyzed using the Alamar blue assay, optical microscopy, and live-dead staining. The data shows that the underlying surface structure itself reduced protein adhesion and also bacterial adhesion, as evidenced by increased antimicrobial activity. It also enhanced cell adhesion to the surfaces. Particularly in combination with the adhesive polycations, the surfaces increased the cell growth compared to the unstructured reference materials. Thus, functionalizing structured surfaces with adhesive polymer could be a valuable tool for improved tissue integration.

Three-Dimensional, Bifunctional Microstructured Polymer Hydrogels Made from Polyzwitterions and Antimicrobial Polymers

Biofilm-associated infections of medical devices are a global problem. For the prevention of such infections, biomaterial surfaces are chemically or topographically modified to slow down the initial stages of biofilm formation. In the bifunctional material here presented, chemical and topographical cues are combined, so that protein and bacterial adhesion as well as bacterial proliferation are effectively inhibited. Upon changes in the surface topography parameters and investigation of the effect of these changes on bioactivity, structure-property relationships are obtained. The target material is obtained by microcontact printing (μCP), a soft lithography method. The antimicrobial component, poly(oxanorbornene)-based synthetic mimics of an antimicrobial peptide (SMAMP), was printed onto a protein-repellent polysulfobetaine hydrogel, so that bifunctional 3D structured polymer surfaces with 1, 2, and 8.5 μm spacing are obtained. These surfaces are characterized with fluorescence microscopy, surface plasmon resonance spectroscopy, atomic force microscopy, and contact angle measurements. Biological studies show that the bifunctional surfaces with 1 and 2 μm spacing are 100% antimicrobially active against Escherichia coli and Staphylococcus aureus, 100% fibrinogen-repellent, and nontoxic to human gingival mucosal keratinocytes. At 8.5 μm spacing, the broad-band antimicrobial activity and the protein repellency are compromised, which indicates that this spacing is above the upper limit for effective simultaneous antimicrobial activity and protein repellency of polyzwitterionic-polycationic materials.

Non-delaminating Polymer Hydrogel Coatings via C,H-Insertion Crosslinking (CHic) - A Case Study of Poly(oxanorbornenes)

Robust, non-delaminating polymer coatings and hydrogels are needed for technical and biomedical applications. This study focusses on surface-attached poly(oxanorbornene) hydrogels obtained by simultaneous UV-activated crosslinking and surface-immobilization. The synthesis and copolymerization of two oxanorbornene monomers carrying the UV-crosslinkers malonic acid diazoester or benzophenone, which can both undergo UV-triggered C,H-insertion crosslinking (CHic), is presented. The crosslinking efficiency and network stability of hydrogels made from these self-crosslinkable polymers are studied and compared to the properties of poly(oxanorbornene) networks obtained by UV-triggered thiol-ene-reactions involving a low molecular weight crosslinker. Smooth, defect-free, non-delaminating hydrogel coatings were obtained by CHic, not only on laboratory model surfaces but also on a technical product.

Simultaneously Antimicrobial, Protein-repellent and Cell-compatible Polyzwitterion Networks: More Insight on Bioactivity and Physical Properties

A poly(oxanorbornene)-based polyzwitterion with primary ammonium and carboxylate groups (PZI) has been reported previously as the first simultaneously antimicrobial and protein-repellent polyzwitterion. Here, additional physical and biological properties of three poly(oxanorbornene)-based polyzwitterions with different functional groups (PZI, the polycarboxybetaine PCB, and the polysulfobetaine PSB) are compared to understand the molecular origins of this unusual bioactivity. Additionally, the three polyzwitterions and the antimicrobial, polycationic SMAMP are exposed to proteins, bacteria suspensions, human plasma and serum. These interactions are investigated by surface plasmon resonance spectroscopy. In protein adhesion studies, neither fibrinogen nor lysozyme adhere irreversibly to PZI, yet reversible interaction with lysozyme is observed at pH 7 and 8. In the presence of bivalent cations, reversible fibrinogen adhesion on PZI and PSB is observed, but not on PCB. This might explain why mammalian cells grow on PZI and PSB, but not on PCB. PZI does not show human plasma adhesion, while PCB and PSB have 0.27 and 0.48 ng mm^-2 adhered plasma, and SMAMP even 6.3 ng mm^-2. Both PZI and SMAMP show strong serum adhesion, while no serum adhered to PCB, and only little to PSB. This could be related to the pH difference between serum and plasma, to which the pH-responsive primary ammonium groups are susceptible, while the permanently charged NR₄ ⁺ groups are unaffected. Both PZI and PCB showed none or only little bacterial adhesion. PCB is also intrinsically antimicrobial against E. coli and S. aureus bacteria and thus is also simultaneously protein-repellent and antimicrobially active. Thus, while the carboxylate groups of PZI and PCB seems to be a prerequisite for the dual antimicrobial activity and protein-repellency, the pH-responsiveness of the primary ammonium group seems to make the PZI molecule vulnerable for protein adhesion in fluids that are slightly out of the physiological range.

Sub-micrometer Sized, 3D-Surface-attached Polymer Networks by Microcontact Printing: Using UV-Crosslinking Efficiency to Tune Structure Height

The lateral dimensions of micro- and nanostructures obtained by microcontact printing (μCP) can be easily varied by selecting stamps with the desired spacing and pattern. However, the height of these structures cannot be tuned as easily, and in most cases only 2D structures are obtained. Here, we show how the chemical cross-linking properties of polymer inks designed for μCP can be used to obtain 3D structures with heights ranging from 3 to 750 nm using the same μCP stamps. This is technologically relevant because the ink concentration affects the quality and resolution of the printed image, and therefore can only be varied in a certain range. By exploiting the cross-linking efficiency to tune the height, an additional parameter is available to reach the desired structure height without compromising the image quality. The inks were made from copolymers containing a low percentage of different UV cross-linkable repeat units: nitrobenzoxadiazole (NBD), coumarin (COU), and/or benzophenone (BP). The base polymer of the here presented model system was an antimicrobially active poly(oxanorbornene) (SMAMP), however the concept should be transferable to many other polymer backbones. We describe the fabrication and characterization of the printed micro- and nanostructures made from pure SMAMP, NBD-SMAMP, coumarin-SMAMP, BP-SMAMP, BP-NBD-SMAMP and BP-coumarin-SMAMP polymer inks. The photo-dimerization of COU during UV irradiation at λ = 254 nm was confirmed by UV-Vis spectroscopy. Since NBD and COU are fluorescent, the polymer could be visualized by fluorescence microscopy. Additionally, their height profiles were measured by atomic force microscopy (AFM). The heights of the 3D surface-attached polymer networks obtained from the here presented polymer inks correlated with the gel-content of the corresponding unstructured polymer layers, and thus with the cross-linking efficiency of the NBD, COU and BP cross-linkers. Due to being covalently cross-linked, these 3D-surface attached polymer structures were solvent-stable and stable in aqueous surroundings.

On the Limits of Benzophenone as Cross-Linker for Surface-Attached Polymer Hydrogels

The synthesis of different photo-reactive poly(alkenyl norbornenes) and poly(oxonorbornenes) containing benzophenone (BP) via ring-opening metatheses polymerization (ROMP) is described. These polymers are UV irradiated to form well-defined surface-attached polymer networks and hydrogels. The relative propensity of the polymers to cross-link is evaluated by studying their gel content and its dependency on BP content, irradiation wavelength (254 or 365 nm) and energy dose applied (up to 11 J·cm-²). Analysis of the UV spectra of the polymer networks demonstrates that the poly(oxonorbornenes) show the expected BP-induced crosslinking behavior at 365 nm, although high irradiation energy doses and BP content are needed. However, these polymers undergo chain scission at 254 nm. The poly(alkenyl norbornenes), on the other hand, do not cross-link at 365 nm, whereas moderate to good cross-linking is observed at 254 nm. UV spectra demonstrate that the cross-linking at 254 nm is due to BP cross-linking combined with a [2 + 2] cylcoaddition of the alkenyl double bonds. This indicates limitations of benzophenone as a universally applicable cross-linking for polymer networks and hydrogels.

Free-standing hydrogel-particle composite membrane with dynamically controlled permeability

The preparation and investigation of a free-standing membrane made from a composite of thermoresponsive poly(N-isopropylacrylamide) (pNIPAAm) and polystyrene nanoparticles (PS NP) with temperature-controlled permeability is reported. The method exploits the light-induced crosslinking of the photo-reactive pNIPAAm-based polymer and mechanical reinforcement of the membrane structure by the polystyrene nanoparticles. About micrometer thick layers were either directly attached to a gold surface or prepared as free-standing layers spanning over arrays of microfluidic channels with a width of about hundred microns by using template stripping. Diffusion of liquid medium, low molecular weight molecules, and large molecular weight proteins contained in blood through the composite membrane was observed with combined surface plasmon resonance (SPR) and optical waveguide spectroscopy (OWS). The swelling ratio, permeability, and nonspecific sorption to these composite membranes were investigated by SPR and OWS as a function of molecular weight of analyte, loading of PS NP in the composite film, and temperature. The authors show successful preparation of a defect-free membrane structure that acts as a thermoresponsive filter with nanoscale pores spanning over an area of several square millimeters. This membrane can be reversibly switched to block or allow the diffusion of low mass molecules to the sensor surface by temperature-triggered swelling and collapsing of the hydrogel component. Blocking of diffusion and low unspecific sorption of proteins contained in blood serum is observed. These features make this platform interesting for potential future applications in continuous monitoring biosensors for the analysis of low molecular weight drug analytes or for advanced cell-on-chip microfluidic studies.

An interpenetrating, microstructurable and covalently attached conducting polymer hydrogel for neural interfaces

This study presents a new conducting polymer hydrogel (CPH) system, consisting of the synthetic hydrogel P(DMAA-co-5%MABP-co-2,5%SSNa) and the conducting polymer (CP) poly(3,4-ethylenedioxythiophene) (PEDOT), intended as coating material for neural interfaces. The composite material can be covalently attached to the surface electrode, can be patterned by a photolithographic process to influence selected electrode sites only and forms an interpenetrating network. The hybrid material was characterized using cyclic voltammetry (CV), impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS), which confirmed a homogeneous distribution of PEDOT throughout all CPH layers. The CPH exhibited a 2,5 times higher charge storage capacity (CSC) and a reduced impedance when compared to the bare hydrogel. Electrochemical stability was proven over at least 1000 redox cycles. Non-toxicity was confirmed using an elution toxicity test together with a neuroblastoma cell-line. The described material shows great promise for surface modification of neural probes making it possible to combine the beneficial properties of the hydrogel with the excellent electronic properties necessary for high quality neural microelectrodes.

STATEMENT OF SIGNIFICANCE

Conductive polymer hydrogels have emerged as a promising new class of materials to functionalize electrode surfaces for enhanced neural interfaces and drug delivery. Common weaknesses of such systems are delamination from the connection surface, and the lack of suitable patterning methods for confining the gel to the selected electrode site. Various studies have reported on conductive polymer hydrogels addressing one of these challenges. In this study we present a new composite material which offers, for the first time, the unique combination of properties: it can be covalently attached to the substrate, forms an interpenetrating network, shows excellent electrical properties and can be patterned via UV-irradiation through a structured mask.

Surface Structuring Meets Orthogonal Chemical Modifications: Toward a Technology Platform for Site-Selectively Functionalized Polymer Surfaces and BioMEMS

A manufacturing process for the site-selective modification of structured (bio)material surfaces with two different polymers/biomolecules is presented. In the first step, a chemical surface contrast is created (e.g., a gold-on-silicon contrast obtained by colloidal lithography), and is combined with two orthogonal surface reactions for polymer/biomolecule immobilization. To demonstrate this, an antimicrobial SMAMP polymer and a protein-repellent polyzwitterion were site-selectively surface-immobilized on the gold-silicon structures. By varying the structure spacing and the surface architecture, structure-property relationships for the interaction of these bifunctional polymer surfaces with bacteria and proteins were obtained (studied by fluorescence microscopy, atomic force microscopy, surface plasmon resonance spectroscopy, and antimicrobial assays). At 1 μm spacing, a fully antimicrobially active bifunctional material was obtained, which also near-quantitatively reduced protein adhesion. As the process is generally applicable to polymers/biomolecules with aliphatic CH-groups, it is an interesting platform technology for site-selectively functionalized bifunctional (Bio)MEMS.

Towards Self-regenerating Antimicrobial Polymer Surfaces

Regeneration of functional polymer surfaces after damage or contamination is an unresolved scientific challenge, and also of practical importance. In this proof-of-concept study, we present a method to regenerate a functional surface property using a polymer multi-layer architecture. This is exemplified using antimicrobially active surfaces. The idea is to shed the top layer of the polymer layer stack, like a reptile shedding its skin. The proof-of-concept stack consists of two antimicrobial layers and a degradable interlayer. Shedding of the top layer is enabled by degrading that interlayer. The shedding process was analyzed by quantitative fluorescence microscopy, ellipsometry, and FTIR spectroscopy. Antimicrobial assays revealed that the functionality of the emerging antimicrobial layer was fully retained after shedding.

Active Control of SPR by Thermoresponsive Hydrogels for Biosensor Applications

The use of thermoresponsive poly(N-isopropylacrylamide)-based hydrogel (pNIPAAm) for rapid tuning of surface plasmon resonance (SPR) is reported. This approach is implemented by using an SPR layer architecture with an embedded indium tin oxide microheater and pNIPAAm film on its top. It takes advantage of rapid thermally induced swelling and collapse of pNIPAAm that is accompanied by large refractive index changes and leads to high thermo-optical coefficient of dn/dT = 2 × 10-2 RIU/K. We show that this material is excellently suited for efficient control of refractive index-sensitive SPR and that it can serve simultaneously as a 3D binding matrix in biosensor applications (if modified with biomolecular recognition elements for a specific capture of target analyte). We demonstrate that this approach enables modulating of the output signal in surface plasmon-enhanced fluorescence spectroscopy biosensors and holds potential for simple time-multiplexing of sensing channels for parallelized readout of fluorescence assays.

Preparation and characterization of light-switchable polymer networks attached to solid substrates

Surface-attached polymer networks that carry light-responsive nitrospiropyran groups in a hydrophilic PDMAA matrix were prepared on planar silicon and glass surfaces and were characterized with respect to their switching behavior under the influence of an external light trigger. Functional polymers bearing light-responsive units as well as photo-cross-linkable benzophenone groups were first synthesized using free radical copolymerization. The number of spiropyran groups in the copolymer was controlled by adjusting the concentration of the respective monomer in the copolymerization feed. The polymer films were prepared by spin-coating the functional polymers from solution and by ultraviolet light (UV)-induced cross-linking utilizing benzophenone photochemistry. On substrates with immobilized benzophenone groups, the complete polymer network is linked to the surface. The dry thickness of the films can be controlled over a wide range from a few nanometers up to more than 1 μm. The integration of such light-switchable organic moieties into a surface-attached polymer network allows one to increase the overall number of light-responsive groups per surface area by adjusting the amount of surface-attached polymer networks. The spiropyran's function in dry (solvent-free) and swollen polymer films can be reversibly switched by UV and visible irradiation. In addition, the switching in water is faster than in the dry state. Therefore, implementing light-responsive spiropyran functions in polymer films linked to solid surfaces could allow for switching of the chemical and optical surface properties in a fast and spatially controlled fashion.

Thin hydrogel films for optical biosensor applications

Hydrogel materials consisting of water-swollen polymer networks exhibit a large number of specific properties highly attractive for a variety of optical biosensor applications. This properties profile embraces the aqueous swelling medium as the basis of biocompatibility, non-fouling behavior, and being not cell toxic, while providing high optical quality and transparency. The present review focuses on some of the most interesting aspects of surface-attached hydrogel films as active binding matrices in optical biosensors based on surface plasmon resonance and optical waveguide mode spectroscopy. In particular, the chemical nature, specific properties, and applications of such hydrogel surface architectures for highly sensitive affinity biosensors based on evanescent wave optics are discussed. The specific class of responsive hydrogel systems, which can change their physical state in response to externally applied stimuli, have found large interest as sophisticated materials that provide a complex behavior to hydrogel-based sensing devices.

Analysis of optical gradient profiles during temperature- and salt-dependent swelling of thin responsive hydrogel films

Surface-attached, cross-linked hydrogel films based on thermoresponsive N-isopropylacrylamide with a dry thickness >1 microm were studied with surface plasmon resonance/optical waveguide mode spectroscopy (SPR/OWS) to monitor temperature-dependent and salt-induced changes of their swelling state. In combination with the reversed Wentzel-Kramers-Brillouin and Bruggeman effective medium approximation and by modeling the hydrogel film as a composite of sublayers with individual complex refractive indices, refractive index/volume fraction gradient profiles perpendicular to the surface are accessible simultaneously with information about local inhomogeneities. Specifically, the imaginary refractive index kappa of each sublayer can be interpreted as a measure for static and dynamic inhomogeneities, which were found to be highest at the volume transition collapse temperature in the layer center. These results indicate that the hydrogel collapse originates rather from the film center than from its boundaries. Upon addition of NaCl to a swollen hydrogel below its transition temperature, comparable optical loss characteristics as for the thermal gel collapse are observed with respect to inhomogeneities. Interestingly, in contrast to the thermally induced layer shrinkage and collapse, swelling increases at intermediate salt concentrations.

Atomic force spectroscopy of thermoresponsive photo-cross-linked hydrogel films

Responsive hydrogel thin films are interesting materials as responsive adhesives or as an active matrix in actuators and sensing applications, and thus, knowledge about their structural and micromechanical properties is of high relevance. Using atomic force spectroscopy, temperature-induced structural and adhesive changes of thermoresponsive hydrogel layers with micrometer thickness based on photo-cross-linked N-isopropylacrylamide (NiPAAm) were investigated in the temperature range of 18-50 degrees C. Grafted onto flat surfaces, these hydrogel layers are restricted to a highly anisotropic swelling and deswelling predominantly perpendicular to the substrate surface, which was monitored and evaluated by force spectroscopy during vertical tip approach and retraction. Analyses of the tip penetration depth yielded quantitative information about the degree of swelling. As a second feature, the critical temperature was found to decrease with increasing cross-linking density. Temperature-dependent measurements with hydrophobic and hydrophilic atomic force microscopy (AFM) tips revealed a strong adhesion to the hydrogel layer in the swollen state, which was reduced upon the layer volume collapse. These observations on the micrometer-thick gel network layers are in contrast to previous reports on ultrathin pNiPAAm brushes and monolayers, which show no adhesion in the swollen state but only in the collapsed state. Furthermore, it was found that the hydrophobicity of the hydrogel probed with a hydrophobic tip continuously increases with temperature over a broad range of at least 30 K.

Viscoelastic response of photo-cross-linked poly(N-isopropylacrylamide) coatings by QCM-D

The viscoelastic behavior of surface-tethered poly(N-isopropylacrylamide), or poly(NIPAAm), networks in contact with aqueous solutions was characterized by the quartz crystal microbalance with dissipation (QCM-D). To avoid ambiguities in the data analysis, four integer multiples of the dry thickness (h = k x 36 nm, where k = 1, 2, 3, 4) were analyzed at the third through ninth overtones of the QCM-D signal. At the third overtone, the networks resembled rigid films and the viscoelastic behavior could not be ascertained. With increasing overtones, however, the films showed deviation from the rigid film limit, allowing for analysis of the viscoelastic parameters. As the network collapsed over the temperature range of 15-40 degrees C, the shear modulus of the network increased by a factor of 24 (from 5 to 120 MPa); the shear viscosity, on the other hand, increased only by a factor of 2 (from 40 to 80 cP). The high values of the shear modulus suggest the QCM-D probes a regime where the polymer mesh does not adequately relax, and the high values of the shear viscosity suggest significant polymer-polymer coil overlap above and below the demixing temperature. Finally, the influence of NaCl on the viscoelastic response was measured. Interestingly, NaCl affects the shear modulus of the networks but not the dynamics.

Model lipid membranes on a tunable polymer cushion

A hydrated, surface-tethered polymer network capable of fivefold change in thickness over a 25-37 degrees C temperature range has been demonstrated via neutron reflectivity and fluorescence microscopy to be a novel support for single lipid bilayers in a liquid environment. As the polymer swells from 170 to 900 A, it promotes both in- and out-of-plane fluctuations of the supported membrane. The cushioned bilayer proved to be very robust, remaining structurally intact for 16 days and many temperature cycles. The promotion of membrane fluctuations offers far-reaching applications for this system as a surrogate biomembrane.

Optical waveguide spectroscopy for the investigation of protein-functionalized hydrogel films

This article reports the implementation of optical waveguide spectroscopy (OWS) for the quantitative time-resolved observation of changes in the swelling behavior and mass density of protein-functionalized hydrogel films. In the experiment, a thin film of an N-isopropylacrylamide (NIPAAm)-based polymer that supported optical waveguide modes is attached to a metallic sensor surface. IgG molecules are in situ immobilized in this gel by using novel coupling chemistry with a charge-attraction scheme based on a tetrafluorophenol sulfonate active ester. The anti-fouling properties of the functionalized hydrogel network and the kinetics of the affinity binding of protein molecules in the gel are investigated.

EPR spectroscopy reveals nanoinhomogeneities in the structure and reactivity of thermoresponsive hydrogels

The dynamic and chemical behavior of solute molecules inside new thermoresponsive hydrogels (photocrosslinked poly(N-isopropylacrylamide) (pNiPAAm) copolymers) is studied by continuous-wave electron paramagnetic resonance spectroscopy. Via addition of paramagnetic tracer molecules (so-called spin probes) a picture is obtained of the thermally induced collapse on the molecular scale, which proceeds over a substantially broader temperature range than indicated by the sharp macroscopic volume transition. The sampling of hydrophilic and hydrophobic environments suggests a discontinuous collapse mechanism with a coexistence of collapsed and expanded network regions. These structural inhomogeneities on the nanoscale also lead to an inhomogeneity in chemical reactivity. The hydrophilic regions form nanoreactors, which strongly accelerate the reaction while the hydrophobic regions act as nanoshelters, in which enclosed spin probes are protected from the decay. The results show that the system consisting of a statistical binary or tertiary copolymer displays remarkably complex behavior that mimics spatial and chemical inhomogeneities observed in functional biopolymers such as enzymes.