Formation of microscale gradients of protein using heterobifunctional photolinkers

Automated detection of patterned single-cells within hydrogel using deep learning

Single-cell analysis has been widely used in various biomedical engineering applications, ranging from cancer diagnostics, and immune response monitoring to drug screening. Single-cell isolation is fundamental for observing single-cell activities and an automatic finding method of accurate and reliable cell detection with few possible human errors is also essential. This paper reports trapping single cells into photo patternable hydrogel microwell arrays and isolating them. Additionally, we present an object detection-based DL algorithm that detects single cells in microwell arrays and predicts the presence of cells in resource-limited environments at the highest possible mAP (mean average precision) of 0.989 with an average inference time of 0.06 s. This algorithm leads to the enhancement of the high-throughput single-cell analysis, establishing high detection precision and reduced experimentation time.

Photochemical approach for multiplexed biofunctionalisation of gallium arsenide

The optoelectronic properties of gallium arsenide (GaAs) hold great promise in biosensing applications, currently being held back by the lack of methodologies reporting the spatially selective functionalisation of this material with multiple biomolecules. Here, we exploit the use of a photoreactive crosslinker - a diazirine derivative - for spatially selective covalent immobilisation of multiple bioreceptors on the GaAs surface. As a proof of principle we show the immobilisation of two proteins: neutravidin and endosulfine alpha protein. X-ray photoelectron spectroscopy results showed the presence of the biomolecules on the GaAs regions selectively exposed to ultraviolet light. The approach presented here is applicable to the covalent attachment of other biomolecules, paving the way for using GaAs as a platform for multiplexed biosensing.

Functionalizing micro-3D-printed protein hydrogels for cell adhesion and patterning

A versatile and dynamic photoconjugation platform is introduced that provides high, 3D spatial resolution for functionalizing micro-3D-printed (μ-3DP) hydrogels. Schwann cells are patterned on μ-3DP hydrogels precisely labeled with RGD, a cell adhesive peptide, demonstrating utility of this platform for cell culture applications.

Generation of a chemical gradient across an array of 256 cell cultures in a single chip

A microfluidic diffusion diluter to create stable chemical gradients across an array of cell cultures was demonstrated. The device enabled concentration based studies to be conducted at 256 different concentrations across individual, low shear cell cultures. A gradient of staurosporine on cells stained with Mitotracker Deep Red (MTDR) showed a concentration-based effect on cell apoptosis across the cell culture array.

Facile photoimmobilization of proteins onto low-binding PEG-coated polymer surfaces

Immobilization of proteins onto polymer surfaces usually requires specific reactive functional groups. Here, we show an easy one-step method to conjugate protein covalently onto almost any polymer surface, including low protein-binding poly(ethylene glycol) (PEG), without the requirement for the presence of specific functional groups. Several types of proteins, including alkaline phosphatase, bovine serum albumin, and polyclonal antibodies, were photoimmobilized onto a PEG-coated polymer surface using a water-soluble benzophenone as photosensitizer. Protein functionality after immobilization was verified for both enzymes and antibodies, and their presence on the surface was confirmed by X-ray photoelectron spectroscopy (XPS) and confocal fluorescence microscopy. Conjugation of capture antibody onto the PEG coating was employed for a simplified ELISA protocol without the need for blocking uncoated surface areas, showing ng/mL sensitivity to a cytokine antigen target. Moreover, spatially patterned attachment of fluorescently labeled protein onto the low-binding PEG-coated surface was achieved with a projection lithography system that enabled the creation of micrometer-sized protein features.

Quantitative photochemical immobilization of biomolecules on planar and corrugated substrates: a versatile strategy for creating functional biointerfaces

Methods for the generation of substratespresenting biomolecules in a spatially controlled manner are enabling tools for applications in biosensor systems, microarray technologies, fundamental biological studies and biointerface science. We have implemented a method to create biomolecular patterns by using light to control the direct covalent immobilization of biomolecules onto benzophenone-modified glass substrates. We have generated substrates presenting up to three different biomolecules patterned in sequence, and demonstrate biomolecular photopatterning on corrugated substrates. The chemistry of the underlying monolayer was optimized to incorporate poly(ethylene glycol) to enable adhesive cell adhesion onto patterned extracellular matrix proteins. Substrates were characterized with contact angle goniometry, AFM, and immunofluorescence microscopy. Importantly, radioimmunoassays were performed to quantify the site density of immobilized biomolecules on photopatterned substrates. Retained function of photopatterned proteins was demonstrated both by native ligand recognition and cell adhesion to photopatterned substrates, revealing that substrates generated with this method are suitable for probing specific cell receptor-ligand interactions. This molecularly general photochemical patterning method is an enabling tool for the creation of substrates presenting both biochemical and topographical variation, which is an important feature of many native biointerfaces.

Creating growth factor gradients in three dimensional porous matrix by centrifugation and surface immobilization

Polycaprolactone (PCL)/Pluronic F127 cylindrical scaffolds with gradually increasing growth factor concentrations were fabricated by the centrifugation of fibril-like PCLs and the subsequent fibril surface immobilization of growth factors. The cylindrical scaffolds exhibited gradually increasing surface areas along the longitudinal direction [from 3.17 ± 0.05 m(2)/g (top position) to 5.42 ± 0.01 m(2)/g (bottom position)]. The growth factors (BMP-7, TGF-β(2) and VEGF(165)) as model bioactive molecules were immobilized onto the fibril surfaces of the scaffolds via heparin binding to produce scaffolds with gradually increasing concentrations of growth factors from the top position (BMP-7, 60.89 ± 2.51; TGF-β(2), 42.85 ± 2.00; VEGF(165), 42.52 ± 3.22 ng/scaffold section) to the bottom position (BMP-7, 181.07 ± 3.21; TGF-β(2), 142.08 ± 2.91; VEGF(165), 112.00 ± 4.00 ng/scaffold section). The released amount of growth factor (VEGF(165)) from the cylindrical scaffold gradually decreased along the longitudinal direction in a sustained manner for up to 35 days, which can allow for a minutely controlled spatial distribution of growth factors in a 3D environment. The 3D porous scaffold with a concentration gradient of growth factors may become a useful tool for basic studies, including in vitro investigations of 3D chemotaxis/haptotaxis for the control of specific biological process. It may also be used as a tissue engineering scaffolding system for a variety of tissues/organs requiring the spatial regulation of growth factors for effective regeneration.

Nanoscale tissue engineering: spatial control over cell-materials interactions

Cells interact with the surrounding environment by making tens to hundreds of thousands of nanoscale interactions with extracellular signals and features. The goal of nanoscale tissue engineering is to harness these interactions through nanoscale biomaterials engineering in order to study and direct cellular behavior. Here, we review two- and three-dimensional (2- and 3D) nanoscale tissue engineering technologies, and provide a holistic overview of the field. Techniques that can control the average spacing and clustering of cell adhesion ligands are well established and have been highly successful in describing cell adhesion and migration in 2D. Extension of these engineering tools to 3D biomaterials has created many new hydrogel and nanofiber scaffold technologies that are being used to design in vitro experiments with more physiologically relevant conditions. Researchers are beginning to study complex cell functions in 3D. However, there is a need for biomaterials systems that provide fine control over the nanoscale presentation of bioactive ligands in 3D. Additionally, there is a need for 2- and 3D techniques that can control the nanoscale presentation of multiple bioactive ligands and that can control the temporal changes in the cellular microenvironment.

Combinatorial and high-throughput screening of biomaterials

Combinatorial and high-throughput methods have been increasingly used to accelerate research and development of new biomaterials. These methods involve creating miniaturized libraries that contain many specimens in one sample in the form of gradients or arrays, followed by automated data collection and analysis. This article reviews recent advances in utilizing combinatorial and high-throughput methods to better understand cell-material interactions, particularly highlighting our efforts at the NIST Polymers Division. Specifically, fabrication techniques to generate controlled surfaces (2D) and 3D cell environments (tissue engineering scaffolds) as well as methods to characterize and analyze material properties and cell-material interactions are described. In conclusion, additional opportunities for combinatorial methods for biomaterials research are noted, including streamlined sample fabrication and characterization, appropriate and automated bioassays, and data analysis.

Poly(L-lysine)-grafted-poly(ethylene glycol)-based surface-chemical gradients. Preparation, characterization, and first applications

A simple dipping process has been used to prepare PEGylated surface gradients from the polycationic polymer poly(L-lysine), grafted with poly(ethylene glycol) (PLL-g-PEG), on metal oxide substrates, such as TiO(2) and Nb(2)O(5). PLL-g-PEG coverage gradients were prepared during an initial, controlled immersion and characterized with variable angle spectroscopic ellipsometry and x-ray photoelectron spectroscopy. Gradients with a linear change in thickness and coverage were generated by the use of an immersion program based on an exponential function. These single-component gradients were used to study the adsorption of proteins of different sizes and shapes, namely, albumin, immunoglobulin G, and fibrinogen. The authors have shown that the density and size of defects in the PLL-g-PEG adlayer determine the amount of protein that is adsorbed at a certain adlayer thickness. In a second step, single-component gradients of functionalized PLL-g-PEG were backfilled with nonfunctionalized PLL-g-PEG to generate two-component gradients containing functional groups, such as biotin, in a protein-resistant background. Such gradients were combined with a patterning technique to generate individually addressable spots on a gradient surface. The surfaces generated in this way show promise as a useful and versatile biochemical screening tool and could readily be incorporated into a method for studying the behavior of cells on functionalized surfaces.

The effect of laminin peptide gradient in enzymatically cross-linked collagen scaffolds on neurite growth

Guided neurite growth is critical in both peripheral nervous system and central nervous system nerve regeneration. Scaffolds that provide structural and guidance cues for neuronal cells have a potential role in neural regeneration application. Type I collagen is suitable to be processed as an engineered scaffold for nerve regeneration because of its biological and structural properties. A few previous studies have shown that cross-linking of collagen scaffolds with microbial transglutaminase improves the mechanical strength and degradation properties of the scaffolds. It was shown that laminin 5 can regulate neurite outgrowth and extension. A motif (PPFLMLLKGSTR) in the human laminin 5 alpha 3 chain is crucial for both integrin alpha 3 beta 1 receptor binding and cell adhesion. In the present work, we studied the guidance effect of a laminin peptide (PPFLMLLKGSTR) gradient in collagen and cross-linked collagen scaffolds on neurite growth. Neurites of rat pheochromocytoma (PC12) cells showed a preferential growth toward the high laminin concentration level on the collagen scaffold, while the incorporation of laminin peptide in the scaffold did not influence neurite length of PC12 cells.

Rapid generation of biologically relevant hydrogels containing long-range chemical gradients

Many biological processes are regulated by gradients of bioactive chemicals. Thus, the generation of materials with embedded chemical gradients may be beneficial for understanding biological phenomena and generating tissue-mimetic constructs. Here we describe a simple and versatile method to rapidly generate materials containing centimeter-long gradients of chemical properties in a microfluidic channel. The formation of chemical gradient was initiated by a passive-pump-induced forward flow and further developed during an evaporation-induced backward flow. The gradient was spatially controlled by the backward flow time and the hydrogel material containing the gradient was synthesized via photopolymerization. Gradients of a cell-adhesion ligand, Arg-Gly-Asp-Ser (RGDS), was incorporated in the poly(ethylene glycol)-diacrylate (PEG-DA) hydrogels to test the response of endothelial cells. The cells attached and spread along the hydrogel material in a manner consistent with the RGDS gradient profile. A hydrogel containing PEG-DA concentration gradient and constant RGDS concentration was also generated. The morphology of cells cultured on such hydrogel changed from round in the lower PEG-DA concentration regions to well-spread in the higher PEG-DA concentration regions. This approach is expected to be a valuable tool to investigate the cell-material interactions in a simple and high-throughput manner and to design graded biomimetic materials for tissue engineering applications.

Gradient lithography of engineered proteins to fabricate 2D and 3D cell culture microenvironments

Spatial patterning of proteins is a valuable technique for many biological applications and is the prevailing tool for defining microenvironments for cells in culture, a required procedure in developmental biology and tissue engineering research. However, it is still challenging to achieve protein patterns that closely mimic native microenvironments, such as gradient protein distributions with desirable mechanical properties. By combining projection dynamic mask lithography and protein engineering with non-canonical photosensitive amino acids, we demonstrate a simple, scalable strategy to fabricate any user-defined 2D or 3D stable gradient pattern with complex geometries from an artificial extracellular matrix (aECM) protein. We show that the elastic modulus and chemical nature of the gradient profile are biocompatible and allow useful applications in cell biological research.

Cell Guidance by 3D-Gradients in Hydrogel Matrices: Importance for Biomedical Applications

Concentration gradients of soluble and matrix-bound guidance cues in the extracellular matrix direct cell growth in native tissues and are of great interest for design of biomedical scaffolds and on implant surfaces. The focus of this review is to demonstrate the importance of gradient guidance for cells as it would be desirable to direct cell growth onto/into biomedical devices. Many studies have been described that illustrate the production and characterization of surface gradients, but three dimensional (3D)-gradients that direct cellular behavior are not well investigated. Hydrogels are considered as synthetic replacements for native extracellular matrices as they share key functions such as 2D- or 3D-solid support, fibrous structure, gas- and nutrition permeability and allow storage and release of biologically active molecules. Therefore this review focuses on current studies that try to implement soluble or covalently-attached gradients of growth factors, cytokines or adhesion sequences into 3D-hydrogel matrices in order to control cell growth, orientation and migration towards a target. Such gradient architectures are especially desirable for wound healing purposes, where defined cell populations need to be recruited from the blood stream and out of the adjacent tissue, in critical bone defects, for vascular implants or neuronal guidance structures where defined cell populations should be guided by appropriate signals to reach their proper positions or target tissues in order to accomplish functional repair.

Direct biophotolithographic method for generating substrates with multiple overlapping biomolecular patterns and gradients

We describe an approach to generate multicomponent surface-immobilized patterns and gradients on the basis of the photochemically controlled covalent coupling of solution-phase biomolecules to benzophenone-modified substrates. Gradients were simply achieved by continuously varying the exposure to nondamaging UV light across the surface with the gradient profile controlled by biomolecule concentration and the spatial and temporal illumination of the surface. Sequential exposure of the same surface in the presence of different biomolecules resulted in overlapping patterns and gradients of proteins and carbohydrates. Finally, we preliminarily demonstrate that the resulting surfaces are suitable for generating model substrates to probe cell-substrate interactions.

Fabrication of thermoresponsive polymer gradients for study of cell adhesion and detachment

A poly(N-isopropylacrylamide) (PNIPAAm) gradient covalently anchored on a silicon substrate with a linear variation of thickness was fabricated by continuous injection of the reaction mixture (NIPAAm, CuBr and its ligand, methanol, and water) into a glass chamber containing a silicon wafer, whose surface had been homogeneously immobilized with bromoisobutyryl bromide (BIBB). Because of the good control of the surface-initiated atom transfer radical polymerization (SI-ATRP) technique, the thickness of the PNIPAAm brushes was linearly proportional to the polymerization time. As a result, the gradient length and sharpness could be easily controlled by the experimental parameters such as the polymerization time and the injection rate. The as-prepared PNIPAAm gradients were characterized by ellipsometry, water contact angle, and atom force microscopy to detect their alteration of the thickness, surface wettability, and morphology, confirming the gradient structure. X-ray photoelectron spectroscopy confirmed the surface composition of the PNIPAAm. In vitro culture of HepG2 cells was implemented on the gradient surfaces, revealing that the cells could adhere at 37 degrees C and could be detached at 24 degrees C when the gradient thickness was in the range of 20-45 nm. The work thus develops a method to fabricate the stable gradient surface with better quality control, and clarifies in a facile manner the appropriate thickness of the PNIPAAm brushes in terms of cell adhesion and detachment.

Strategies and applications for incorporating physical and chemical signal gradients in tissue engineering

From embryonic development to wound repair, concentration gradients of bioactive signaling molecules guide tissue formation and regeneration. Moreover, gradients in cellular and extracellular architecture as well as in mechanical properties are readily apparent in native tissues. Perhaps tissue engineers can take a cue from nature in attempting to regenerate tissues by incorporating gradients into engineering design strategies. Indeed, gradient-based approaches are an emerging trend in tissue engineering, standing in contrast to traditional approaches of homogeneous delivery of cells and/or growth factors using isotropic scaffolds. Gradients in tissue engineering lie at the intersection of three major paradigms in the field-biomimetic, interfacial, and functional tissue engineering-by combining physical (via biomaterial design) and chemical (with growth/differentiation factors and cell adhesion molecules) signal delivery to achieve a continuous transition in both structure and function. This review consolidates several key methodologies to generate gradients, some of which have never been employed in a tissue engineering application, and discusses strategies for incorporating these methods into tissue engineering and implant design. A key finding of this review was that two-dimensional physicochemical gradient substrates, which serve as excellent high-throughput screening tools for optimizing desired biomaterial properties, can be enhanced in the future by transitioning from two dimensions to three dimensions, which would enable studies of cell-protein-biomaterial interactions in a more native tissue-like environment. In addition, biomimetic tissue regeneration via combined delivery of graded physical and chemical signals appears to be a promising strategy for the regeneration of heterogeneous tissues and tissue interfaces. In the future, in vivo applications will shed more light on the performance of gradient-based mechanical integrity and signal delivery strategies compared to traditional tissue engineering approaches.

Screening the cellular microenvironment: a role for microfluidics

The cellular microenvironment is an increasingly discussed topic in cell biology as it has been implicated in the progression of cancer and the maintenance of stem cells. The microenvironment of a cell is an organized combination of extracellular matrix (ECM), cells, and interstitial fluid that influence cellular phenotype through physical, mechanical, and biochemical mechanisms. Screening can be used to map combinations of cells and microenvironments to phenotypic outcomes in a way that can help develop more predictive in vitro models and to better understand phenotypic mechanisms from a systems biology perspective. This paper examines microenvironmental screening in terms of outcomes and benefits, key elements of the screening process, challenges for implementation, and a possible role for microfluidics as the screening platform. To assess microfluidics for use in microenvironmental screening, examples and categories of micro-scale and microfluidic technology are highlighted. Microfluidic technology shows promise for simultaneous control of multiple parameters of the microenvironment and can provide a base for scaling advanced cell-based experiments into automated high-throughput formats.

Miniaturized system of neurotrophin patterning for guided regeneration

Understanding the fundamentals of cell behaviour is imperative for designing and improving engineering strategies for regenerative medicine. By combining the precision of confocal microscopy with photochemistry, nerve growth factor (NGF) was chemically immobilized on chitosan films either in distinct areas or as concentration gradients. Using rhodamine as a proxy for NGF, a series of immobilized concentration gradients were created, using the number of rastering scans within a defined area and the distance between each area as a way to control the resulting gradient. The same photochemistry was applied to create NGF patterns on chitosan films which were visualized by immunostaining, and the immobilized NGF remained bioactive as demonstrated with a neuron survival assay. Neuron survival was 73.2+/-1.3% after 3 days of culture on chitosan films with 30 ng/cm(2) of homogenously immobilized NGF, which was comparable to 74.8+/-3.4% neuron survival on chitosan with 50 ng/ml of soluble NGF present. Interestingly, when neurons were plated on a chitosan film that had distinct immobilized NGF-patterned areas surrounded by unmodified chitosan, the neurons remained predominantly as single cells in the NGF-patterned regions, but formed aggregates outside of these patterns on the plain chitosan film. Thus, the immobilized NGF pattern influenced neuron behaviour and can be used to further probe mechanisms of other neuron behaviour such as axon guidance. Importantly, the versatility of the confocal laser patterning technique reported here can be extended to other factors to elucidate fundamental cell functions, and hence design strategies in regenerative medicine.

Surface-bound soft matter gradients

This feature article describes the progress realized over the past half century in the field of surface-bound gradient structures created on or from soft materials (oligomers and/or polymers), or those enabling the study of the behavior of soft materials. By highlighting our work in the field and accounting for the contribution of other groups, we emphasize the exceptional versatility of gradient assemblies in facilitating fast screening of physicochemical phenomena, acting as "recording media" for monitoring a process, and playing a key role in the design and fabrication of surface-bound molecular and macromolecular motors capable of directing a transport phenomenon.

Surface-chemical and -morphological gradients

Surface gradients of chemistry or morphology represent powerful tools for the high-throughput investigation of interfacial phenomena in the areas of physics, chemistry, materials science and biology. A wide variety of methods for the fabrication of such gradients has been developed in recent years, relying on principles ranging from diffusion to time-dependent irradiation in order to achieve a gradual change of a particular parameter across a surface. In this review we have endeavoured to cover the principal fabrication approaches for surface-chemical and surface-morphological gradients that have been described in the literature, and to provide examples of their applications in a variety of different fields.

Endothelial cell migration on surface-density gradients of fibronectin, VEGF, or both proteins

Cell migration is essential to many physiological processes, including angiogenesis, which is critical to the success of implanted biomaterials and tissue-engineered constructs. Gradients play an important role in cell migration. Previous work on cell migration has been mostly executed either in the concentration gradients of stimuli (e.g., VEGF) in bulk or hydrogels or on the surface-density gradients of ECM proteins (e.g., fibronectin) or small ligands (e.g., RGD). Little work has been done to investigate how cell migration responds to the surface-density gradients of growth factors. No work has been done to study how the surface gradients of both adhesive proteins and growth factors influence cell migration. In this work, we studied the effect of the surface-density gradients of fibronectin (FN), VEGF, or both proteins on endothelial cell migration. Gradients with different slopes were prepared to study how the gradient slope affects cell migration. The gradients were generated by first forming a counter-propagating C15COOH/C11OH self-assembled monolayer (SAM) gradient using a surface electrochemistry approach, followed by activating the -COOH moieties and covalently immobilizing proteins onto the surface. Fourier transform infrared spectra and X-ray photoelectron spectroscopy were used to characterize the SAM and protein gradients, respectively. A free cell migration assay using bovine aortic endothelial cells was performed on various gradient surfaces or on surfaces with uniform protein density. Results showed that cells on the surface-density gradients of FN, VEGF, or both proteins moved faster along the gradient direction than on the respective uniform control surface after 24-h cell culture. It is also shown that for each protein or protein combination, the directional cell displacement was not statistically different between two gradients with different slopes. Results show that the directional cell migration was increased by about 2-fold on the VEGF gradient as compared to the FN gradient and was further increased by another 2-fold on the combined gradients of both proteins as compared to the VEGF gradient alone. This is the first work to create surface-density gradients of VEGF and the first study to generate a combined surface gradient of growth factor and ECM protein to investigate their effect on cell migration on surfaces. This work broadens our understanding of the directional movement of endothelial cells. Our findings provide useful information for directing cell migration into tissue-engineered constructs and can be potentially used for those applications where cell migration is critical, such as angiogenesis.

Covalent surface chemistry gradients for presenting bioactive peptides

The activation of surfaces by covalent attachment of bioactive moieties is an important strategy for improving the performance of biomedical materials. Such techniques have also been used as tools to study cellular responses to particular chemistries of interest. The creation of gradients of covalently bound chemistries is a logical extension of this technique. Gradient surfaces may permit the rapid screening of a large range of concentrations in a single experiment. In addition, the biological response to the gradient itself may provide new information on receptor requirements and cell signaling. The current work describes a rapid and flexible technique for the covalent addition of bioactive peptide gradients to a surface or gel and a simple fluorescence technique for assaying the gradient. In this technique, bioactive peptides with a terminal cysteine are bound via a heterobifunctional coupling agent to primary amine-containing surfaces and gels. A gradient in the coupling agent is created on the surfaces or gels by varying the residence time of the coupling agent across the surface or gel, thereby controlling the extent of reaction. We demonstrate this technique using poly(l-lysine)-coated glass surfaces and fibrin gels. Once the surface or gel has been activated by the addition of the coupling agent gradient, the bioactive peptide is added. Quantitation of the gradient is achieved by measuring the reaction kinetics of the coupling agent with the surface or gel of interest. This can be done either by fluorescently labeling the coupling agent (in the case of surfaces) or by spectrophotometrically detecting the release of pyridine-2-thione, which is produced when the thiol-reactive portion of the coupling agent reacts. By these methods, we can obtain reasonably precise estimates for the peptide gradients without using expensive spectroscopic or radiolabeling techniques. Validation with changes in fibroblast cell migration behavior across a bioactive peptide gradient illustrates preservation of peptide function as well as the usefulness of this technique.

The movement of a water droplet on a gradient surface prepared by photodegradation

A hydrophobic to hydrophilic gradient surface was prepared using the tuned photodegradation of an alkylsilane self-assembled monolayer (SAM) using irradiation of vacuum ultraviolet light (wavelength=172 nm). The water contact angle on the photodegraded SAM surface was adjusted using the intensity and time photoirradiation parameters. The formation of a gradient was confirmed by fluorescent labeling. The water drop moved from the hydrophobic to hydrophilic surface with a velocity that depended on the gradient. The higher the gradient, the faster the water moved. For the first time, we have prepared a gradient surface using photodegradation where the movement of a water drop was regulated by the degree of gradation. Considering that the photodegradation technique can be applied to various surfaces and to lithography, this technique will be useful for various material surfaces.

Surface engineering approaches to micropattern surfaces for cell-based assays

The ability to produce patterns of single or multiple cells through precise surface engineering of cell culture substrates has promoted the development of cellular bioassays that provide entirely new insights into the factors that control cell adhesion to material surfaces, cell proliferation, differentiation and molecular signaling pathways. The ability to control shape and spreading of attached cells and cell-cell contacts through the form and dimension of the cell-adhesive patches with high precision is important. Commitment of stem cells to different specific lineages depends strongly on cell shape, implying that controlled microenvironments through engineered surfaces may not only be a valuable approach towards fundamental cell-biological studies, but also of great importance for the design of cell culture substrates for tissue engineering. Furthermore, cell patterning is an important tool for organizing cells on transducers for cell-based sensing and cell-based drug discovery concepts. From a material engineering standpoint, patterning approaches have greatly profited by combining microfabrication technologies, such as photolithography, with biochemical functionalization to present to the cells biological cues in spatially controlled regions where the background is rendered non-adhesive ("non-fouling") by suitable chemical modification. The focus of this review is on the surface engineering aspects of biologically motivated micropatterning of two-dimensional (flat) surfaces with the aim to provide an introductory overview and critical assessment of the many techniques described in the literature. In particular, the importance of non-fouling surface chemistries, the combination of hard and soft lithography with molecular assembly techniques as well as a number of less well known, but useful patterning approaches, including direct cell writing, are discussed.

Length scale heterogeneity in lateral gradients of poly(N-isopropylacrylamide) polymer brushes prepared by surface-initiated atom transfer radical polymerization coupled with in-plane electrochemical potential gradients

We report the preparation and characterization of poly(N-isopropylacrylamide) (PNIPAAm) polymer brushes exhibiting controlled lateral variations in the patchiness of polymer chains. These gradients were achieved through an atom transfer radical polymerization (ATRP) grafting-from approach utilizing surfaces on which the spatial profile of the initiator density was carefully controlled. Initiator density gradients were formed on Au by first preparing a hexadecanethiol (HDT) density gradient, by reductive desorption using a laterally anisotropic electrochemical gradient. The bare areas in the original HDT gradient were then back-filled with a disulfide initiator, (BrC(CH3)2COO(CH2)11S)2. The initiator coverage was characterized by X-ray photoelectron spectroscopy (XPS). Then, surface-initiated ATRP was utilized to transfer the initiator density gradient into gradients of PNIPAAm chain density. Ellipsometry, surface plasmon resonance (SPR), and atomic force microscopy (AFM) were used to characterize these PNIPAAm density gradients. The defining characteristic of the PNIPAAm gradients is the evolution of the morphology from discontinuous mushroom structures at extremely low grafting densities to heterogeneous patchy structures at intermediate grafting densities. The size of the patchy domains gradually increases, until at a high grafting density region, the morphology evolves to a smoother, presumably more extended, structure.

Anisotropic hydrogel thickness gradient films derivatized to yield three-dimensional composite materials

We report the preparation of in-plane density gradients of amino-terminated molecules and gold particles through derivatization of laterally varying thickness gradients of poly(acrylic acid) (PAA) or poly(acrylamide) (PAAm) films. PAA and PAAm gradients were formed by Zn(II)-catalyzed electropolymerization of acrylic acid (AA) or acrylamide (AAm) in the presence of an in-plane electrochemical potential gradient applied to Au or indium-tin-oxide (ITO) working electrodes. PAA thickness gradients were converted into density gradients of fluorocarbons or biocompatible groups by derivatizing with NH(2)CH(2)(CF(2))(6)CF(3) or an Arg-Gly-Asp (RGD)-containing peptide, respectively. X-ray photoelectron spectroscopy (XPS) and XPS imaging were used to characterize the modified PAA gradients. Transition regions as narrow as 104 mum were achieved for fluorocarbon gradients. PAAm gradients were treated with gold particles to form a density gradient of gold particles. Surface plasmon resonance imaging and scanning electron microscopy (SEM) as well as UV-visible absorption measurements were used to characterize the gold particle density gradients. It is likely that the gold particles were attached both on the surface and inside the PAAm film.

A technique for preparing protein gradients on polymeric surfaces:

A technique for preparing micropatterns and gradients of proteins on polymeric substrates has been developed in this work. Peroxides were generated on the substrate surface by UV preirradiation and they initiated graft polymerization of acrylic acid (AA) onto the surface upon a second UV irradiation. Micropatterns and gradients of poly(acrylic acid) (PAA) were formed when the substrate was placed under or moved with respect to a photomask during UV preirradiation. Protein micropatterns and gradients were fabricated on the surface by covalently linking to the carboxyl groups on PAA chains. To test cell response to the protein gradient surfaces, PC12 pheochromocytoma cells were cultured on laminin-bound substrates in serum-free medium supplemented with nerve growth factor (NGF). It is found that both the attachment and neurite outgrowth behaviors of PC12 cells were dependent on the surface laminin density. However, the unreacted carboxyl groups on the polymer surface negatively affected PC12 cells. This weakened the positive influence from laminin.

Molecular gradients: an efficient approach for optimizing the surface properties of biomaterials and biochips

A variety of molecular gradients of alkanethiols with the structure HS-(CH(2))(m)-X (m = 15; X = COOH, CH(2)NH(2), or CH(3)) and oligo(ethylene glycol)-terminated alkanethiols with the structures HS-(CH(2))(15)-CO-NH-Eg(n) (n = 2, 4, or 6), HS-(CH(2))(15)-CO-NH-Eg(2)-(CH(2))(2)-NH-CO-(CH(2))(4)-biotin, and HS-(CH(2))(15)-CO-NH-Eg(6)-CH(2)-COOH were prepared on polycrystalline gold films. These gradients were designed to serve as model surfaces for fundamental studies of protein adsorption and immobilization phenomena. Ellipsometry, infrared spectroscopy, and X-ray photoelectron spectroscopy, operating in scanning mode, were used to monitor the layer composition, gradient profiles, tail group conformation, and overall structural quality of the gradient assemblies. The gradient profiles were found to be 4-10 mm wide, and they increased in width with increasing difference in molecular complexity between the thiols used to form the gradient. The oligo(ethylene glycol) thiols are particularly interesting because they can be used to prepare so-called conformational gradients, that is, gradients that display a variation in oligo(ethylene glycol) chain conformation from all trans on the extreme Eg(2,4) sides, via an amorphous-like phase in the mixing regimes, to helical at the extreme Eg(6) sides. We demonstrate herein a series of experiments where the above gradients are used to evaluate nonspecific binding of the plasma protein fibrinogen, and in agreement with previous studies, the highest amounts of nonspecifically bound fibrinogen were observed on all-trans monolayers, that is, on the extreme Eg(2,4) sides. Moreover, gradients between Eg(2) and a biotinylated analogue have been prepared to optimize the conditions for the immobilization of streptavidin. Ellipsometry and infrared spectroscopy reveal high levels of immobilization over a fairly broad range of compositions in the gradient regime, with a maximum between 50 and 60% of the biotinylated analogue in the monolayer. A pI gradient composed of (NH(3)(+)/COO(-))-terminated thiols was also prepared and evaluated with respect to its ability to separate differently charged proteins, pepsin, and lysozyme, on a solid surface.

Cellular alignment by grafted adhesion peptide surface density gradients

The extracellular matrix and extracellular matrix-associated proteins play a major role in growth and differentiation of tissues and organs. To date, few methods have been developed that allow researchers to examine the affect of surface density gradients of adhesion molecules in a controlled manner. Fibroblasts cultured on surfaces with a surface density gradient of RGD peptide aligned parallel to the gradient while fibroblasts on constant density RGD surfaces spread but did not align as has been shown in numerous earlier studies. Not only did fibroblasts align on the gradient surfaces, but they also showed significantly greater elongation than on constant density peptide surfaces or on control surfaces. This type of method is easy to replicate and can be used by laboratories interested in investigating alignment of various cell types without mechanical force or other stimulation, and without cell-cell interaction or for investigation of affects of surface density gradients of molecules on cellular biochemistry and biophysics. This method also has potential applications for developing scaffolds for tissue engineering applications where cellular alignment is necessary.

Theoretical model for photochemical or thermally activated immobilization of macromolecules

The covalent immobilization of macromolecules on surfaces and within 3-dimensional networks is quantitatively described using a model based on Poisson statistics. This model determines the immobilized density or layer thickness as a function of molecular weight of the macromolecule or radiant exposure prior to and following the surface deposition of the macromolecule. Measurements of immobilized layer thickness provide first-order rate constants for decomposition of the surface-bound linker molecules and an estimate of the surface-bound linker density. The model predicts the relative density of immunocomplexed antibodies as a function of the irradiation time used to immobilize antigens. By providing the average number of bonds to the immobilized molecule, the model enables studies of the effect of multiple bonds on the activity of biomolecules. Experimental data by the authors and from the literature validate the model.

Anisotropic In-Plane Gradients of Poly(acrylic acid) Formed by Electropolymerization with Spatiotemporal Control of the Electrochemical Potential

Laterally varying thickness gradients of poly(acrylic acid) (PAA) were formed by Zn(II)-catalyzed electropolymerization of acrylic acid (AA) in the presence of an in-plane electrochemical potential gradient applied to Au working electrodes. In the static potential gradient (SPG) approach, two ends of a Au working electrode were clamped at distinct potentials for the duration of the electropolymerization process, thereby generating a time-independent in-plane electrochemical potential gradient, V(x). A dynamic potential gradient (DPG) approach was also used, in which the two end potentials were varied in time, while maintaining a constant voltage offset, to generate an in-plane electrochemical potential gradient, V(x,t). Because the kinetics of heterogeneous electron transfer vary with the local overpotential, these two methods produce PAA films with laterally varying thickness gradients, although they exhibit different spatial characteristics. X-ray photoelectron spectroscopy (XPS) and surface plasmon resonance (SPR) imaging were used to characterize the PAA gradients. The in-plane thickness variations of PAA gradients formed by both SPG and DPG approaches agree with predictions of the Butler-Volmer equation at small absolute overpotentials, while at large (negative) overpotentials, mass transport dominates, and the thickness reaches a plateau value independent of local potential. DPG-produced PAA gradients are generally broader than SPG gradients with the same initial potential and comparable effective growth time, indicating that the DPG approach is more suitable for formation of thicker gradients.

Photochemical immobilization of proteins on microwave–synthesized photoreactive polymers

We report a rapid and versatile procedure for the preparation of photoreactive polymers and light-induced immobilization of proteins onto such polymers. Photoreactive controlled-pore glass, silica gel, glass slide, and polystyrene microtiter plate are prepared in 40-60s by microwave irradiation of the respective amino polymers and 1-fluoro-2-nitro-4-azidobenzene. Azido group, now part of the polymer, yields highly reactive nitrene under ultraviolet (UV) light at 365 nm. Thus, when photoreactive polymer and horseradish peroxidase or glucose oxidase are exposed to UV light, the reactive nitrene immobilizes the protein molecules in 10 to 20 min through covalent bonding. As nitrene has a property of inserting into C-H bond, the method may find potential applications for immobilization of biomolecules irrespective of their functional groups.

Quantitative Submonolayer Spatial Mapping of Arg-Gly-Asp-Containing Peptide Organomercaptan Gradients on Gold with Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

Peptides containing the tripeptide sequence Arg-Gly-Asp (RGD) have the ability to bind to members of the integrin superfamily of cell-surface receptors and direct cellular adhesion and haptotaxis. The goal of this work is the development of a rapid and effective method for the quantitative submonolayer spatial composition mapping of surfaces displaying molecular assemblies of RGD-containing organomercaptan peptides on a Au surface using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS). Quantitation of the RGD peptide is achieved by determining the peak intensity of the protonated molecular ion, (M + H)+, relative to the (M + H)+ peak for an internal standard, which is similar chemically but with glutamic acid (E) substituted for aspartic acid (D). Using optimized sample preparation procedures, a bilinear calibration was obtained between the quantitative peak intensity ratio and the mole fraction of the RGD-containing peptide. Quantitative compositions were determined with relative standard deviations of <10%, even in the presence of 10x spot-to-spot variations in the absolute signal intensities, by using this internal standard approach. This MALDI-MS quantitative analysis method was employed to probe variable-width two-component counterpropagating electrochemically generated gradients of the two peptides, prepared by coupling in-plane electrochemical potential gradients with the electrosorption reactions of organothiols to vary the composition laterally. The measured lateral composition profiles match the quasi-linear potential gradient model and yield profiles that overlap to a high degree of fidelity in potential space. Thus, MALDI-MS spatial composition mapping should become a powerful tool for the preparation of designed surfaces facilitating the study of cellular adhesion and motility and cell-cell interactions.

A Simple, Reproducible Approach to the Preparation of Surface-Chemical Gradients

We demonstrate a very simple and reproducible preparative approach for the fabrication of surface-chemical gradients. A surface concentration gradient of adsorbed methyl- or hydroxyl-terminated thiolates was achieved upon gradually immersing a gold-coated substrate into a very dilute thiol solution (0.0033 mM) by means of a linear-motion drive. Subsequent immersion of the substrate into the complementary thiol solution provided a hydrophobicity gradient with a large range (50° of the water-contact angle) and over a significant distance (35 mm). The self-assembled monolayer gradient produced in this way also displayed a high packing density, as demonstrated by dynamic contact-angle and X-ray photoelectron spectroscopy measurements.