Poly(dimethylsiloxane) contamination in microcontact printing and its influence on patterning oligonucleotides

Comprehensive fluorescence profiles of contamination-prone foods applied to the design of microcontact-printed in situ functional oligonucleotide sensors

With both foodborne illness and food spoilage detrimentally impacting human health and the economy, there is growing interest in the development of in situ sensors that offer real-time monitoring of food quality within enclosed food packages. While oligonucleotide-based fluorescent sensors have illustrated significant promise, the development of such on-food sensors requires consideration towards sensing-relevant fluorescence properties of target food products-information that has not yet been reported. To address this need, comprehensive fluorescence profiles for various contamination-prone food products are established in this study across several wavelengths and timepoints. The intensity of these food backgrounds is further contextualized to biomolecule-mediated sensing using overlaid fluorescent oligonucleotide arrays, which offer perspective towards the viability of distinct wavelengths and fluorophores for in situ food monitoring. Results show that biosensing in the Cyanine3 range is optimal for all tested foods, with the Cyanine5 range offering comparable performance with meat products specifically. Moreover, recognizing that mass fabrication of on-food sensors requires rapid and simple deposition of sensing agents onto packaging substrates, RNA-cleaving fluorescent nucleic acid probes are successfully deposited via microcontact printing for the first time. Direct incorporation onto food packaging yields cost-effective sensors with performance comparable to ones produced using conventional deposition strategies.

Microfluidic delivery of cutting enzymes for fragmentation of surface-adsorbed DNA molecules

We describe a method for fragmenting, in-situ, surface-adsorbed and immobilized DNAs on polymethylmethacrylate(PMMA)-coated silicon substrates using microfluidic delivery of the cutting enzyme DNase I. Soft lithography is used to produce silicone elastomer (Sylgard 184) gratings which form microfluidic channels for delivery of the enzyme. Bovine serum albumin (BSA) is used to reduce DNase I adsorption to the walls of the microchannels and enable diffusion of the cutting enzyme to a distance of 10mm. Due to the DNAs being immobilized, the fragment order is maintained on the surface. Possible methods of preserving the order for application to sequencing are discussed.

PDMS as a Substrate for Lipid Bilayers

PDMS (polydimethylsiloxane) is a cheap, optically clear polymer that is elastic and can be easily and quickly fabricated into a wide array of microscale and nanoscale architectures, making it a versatile substrate for biophysical experiments on cell membranes. It is easy to imagine many new experiments will be devised that require a bilayer to be placed upon a substrate that is flexible or easily cast into a desired geometry, such as in lab-on-a-chip, organ-on-chip, and microfluidic applications, or for building accurate membrane models that replicate the surface structure and elasticity of the cytoskeleton. However, PDMS has its limitations, and the extent to which the behavior of membranes is affected on PDMS has not been fully explored. We use AFM and fluorescence optical microscopy to investigate the use of PDMS as a substrate for the formation and study of supported lipid bilayers (SLBs). Lipid bilayers form on plasma-treated PDMS and show free diffusion and normal phase transitions, confirming its suitability as a model bilayer substrate. However, lipid-phase separation on PDMS is severely restricted due to the pinning of domains to surface roughness, resulting in the cessation of lateral hydrodynamic flow. We show the high-resolution porous structure of PDMS and the extreme smoothing effect of oxygen plasma treatment used to hydrophilize the surface, but this is not flat enough to allow domain formation. We also observe bilayer degradation over hour timescales, which correlates with the known hydrophobic recovery of PDMS, and establish a critical water contact angle of 30°, above which bilayers degrade or not form at all. Care must be taken as incomplete surface oxidation and hydrophobic recovery result in optically invisible membrane disruption, which will also be transparent to fluorescence microscopy and lipid diffusion measurements in the early stages.

Nanomechanical Stability of Laterally Heterogeneous Films of Corrosion Inhibitor Molecules Obtained by Microcontact Printing on Au Model Substrates

Self-assembled monolayers of corrosion inhibitors of the mercaptobenzimidazole family, SH-BimH, SH-BimH-5NH₂, and SH-BimH-5OMe, were formed on template-stripped ultraflat Au surfaces using microcontact printing, and subsequently analyzed using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and AFM-force spectroscopy (AFM-FS) using a quantitative imaging (QI) mode. Printing of all used inhibitor molecules resulted in clear patterns and in slightly more compact films compared to immersion. The stability of the monolayers is further probed by AFM-FS. Adhesion values of laterally heterogeneous inhibitor-modified surfaces compared to bare Au surfaces, nonpatterned areas, and fully covered surfaces are analyzed and discussed. Microcontact printing confers a superior nanomechanical stability to imidazole-modified films of the printed surface patches as compared to homogeneously covered surfaces by immersion into the inhibitor solution.

Topographically Flat Nanoplasmonic Sensor Chips for Biosensing and Materials Science

Nanoplasmonic sensors typically comprise arrangements of noble metal nanoparticles on a dielectric support. Thus, they are intrinsically characterized by surface topography with corrugations at the 10-100 nm length scale. While irrelevant in some bio- and chemosensing applications, it is also to be expected that the surface topography significantly influences the interaction between solids, fluids, nanoparticles and (bio)molecules, and the nanoplasmonic sensor surface. To address this issue, we present a wafer-scale nanolithography-based fabrication approach for high-temperature compatible, chemically inert, topographically flat, and laterally homogeneous nanoplasmonic sensor chips. We demonstrate their sensing performance on three different examples, for which we also carry out a direct comparison with a traditional nanoplasmonic sensor with representative surface corrugation. Specifically, we (i) quantify the film-thickness dependence of the glass transition temperature in poly(methyl metacrylate) thin films, (ii) characterize the adsorption and specific binding kinetics of the avidin-biotinylated bovine serum albumin protein system, and (iii) analyze supported lipid bilayer formation on SiO₂ surfaces.

Automated and Multiplexed Soft Lithography for the Production of Low-Density DNA Microarrays

Microarrays are established research tools for genotyping, expression profiling, or molecular diagnostics in which DNA molecules are precisely addressed to the surface of a solid support. This study assesses the fabrication of low-density oligonucleotide arrays using an automated microcontact printing device, the InnoStamp 40(®). This automate allows a multiplexed deposition of oligoprobes on a functionalized surface by the use of a MacroStamp(TM) bearing 64 individual pillars each mounted with 50 circular micropatterns (spots) of 160 µm diameter at 320 µm pitch. Reliability and reuse of the MacroStamp(TM) were shown to be fast and robust by a simple washing step in 96% ethanol. The low-density microarrays printed on either epoxysilane or dendrimer-functionalized slides (DendriSlides) showed excellent hybridization response with complementary sequences at unusual low probe and target concentrations, since the actual probe density immobilized by this technology was at least 10-fold lower than with the conventional mechanical spotting. In addition, we found a comparable hybridization response in terms of fluorescence intensity between spotted and printed oligoarrays with a 1 nM complementary target by using a 50-fold lower probe concentration to produce the oligoarrays by the microcontact printing method. Taken together, our results lend support to the potential development of this multiplexed microcontact printing technology employing soft lithography as an alternative, cost-competitive tool for fabrication of low-density DNA microarrays.

Human iPSC-derived endothelial cell sprouting assay in synthetic hydrogel arrays

Activation of vascular endothelial cells (ECs) by growth factors initiates a cascade of events during angiogenesis in vivo consisting of EC tip cell selection, sprout formation, EC stalk cell proliferation, and ultimately vascular stabilization by support cells. Although EC functional assays can recapitulate one or more aspects of angiogenesis in vitro, they are often limited by undefined substrates and lack of dependence on key angiogenic signaling axes. Here, we designed and characterized a chemically-defined model of endothelial sprouting behavior in vitro using human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs). We rapidly encapsulated iPSC-ECs at high density in poly(ethylene glycol) (PEG) hydrogel spheres using thiol-ene chemistry and subsequently encapsulated cell-dense hydrogel spheres in a cell-free hydrogel layer. The hydrogel sprouting array supported pro-angiogenic phenotype of iPSC-ECs and supported growth factor-dependent proliferation and sprouting behavior. iPSC-ECs in the sprouting model responded appropriately to several reference pharmacological angiogenesis inhibitors of vascular endothelial growth factor, NF-κB, matrix metalloproteinase-2/9, protein kinase activity, and β-tubulin, which confirms their functional role in endothelial sprouting. A blinded screen of 38 putative vascular disrupting compounds from the US Environmental Protection Agency's ToxCast library identified six compounds that inhibited iPSC-EC sprouting and five compounds that were overtly cytotoxic to iPSC-ECs at a single concentration. The chemically-defined iPSC-EC sprouting model (iSM) is thus amenable to enhanced-throughput screening of small molecular libraries for effects on angiogenic sprouting and iPSC-EC toxicity assessment.

STATEMENT OF SIGNIFICANCE

Angiogenesis assays that are commonly used for drug screening and toxicity assessment applications typically utilize natural substrates like Matrigel(TM) that are difficult to spatially pattern, costly, ill-defined, and may exhibit lot-to-lot variability. Herein, we describe a novel angiogenic sprouting assay using chemically-defined, bioinert poly(ethylene glycol) hydrogels functionalized with biomimetic peptides to promote cell attachment and degradation in a reproducible format that may mitigate the need for natural substrates. The quantitative assay of angiogenic sprouting here enables precise control over the initial conditions and can be formulated into arrays for screening. The sprouting assay here was dependent on key angiogenic signaling axes in a screen of angiogenesis inhibitors and a blinded screen of putative vascular disrupting compounds from the US-EPA.

Polyethylene Glycol Coatings on Plastic Substrates for Chemically Defined Stem Cell Culture

Human mesenchymal stem cells (hMSCs) are a widely available and clinically relevant cell type with a host of applications in regenerative medicine. Current clinical expansion methods can lead to selective changes in hMSC phenotype potentially resulting from relatively undefined cell culture surfaces. Chemically defined synthetic surfaces can aid in understanding the influence of cell-material interactions on stem cell behavior. Here, a thin copolymer coating for hMSC culture on plastic substrates is developed. The random copolymer is synthesized by living free radical polymerization and characterized in solution before application to the substrate, ensuring a homogeneous coating and limiting the sample-to-sample variations. The ability to coat multiple substrate types and cover large surface areas is reported. Arg-Gly-Asp-containing peptides are incorporated into the coating under aqueous conditions via their lysine or cysteine side chains, resulting in amide and thioester linkages, respectively. Stability studies show amide linkages to be stable and thioester linkages to be labile under standard serum-containing culture conditions. In addition, chemically defined passaging of hMSCs using only ethylenediaminetetraacetic acid on polystyrene dishes is shown. After passage, the hMSCs can be seeded back onto the same plate, indicating potential reusability of the coating.

Strong hydrophobizer: laterally chemisorbed low-molecular-weight polydimethylsiloxane

We introduce a siloxane chain-based hydrophobizer that exhibits superior thermal and chemical stability compared to the conventional hydrophobizing silane agent under conditions of over 300 °C and pH 2–13.

Negative printing by soft lithography

In inkless microcontact printing (IμCP) by soft lithography, the poly(dimethylsiloxane) (PDMS) stamp transfers uncured polymer to a substrate corresponding to its pattern. The spontaneous diffusion of PDMS oligomers to the surface of the stamp that gives rise to this deleterious side effect has been leveraged to fabricate a variety of devices, such as organic thin film transistors, single-electron devices, and biomolecular chips. Here we report an anomalous observation on a partially cured PDMS stamp where the transfer of oligomers onto Au occurred on regions that were not in contact with the stamp, while the surface in contact with the stamp was pristine with no polymer. On the SiO2 surface of the same chip, as expected, the transfer of PDMS occurred exclusively on regions in contact with the stamp. The printing on Au was quantified by a novel method where the submonolayer of PDMS transfer was measured by probing the local electrochemical passivation of the Au. The local transfer of polymer on SiO2 (and also Au) was measured by selective deposition of Au nanoparticle necklaces that exclusively deposited on PDMS at submonolayer sensitivity. It was discovered that the selectivity and sharpness of PDMS deposition on Au for inkless printing (i.e., negative) is significantly better than the traditional (positive) microcontact printing where the stamp is "inked" with low molecular weight PDMS.

Cell division orientation on biospecific peptide gradients

An assay was developed for determining cell division orientation on gradients. The methodology is based on permeating microfluidic devices with alkanethiols and subsequent printing of cell adhesive peptide gradient self-assembled monolayers (SAMs) for examining oriented cell divisions. To our knowledge, there has been no study examining the correlation between cell division orientations based on an underlying ligand gradient. These results implicate an important role for how the extracellular matrix may control cell division. These surfaces would allow for a range of cell behavior (polarization, migration, division, differentiation) studies on tailored biospecific gradients and as a potential biotechnological platform to assess small molecule perturbations of cell function.

Microdevice arrays of high aspect ratio poly(dimethylsiloxane) pillars for the investigation of multicellular tumour spheroid mechanical properties

We report the design, fabrication and evaluation of an array of microdevices composed of high aspect ratio PDMS pillars, dedicated to the study of tumour spheroid mechanical properties. The principle of the microdevice is to confine a spheroid within a circle of micropillars acting as peripheral flexible force sensors. We present a technological process for fabricating high aspect ratio micropillars (300 μm high) with tunable feature dimensions (diameter and spacing) enabling production of flexible PDMS pillars with a height comparable to spheroid sizes. This represents an upscale of 10 along the vertical direction in comparison to more conventional PDMS pillar force sensors devoted to single cell studies, while maintaining their force sensitivity in the same order of magnitude. We present a method for keeping these very high aspect ratio PDMS pillars stable and straight in liquid solution. We demonstrate that microfabricated devices are biocompatible and adapted to long-term spheroid growth. Finally, we show that the spheroid interaction with the micropillars' surface is dependent on PDMS cellular adhesiveness. Time-lapse recordings of growth-induced micropillars' bending coupled with a software program to automatically detect and analyse micropillar displacements are presented. The use of these microdevices as force microsensors opens new prospects in the fields of tissue mechanics and pharmacological drug screening.

Comparison of blend morphologies of the nano-patterned photoactive films via two different techniques: thermal-assisted and solvent-assisted soft-nanoimprint lithography

We reports the comparison of blend morphologies of nano-patterned photoactive films via two different techniques and its effects on the performance of OPVC.

Differential effects of cell adhesion, modulus and VEGFR-2 inhibition on capillary network formation in synthetic hydrogel arrays

Efficient biomaterial screening platforms can test a wide range of extracellular environments that modulate vascular growth. Here, we used synthetic hydrogel arrays to probe the combined effects of Cys-Arg-Gly-Asp-Ser (CRGDS) cell adhesion peptide concentration, shear modulus and vascular endothelial growth factor receptor 2 (VEGFR2) inhibition on human umbilical vein endothelial cell (HUVEC) viability, proliferation and tubulogenesis. HUVECs were encapsulated in degradable poly(ethylene glycol) (PEG) hydrogels with defined CRGDS concentration and shear modulus. VEGFR2 activity was modulated using the VEGFR2 inhibitor SU5416. We demonstrate that synergy exists between VEGFR2 activity and CRGDS ligand presentation in the context of maintaining HUVEC viability. However, excessive CRGDS disrupts this synergy. HUVEC proliferation significantly decreased with VEGFR2 inhibition and increased modulus, but did not vary monotonically with CRGDS concentration. Capillary-like structure (CLS) formation was highly modulated by CRGDS concentration and modulus, but was largely unaffected by VEGFR2 inhibition. We conclude that the characteristics of the ECM surrounding encapsulated HUVECs significantly influence cell viability, proliferation and CLS formation. Additionally, the ECM modulates the effects of VEGFR2 signaling, ranging from changing the effectiveness of synergistic interactions between integrins and VEGFR2 to determining whether VEGFR2 upregulates, downregulates or has no effect on proliferation and CLS formation.

A chemically-defined screening platform reveals behavioral similarities between primary human mesenchymal stem cells and endothelial cells

Chemically defined substrates, which rigorously control protein-surface and cell-surface interactions, can be used to probe the effects of specific biomolecules on cell behavior. Here we combined a chemically-defined, array-based format with automated, time-lapse microscopy to efficiently screen cell-substrate interactions. Self-assembled monolayers (SAMs) of alkanethiolates bearing oligo(ethylene glycol) units and reactive terminal groups were used to present cell adhesion peptides while minimizing non-specific protein interactions. Specifically, we describe rapid fabrication of arrays of 1 mm spots, which present varied densities of the integrin-binding ligand Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP). Results indicate that cell attachment, cell spreading, and proliferation exhibit strong dependencies on GRGDSP density for both human mesenchymal stem cells (hMSCs) and human umbilical vein endothelial cells (HUVECs). Furthermore, relative spreading and proliferation over a broad range of GRGDSP densities were similar for both primary cell types, and detailed comparison between cell behaviors identified a 1 : 1 correlation between spreading and proliferation for both HUVECs and hMSCs. Finally, time-lapse microscopy of SAM arrays revealed distinct adhesion-dependent migratory behaviors for HUVECs and hMSCs. These results demonstrate the benefits of using an array-based screening platform for investigating cell function. While the proof-of-concept focuses on simple cellular properties, the quantitative similarities observed for hMSCs and HUVECs provides a direct example of how phenomena that would not easily be predicted can be shown to correlate between different cell types.

Differential effects of a soluble or immobilized VEGFR-binding peptide

Regulating endothelial cell behavior is a key step in understanding and controlling neovascularization for both pro-angiogenic and anti-angiogenic therapeutic strategies. Here, we characterized the effects of a covalently immobilized peptide mimic of vascular endothelial growth factor, herein referred to as VEGF receptor-binding peptide (VR-BP), on human umbilical vein endothelial cell (HUVEC) behavior. Self-assembled monolayer arrays presenting varied densities of covalently immobilized VR-BP and varied densities of the fibronectin-derived cell adhesion peptide Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) were used to probe for changes in HUVEC attachment, proliferation and tubulogenesis. In a soluble form, VR-BP exhibited pro-angiogenic effects in agreement with previous studies, indicated by increases in HUVEC proliferation. However, when presented to cells in an insoluble context, covalently immobilized VR-BP inhibited several pro-angiogenic HUVEC behaviors, including attachment and proliferation, and also inhibited HUVEC response to soluble recombinant VEGF protein. Furthermore, substrates with covalently immobilized VR-BP also modulated HUVEC tubulogenesis when a matrigel overlay assay was used to provide cells with a pseudo-three dimensional environment. Taken together, these results demonstrate that the context in which ligands are presented to cell surface receptors strongly influences their effects, and that the same ligand can be an agonist or an antagonist depending on the manner of presentation to the cell.

Combinatorial screening of chemically defined human mesenchymal stem cell culture substrates

Self-assembled monolayers (SAMs) of alkanethiolates on gold are chemically defined substrates that can be used to evaluate the effects of an immobilized biomolecule. However, the types of biomolecules that can influence stem cell behavior are numerous and inter-related, and efficient experimental formats are a critical need. Here we employed a SAM array technology to investigate the effects of multiple, distinct peptides and peptide combinations on human mesenchymal stem cell (hMSC) behavior. Specifically, we characterized the conjugation of peptide mixtures to SAM arrays and then investigated the combined effects of a bone morphogenic protein receptor-binding peptide (BR-BP), a heparin proteoglycan-binding peptide (HPG-BP), and varied densities of the integrin-binding ligand Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) on hMSC surface coverage and alkaline phosphatase activity. Results indicate that an amine reactive fluorescent probe can be used to characterize peptide composition after immobilization in SAM array spots. Furthermore, hMSC response to BR-BP and HPG-BP is dependent on GRGDSP density and at day 7, hMSC alkaline phosphatase expression is highly dependent on GRGDSP density. Taken together, we demonstrate how a SAM array approach can be used to probe the combinatorial effects of multiple peptides and motivate further investigations into potential synergies between cell adhesion and other bioactive peptides.

Universal nanopatternable interfacial bonding

A nanopatternable polydimethylsiloxane (PDMS) oligomer layer is demonstrated as an interfacial adhesive for its intrinsic transferability and universal adhesiveness. Utilizing the well-established surface modification and bonding techniques of PDMS surfaces, irreversible bonding is formed (up to 400 kPa) between a wide range of substrate pairs, representing ones within and across different materials categories, including metals, ceramics, thermoset, and thermoplastic polymers.

High-density fabrication of normally closed microfluidic valves by patterned deactivation of oxidized polydimethylsiloxane

The use of polydimethylsiloxane (PDMS) in microfluidic devices is extensive in academic research. One of the most fundamental treatments is to expose PDMS to plasma oxidation in order to render its surface temporarily hydrophilic and capable of permanent bonding. Here, we show that changes in the surface chemistry induced by plasma oxidation can spatially be counteracted very cleanly and reliably in a scalable manner by subsequent microcontact printing of residual oligomers from a PDMS stamp. We characterize the surface modifications through contact angle, atomic force microscopy, X-ray photoelectron spectroscopy, and bond-strength measurements. We utilize this approach for negating the bonding of a flexible membrane layer within an elastomeric valve and demonstrate its effectiveness by integration of over one thousand normally closed elastomeric valves within a single substrate. In addition, we demonstrate that surface energy patterning can be used for "open microfluidic" applications that utilize spatial control of surface wetting.

Hydrophobically recovered and contact printed siloxane oligomers for general-purpose surface patterning

Hydrophobic recovery of elastomeric polydimethylsiloxane (PDMS) has been well-known in various fields, such as microcontact printing (microCP), microfluidics, and electric insulation, etc., which has been believed to be due to the transfer of out-diffused siloxane oligomers in PDMS. The recovery phenomenon has been used to control surface energy of a substrate, due partly to its nanoscale thickness. In this work, we extend the use of recovered oligomers to a general-purpose surface patterning process, in combination with both dry and wet pattern transfer processes. The out-diffused and transfer-printed oligomers play exactly the same role of "ink" in the conventional microCP; thus, the present method can be termed as "inkless" microcontact printing (ImicroCP). Also, the detailed nature of recovered oligomers has been investigated, and they are found to have a molecular weight approximately 10 times larger than that of pristine, uncured PDMS oligomers. And the molecular weight distribution is very broad with a polydispersity index of approximately 15. Then, we present and discuss various aspects of the ImicroCP process, such as pattern transfer onto substrate via wet or dry etching, effect of process variables on printing results, minimum feature size achieved by the technique, repeated printing with the same stamp, and the generation of more complex patterns from simpler ones by applying multiple ImicroCP.

Microfluidic permeation printing of self-assembled monolayer gradients on surfaces for chemoselective ligand immobilization applied to cell adhesion and polarization

To study complex cell behavior on model surfaces requires biospecific interactions between the interfacing cell and material. Developing strategies to pattern well-defined molecular gradients on surfaces is difficult but critical for studying cell adhesion, polarization, and directed cell migration. We introduce a new strategy, microfluidic SPREAD (Solute PeRmeation Enhancement And Diffusion) for inking poly(dimethylsiloxane) (PDMS) microfluidic cassettes with a gradient of alkanethiol. Using SPREAD, an oxyamine-terminated alkanethiol is able to permeate into a PDMS microfluidic cassette, creating a chemical gradient, which can subsequently be transfer printed onto a gold surface to form the corresponding chemoselective gradient of oxyamine-alkanethiol self-assembled monolayer (SAM). By first patterning regions of the gold surface with a protective SAM using microfluidic lithography, directional gradients can be stamped exclusively onto unprotected bare gold regions to form single cell gradient microarrays. The microfluidic SPREAD strategy can also be extended to print micrometer-sized islands of radial SAM gradients with excellent geometric resolution. The immobilization of a cell adhesive Arg-Gly-Asp (RGD)-ketone peptide to the SPREAD stamped oxyamine-alkanethiol SAMs provides a stable interfacial oxime linkage for biospecific studies of cell adhesion, polarity, and migration.

Guiding neuron development with planar surface gradients of substrate cues deposited using microfluidic devices

Wiring the nervous system relies on the interplay of intrinsic and extrinsic signaling molecules that control neurite extension, neuronal polarity, process maturation and experience-dependent refinement. Extrinsic signals establish and enrich neuron-neuron interactions during development. Understanding how such extrinsic cues direct neurons to establish neural connections in vitro will facilitate the development of organized neural networks for investigating the development and function of nervous system networks. Producing ordered networks of neurons with defined connectivity in vitro presents special technical challenges because the results must be compliant with the biological requirements of rewiring neural networks. Here we demonstrate the ability to form stable, instructive surface-bound gradients of laminin that guide postnatal hippocampal neuron development in vitro. Our work uses a three-channel, interconnected microfluidic device that permits the production of adlayers of planar substrates through the combination of laminar flow, diffusion and physisorption. Through simple flow modifications, a variety of patterns and gradients of laminin (LN) and fluorescein isothiocyanate-conjugated poly-l-lysine (FITC-PLL) were deposited to present neurons with an instructive substratum to guide neuronal development. We present three variations in substrate design that produce distinct growth regimens for postnatal neurons in dispersed cell cultures. In the first approach, diffusion-mediated gradients of LN were formed on cover slips to guide neurons toward increasing LN concentrations. In the second approach, a combined gradient of LN and FITC-PLL was produced using aspiration-driven laminar flow to restrict neuronal growth to a 15 microm wide growth zone at the center of the two superimposed gradients. The last approach demonstrates the capacity to combine binary lines of FITC-PLL in conjunction with surface gradients of LN and bovine serum albumin (BSA) to produce substrate adlayers that provide additional levels of control over growth. This work demonstrates the advantages of spatio-temporal fluid control for patterning surface-bound gradients using a simple microfluidics-based substrate deposition procedure. We anticipate that this microfluidics-based patterning approach will provide instructive patterns and surface-bound gradients to enable a new level of control in guiding neuron development and network formation.

Hysteresis and reversibility of a superhydrophobic photopatternable silicone elastomer

We report upon the wetting property of layers of a micropatterned photodefinable silicon elastomer, PDSE, repetitively and alternatively treated with oxygen plasma and temperature cycles. At low power plasma treatments, we observed a hysteresis in terms of contact angle between phases lowering the contact angle and phases of recovery. As opposed to high power plasma for which we show that by generating fissures on the surface, the structure can be cycled between superhydrophobic and superhydrophilic states. The plasma-generated diffusion paths were characterized by electron microscopy and were found to be directly related to the recovery of the wetting properties of the plasma treated layers of PDSE. The cycling between the superhydrophobic and superhydrophilic states was dependent on the power of the applied plasma as well as the condition during the contact angle recovery amplified by a temperature-controlled baking step.

Aminosilane micropatterns on hydroxyl-terminated substrates: fabrication and applications

The technique to pattern aminosilanes on hydroxyl-terminated substrates will open up extensive applications in many fields. There are some existing methods to pattern aminosilanes, in particular, (3-aminopropyl)triethoxysilane (APTES) on SiO(2) and glass substrates through indirect routes. However, few reports focus on the direct patterning of APTES by microcontact printing (microCP), due to the volatility of "inks" which consist of APTES and organic solvents. This report shows that high-quality APTES patterns on hydroxyl-terminated substrates can be directly obtained by microCP using an APTES aqueous solution as "ink". Gold nanoparticles (Au NPs) have been used to verify the presence and quality of APTES patterns on which they are selectively adsorbed. Thus-obtained Au NP patterns can serve as templates for the growth of ZnO nanostructures. Lectins are also successfully immobilized on the APTES patterns, with glutaraldehyde as linker. We believe that our method will serve as a general approach and find a wide range of applications in the fabrication of patterns and devices.

Patterning discrete stem cell culture environments via localized self-assembled monolayer replacement

Self-assembled monolayers (SAMs) of alkanethiolates on gold have become an important tool for probing cell-material interactions. Emerging studies in stem cell biology are particularly reliant on well-defined model substrates, and rapid, highly controllable fabrication methods may be necessary for characterizing the wide array of stem cell-material interactions. Therefore, this study describes a rapid method for creating SAM cell culture substrates with multiple discrete regions of controlled peptide identity and density. The approach uses a NaBH(4) solution to selectively remove regions of bioinert, hydroxyl-terminated oligo(ethylene glycol) alkanethiolate SAM and then locally replace them with mixed SAMs of hydroxyl- and carboxylic acid-terminated oligo(ethylene glycol) alkanethiolates. The cell adhesion peptide Arg-Gly-Asp-Ser-Pro (RGDSP) was then covalently linked to carboxylic acid-terminated mixed SAM regions to create cell adhesive environments within a bioinert background. SAM preparation and peptide immobilization were characterized using polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS), as well as assays to monitor conjugation of a fluorescently labeled peptide. This "localized SAM replacement" method was achieved using an array of microchannels, which facilitated rapid and simple processing. Results indicate that immobilized RGDSP promoted spatially localized attachment of human mesenchymal stem cells (hMSCs) within specified regions, while maintaining a stable, bioinert background in serum-containing cell culture conditions for up to 14 days. Cell attachment to patterned regions presenting a range of cell adhesion peptide densities demonstrated that peptide identity and density strongly influence hMSC spreading and focal adhesion density. These substrates contain discrete, well-defined microenvironments for stem cell culture, which could ultimately enable high-throughput screening for the effects of immobilized signals on stem cell phenotype.

Nanoscale patterns of dendrimers obtained by soft lithography using elastomeric stamps spontaneously structured by plasma treatment

It is well established that polydimethylsiloxane (PDMS) stamps submitted to an adequate plasma treatment spontaneously develop an ordered surface roughness. In this work, we show that thin layers made of polyamidoamine (PAMAM) dendrimers can be patterned at the nanoscale using these buckled PDMS stamps. The structures accurately reproduce the self-assembled waves observed on the stamp surface after plasma treatment. We discuss the involved transfer of molecules from the stamp to the surface, which relies on molding rather than on printing. Self-assembled networks of lines made of dendrimers with submicrometric pitch can therefore be produced using this process at very low cost without any advanced lithography method for generating hard molds.

Top-down particle fabrication: control of size and shape for diagnostic imaging and drug delivery

This review discusses rational design of particles for use as therapeutic vectors and diagnostic imaging agent carriers. The emerging importance of both particle size and shape is considered, and the adaptation and modification of soft lithography methods to produce nanoparticles are highlighted. To this end, studies utilizing particles made via a process called Particle Replication In Non-wetting Templates are discussed. In addition, insights gained into therapeutic cargo and imaging agent delivery from related types of polymer-based carriers are considered.

Effect of surface free energy on PDMS transfer in microcontact printing and its application to ToF-SIMS to probe surface energies

Although polydimethylsiloxane (PDMS) transfer during microcontact printing (microCP) has been observed in previous reports, which generally focused on only one or a few different substrates, in this work we investigate the extent of PDMS transfer onto a series of surfaces with a wide range of hydrophobicities using an uninked, unpatterned PDMS stamp. These surfaces include clean silicon, clean titanium, clean gold, "dirty" silicon, polystyrene, Teflon, surfaces modified with PEG, amino, dodecyl, and hexadecyl monolayers, and also two loose molecular materials. The PDMS transferred onto planar surfaces is, in general, easily detected by wetting and spectroscopic ellipsometry. More importantly, it is detected by time-of-flight secondary ion mass spectrometry (ToF-SIMS) because of the sensitivity of this technique to PDMS. The effect of surface free energy on PDMS transfer in microcontact printing is investigated, and the relationship between the amount of PDMS in ToF-SIMS spectra and the surface tensions of initial surfaces is revealed. We show that PDMS transfer can be applied as a probe of surface free energies using ToF-SIMS, where PDMS preferentially transfers onto more hydrophilic surface features during stamping, with little being transferred onto very hydrophobic surface features. Multivariate curve resolution (MCR) analysis of the ToF-SIMS image data further confirms and clarifies these results. Our data lend themselves to the hypothesis that it is the free energy of the surface that plays a major role in determining the degree of PDMS transfer during microCP.

Superhydrophobic polyimide films with a hierarchical topography: combined replica molding and layer-by-layer assembly

Artificial superhydrophobic surfaces with a hierarchical topography were fabricated by using layer-by-layer assembly of polyelectrolytes and silica nanoparticles on microsphere-patterned polyimide precursor substrates followed with thermal and fluoroalkylsilane treatment. In this special hierarchical topography, micrometer-scale structures were provided by replica molding of polyamic acid using two-dimensional arrays of polystyrene latex spheres as templates, and nanosized silica particles were then assembled on these microspheres to construct finer structures at the nanoscale. Heat treatment was conducted to induce chemical cross-linking between polyelectrolytes and simultaneously convert polyamic acid to polyimide. After surface modification with fluoroalkylsilane, the as-prepared highly hydrophilic surface was endowed with superhydrophobicity due to the bioinspired combination of low surface energy materials and hierarchical surface structures. A superhydrophobic surface with a static water contact angle of 160 degrees and sliding angle of less than 10 degrees was obtained. Notably, the polyimide microspheres were integrated with the substrate and were mechanically stable. In addition, the chemical and mechanical stability of the polyelectrolyte/silica nanoparticle multilayers could be increased by heat-induced cross-linking between polyelectrolytes to form nylon-like films, as well as the formation of interfacial chemical bonds.

Microfluidic lithography of SAMs on gold to create dynamic surfaces for directed cell migration and contiguous cell cocultures

A straightforward, flexible, and inexpensive method to create patterned self-assembled monolayers (SAMs) on gold using microfluidics-microfluidic lithography-has been developed. Using a microfluidic cassette, alkanethiols were rapidly patterned on gold surfaces to generate monolayers and mixed monolayers. The patterning methodology is flexible and, by controlling the solvent conditions and thiol concentration, permeation of alkanethiols into the surrounding PDMS microfluidic cassette can be advantageously used to create different patterned feature sizes and to generate well-defined SAM surface gradients with a single microfluidic chip. To demonstrate the utility of microfluidic lithography, multiple cell experiments were conducted. By patterning cell adhesive regions in an inert background, a combination of selective masking of the surface and centrifugation achieved spatial and temporal control of patterned cells, enabling the design of both dynamic surfaces for directed cell migration and contiguous cocultures. Cellular division and motility resulted in directed, dynamic migration, while the centrifugation-aided seeding of a second cell line produced contiguous cocultures with multiple sites for heterogeneous cell-cell interactions.