Xiaopei Deng

Ultrasmall implantable composite microelectrodes with bioactive surfaces for chronic neural interfaces

Implantable neural microelectrodes that can record extracellular biopotentials from small, targeted groups of neurons are critical for neuroscience research and emerging clinical applications including brain-controlled prosthetic devices. The crucial material-dependent problem is developing microelectrodes that record neural activity from the same neurons for years with high fidelity and reliability. Here, we report the development of an integrated composite electrode consisting of a carbon-fibre core, a poly(p-xylylene)-based thin-film coating that acts as a dielectric barrier and that is functionalized to control intrinsic biological processes, and a poly(thiophene)-based recording pad. The resulting implants are an order of magnitude smaller than traditional recording electrodes, and more mechanically compliant with brain tissue. They were found to elicit much reduced chronic reactive tissue responses and enabled single-neuron recording in acute and early chronic experiments in rats. This technology, taking advantage of new composites, makes possible highly selective and stealthy neural interface devices towards realizing long-lasting implants.

Structurally Controlled Bio‐hybrid Materials Based on Unidirectional Association of Anisotropic Microparticles with Human Endothelial Cells

Biocompatible anisotropic polymer particles with bipolar affinity towards human endothelial cells are a novel type of building blocks for microstructured bio-hybrid materials. Functional polarity due to two biologically distinct hemispheres has been achieved by synthesis of anisotropic particles via electro-hydrodynamic co-jetting of two different polymer solutions and subsequent selective surface modification.

Bio-orthogonal “Double-Click” Chemistry Based on Multifunctional Coatings

As a consequence of recent progress in biotechnology, regenerative medicine, and developments concerning medical implants, an increasing need for precise and flexible conjugation methods has emerged. For the immobilization of biomolecules, chemical reactions with high specificity towards the molecule of interest, mild reaction conditions compatible with physiological milieu, and rapid as well as quantitative conversion are essential. If defined immobilization of two or more biomolecules on the same surface in controlled ratios is required, the individual reactions not only need to be orthogonal with respect to ongoing biological events, but also with respect to each other. This prerequisite puts major constraints on the type of chemical reactions that can be exploited for bio-orthogonal immobilization. The development of bio-orthogonal reaction schemes has been heavily influenced by the concept of “click” chemistry, which was first introduced by Sharpless and co-workers in 2001. As the archetypal example of click chemistry, the copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition of azides and terminal alkynes (CuAAC) has since been widely used as a surfacemodification strategy. CuAAC is a highly efficient reaction under mild conditions, with complete regioselectivity for the 1,4-triazole product. Triazoles are stable linkers that are resistant to hydrolysis, oxidation, or reduction. Initial work was conducted on model surfaces, such as gold and silicon, but was recently extended to a wide range of different substrates. Chemical vapor deposition (CVD) polymerization is a versatile coating process that effectively decouples the surface chemistry from the bulk composition. The CVD polymerization of functionalized [2.2]paracyclophanes can result in functionalized poly(p-xylylene) coatings with a wide range of different groups including active esters, aldehydes, ketones, or anhydrides. These coatings have been used for the immobilization of proteins, peptides, DNA, and cells. CVD coatings can be conformally deposited on a broad range of materials with different geometry and are useful for applications including functional electrically conductive polymer films, polymer gradients, protein-resistant surfaces, solventless adhesive bonding, 3D photoresists, and polymer/carbon nanotube composites. The CVD process has been successfully applied for the deposition of alkyne-functionalized polymers on a range of different substrates and can even support microand nanopatterning by CuAAC. 9] Despite the success of CuAAC, the requirement of a potentially cytotoxic copper catalyst may limit its biomedical applications. To develop Cu-free click chemistry, alkynes were activated by applying ring-strain, incorporating an electron-withdrawing group, or both. Bertozzi and co-workers conducted extensive studies on the synthesis of cyclooctyne derivatives for copper-free azide– alkyne cycloadditions. They successfully improved the cyclooctyne reactivity by introducing electron-withdrawing fluorine atoms and used the copper-free click reactions for selective modifications of biomolecules and living cells. Boons and co-workers achieved a similar rate enhancement by fusing two aryl rings to the cyclooctyne scaffold. The strain-promoted cycloaddition of functionalized cyclooctynes to azides is an efficient reaction, but their challenging synthesis has prevented them from being more widely investigated and applied to bioimmobilization. Herein, we report a synthetically straightforward approach towards reactive coatings for copper-free 1,3dipolar cycloadditions and demonstrate a bio-orthogonal reaction scheme based on two sequential click reactions. Our approach is based on CVD polymerization of appropriately functionalized [2.2]paracylophanes. The development of a CVD coating that presents alkyne groups capable of copperfree click chemistry poses a number of challenges: 1) The reactive groups must readily react with azide groups at room temperature in benign solvents (e.g., water); 2) The functional groups have to be compatible with the processing conditions during CVD polymerization without decomposition or side reactions; 3) The precursors should be accessible by straightforward synthesis. Herein, we chose to synthesize [2.2]paracyclophane-4-methyl propiolate, which provides an electron-deficient alkynyl group for the spontaneous reaction with azide groups even in the absence of a catalyst. Neighboring electron-withdrawing groups are known to increase the reactivity of alkyne groups. Functional moieties such as sulfonyl and carbonyl groups were investigated in different studies. Applications of electrondeficient alkyne moieties include DNA modification, gold nanoparticle functionalization, or hydrogel crosslinking. As shown in Scheme 1, [2.2]paracyclophane-4-methyl propiolate was sublimed at 100 8C and 0.3 mbar and then subjected to thermal pyrolysis at 510 8C in vacuum to generate [*] X. Deng, Dr. C. Friedmann, Prof. Dr. J. Lahann Departments of Chemical Engineering, Materials Science and Engineering, Macromolecular Science and Engineering and Biomedical Engineering, University of Michigan Ann Arbor, 48109 (USA) and Institute of Functional Interfaces, Karlsruhe Institute of Technology Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen (Germany) Fax: (+ 1)734-764-7453 E-mail: lahann@umich.edu

Substrate-independent dip-pen nanolithography based on reactive coatings

We report that nanostructuring via dip-pen nanolithography can be used for modification of a broad range of different substrates (polystyrene, Teflon, stainless steel, glass, silicon, rubber, etc.) without the need for reconfiguring the underlying printing technology. This is made possible through the use of vapor-based coatings that can be deposited on these substrates with excellent conformity, while providing functional groups for subsequent spatially directed click chemistry via dip-pen nanolithography. Pattern quality has been compared on six different substrates demonstrating that this approach indeed results in a surface modification protocol with potential use for a wide range of biotechnological applications.

BSA-modified polyethersulfone membrane: preparation, characterization and biocompatibility.

A polyethersulfone (PES) membrane was modified by blending with a co-polymer of acrylic acid (AA) and N-vinyl pyrrolidone (VP), followed by immobilization of bovine serum albumin (BSA) onto the surface. The scanning electron microscopy results showed that PES had good miscibility with the co-polymer. X-ray photoelectron spectroscopy confirmed the existence of P(VP-AA) co-polymer on the surface of the blended membrane and the existence of BSA after the immobilization process. The amount of BSA immobilized on the surface of the membranes was determined. It was found that the protein adsorption amounts from BSA, human plasma fibrinogen and diluted human plasma solutions decreased significantly after modification. According to the circular dichroism results, the proteins kept more α-helix conformation in the modified membranes than in the pure PES membrane. The number of the adhered platelets was reduced, and the morphology change for the adherent platelets was also suppressed by the modification with BSA. The SEM morphological observation of the cells and the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay demonstrated that the BSA-modified PES membrane surface promoted endothelial cell adhesion and proliferation.

The effects of Runx2 immobilization on poly (ɛ-caprolactone) on osteoblast differentiation of bone marrow stromal cells in vitro

In vivo regenerative gene therapy is a promising approach for bone regeneration and can help to address cell-source limitations through surgical implantation of osteoinductive materials and subsequent recruitment of host-derived cells. Localized viral delivery may reduce the risk of virus dispersion, enhance transduction efficiency, and reduce administration/injection dosing, which subsequently increases patient safety. In this manuscript, we present a custom-tailored strategy to immobilize adenovirus expressing runt-related transcription factor 2 (AdRunx2) by using reactive polymer coatings to enhance in vitro osteoblast differentiation of bone marrow stromal cells (BMSCs). A thin polymer film of poly[p-xylylene carboxylic acid pentafluorophenol ester-co-p-xylylene] equipped with amine-reactive active ester groups was deposited on the surface of poly (epsilon-caprolactone) (PCL) using the chemical vapor deposition (CVD) polymerization technique and then anti-adenovirus antibody was conjugated on the material with an amide chemical bond. Following antibody conjugation, AdRunx2 was conjugated to the PCL surface through antibody-antigen interaction. Osteoblast differentiation of BMSCs was induced by incubation in osteogenic medium. Alkaline phosphatase (ALP) activity, calcium deposition, and matrix mineralization were confirmed as markers of osteoblast formation. Incubation of the BMSCs in the presence of AdRunx2 modified PCL resulted in a 6.5-fold increase in ALP activity and significant increases in matrix mineralization when compared to controls. These results demonstrate that adenovirus vectors driving the expression of transcription factors can be delivered directly from biomaterials to direct cell differentiation.

Co-immobilization of biomolecules on ultrathin reactive chemical vapor deposition coatings using multiple click chemistry strategies.

Immobilization of biomolecules, such as proteins or sugars, is a key issue in biotechnology because it enables the understanding of cellular behavior in more biological relevant environment. Here, poly(4-ethynyl-p-xylylene-co-p-xylylene) coatings have been fabricated by chemical vapor deposition (CVD) polymerization in order to bind bioactive molecules onto the surface of the material. The control of the thickness of the CVD films has been achieved by tuning the amount of precursor used for deposition. Copper-catalyzed Huisgen cycloaddition has then been performed via microcontact printing to immobilize various biomolecules on the reactive coatings. The selectivity of this click chemistry reaction has been confirmed by spatially controlled conjugation of fluorescent sugar recognizing molecules (lectins) as well as cell adhesion onto the peptide pattern. In addition, a microstructured coating that may undergo multiple click chemistry reactions has been developed by two sequential CVD steps. Poly(4-ethynyl-p-xylylene-co-p-xylylene) and poly(4-formyl-p-xylylene-co-p-xylylene) have been patterned via vapor-assisted micropatterning in replica structures (VAMPIR). A combination of Huisgen cycloaddition and carbonyl-hydrazide coupling was used to spatially direct the immobilization of sugars on a patterned substrate. This work opens new perspectives in tailoring microstructured, multireactive interfaces that can be decorated via bio-orthogonal chemistry for use as mimicking the biological environment of cells.

A Generic Strategy for Co-Presentation of Heparin-Binding Growth Factors Based on CVD Polymerization

A multifunctional copolymer with both aldehyde and alkyne groups is synthesized by chemical vapor deposition (CVD) for orthogonal co-immobilization of biomolecules. Surface analytical methods including FTIR and XPS are used to confirm the surface modification. Heparin-binding growth factors [basic fibroblast growth factor (bFGF) in this study] can be immobilized through interaction with heparin, which was covalently attached to the CVD surface through an aldehyde-hydrazide reaction. In parallel, an alkyne-azide reaction is used to orthogonally co-immobilize an adhesion peptide as the second biomolecule.

Chemical-vapor-deposition-based polymer substrates for spatially resolved analysis of protein binding by imaging ellipsometry.

Biomolecular interactions between proteins and synthetic surfaces impact diverse biomedical fields. Simple, quantitative, label-free technologies for the analysis of protein adsorption and binding of biomolecules are thus needed. Here, we report the use of a novel type of substrate, poly-p-xylylene coatings prepared by chemical vapor deposition (CVD) polymerization, for surface plasmon resonance enhanced ellipsometry (SPREE) studies and assess the reactive coatings as spatially resolved biomolecular sensing arrays. Prior to use in binding studies, reactive coatings were fully characterized by Fourier transform infrared spectroscopy, electrochemical impedance spectroscopy, and ellipsometry. As a result, the chemical structure, thickness, and homogeneous coverage of the substrate surface were confirmed for a series of CVD-coated samples. Subsequent SPREE imaging and fluorescence microscopy indicated that the synthetic substrates supported detectable binding of a cascade of biomolecules. Moreover, analysis revealed a useful thickness range for CVD films in the assessment of protein and/or antigen-antibody binding via SPREE imaging. With a variety of functionalized end groups available for biomolecule immobilization and ease of patterning, CVD thin films are useful substrates for spatially resolved, quantitative binding arrays.

Selective and Reversible Binding of Thiol-Functionalized Biomolecules on Polymers Prepared via Chemical Vapor Deposition Polymerization

We use chemical vapor deposition polymerization to prepare a novel dibromomaleimide-functionalized polymer for selective and reversible binding of thiol-containing biomolecules on a broad range of substrates. We report the synthesis and CVD polymerization of 4-(3,4-dibromomaleimide)[2.2]paracyclophane to yield nanometer thick polymer coatings. Fourier transformed infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the chemical composition of the polymer coating. The reactivity of the polymer coating toward thiol-functionalized molecules was confirmed using fluorescent ligands. As a proof of concept, the binding and subsequent release of cysteine-modified peptides from the polymer coating were also demonstrated via sum frequency generation spectroscopy. This reactive polymer coating provides a flexible surface modification approach to selectively and reversibly bind biomolecules on a broad range of materials, which could open up new opportunities in many biomedical sensing and diagnostic applications where specific binding and release of target analytes are desired.