A microfluidic platform for integrated synthesis and dynamic light scattering measurement of block copolymer micelles.

Microfluidic devices were developed that integrate the synthesis of well defined block copolymers and dynamic light scattering (DLS) measurement of their micelle formation. These metal devices were designed to operate in contact with organic solvents and elevated temperatures for long periods, and thus were capable of continuous in-channel atom transfer radical polymerization (ATRP) of styrene and (meth)acrylate homopolymers and block copolymers. These devices were equipped with a miniaturized fiber optic DLS probe that included several technology improvements, including a measurement volume of only 4 microlitres, simple alignment, and reduced multiple scattering. To demonstrate the integrated measurement, poly(methyl methacrylate-b-lauryl methacrylate) and poly(methyl methacrylate-b-octadecyl methacrylate) block copolymers were processed on the device with a selective solvent, dodecane, to induce micelle formation. The in situ DLS measurements yielded the size and aggregation behavior of the micelles. For example, the block copolymer solutions formed discrete micelles (D(H) approximately = 25 nm) when the corona block was sufficiently long (f(MMA) < 0.51), but the micelles aggregated when this block was short. This study demonstrates the utility of these new devices for screening the solution behavior of custom synthesized polymeric surfactants and additives.

Effect of temperature on the morphology and kinetics of surface pattern formation in thin block copolymer films.

Hole formation and growth on the top layer of thin symmetric diblock copolymer films, forming an ordered lamellar structure parallel to the solid substrate (silicon wafer) within these films, is investigated as a function of time (t), temperature (T), and film thickness (l), using a high-throughput experimental technique. The kinetics of this surface pattern formation process is interpreted in terms of a first-order reaction model with a time-dependent rate constant determined uniquely by the short-time diffusive growth kinetics characteristic of this type of ordering process. On the basis of this model, we conclude that the average hole size, lambda(h), approaches a steady-state value, lambda(h)(t-->infinity) identical with lambda(h,infinity)(T), after long annealing times. The observed change in lambda(h,infinity)(T) with temperature is consistent with a reduction of the surface elasticity (Helfrich elastic constant) of the outer block copolymer layer with increasing temperature. We also find that the time constant, tau(T), characterizing the rate at which lambda(h)(t) approaches lambda(h,infinity)(T), first decreases and then increases with increasing temperature. This temperature variation of tau(T) is attributed to two basic competing effects that influence the rate of ordering in block copolymer materials: the reduction in molecular mobility at low temperatures associated with glass formation and a slowing of the rate of ordering due to fluctuation effects associated with an approach to the block copolymer film disordering temperature (T(d)) from below.

Miniaturized dynamic light scattering instrumentation for use in microfluidic applications.

Five designs for a miniaturized dynamic light scattering (DLS) instrument are described that incorporate microfluidic flow of the sample volume and fiber optic probes directly embedded into the sample. These instruments were demonstrated to accurately determine the size of 10-100 nm particles dispersed in organic and aqueous solvents with most sample sizes less than 150 microl. Small stir bars were incorporated directly into the instruments, and enabled blending of different solutions immediately prior to DLS measurements. Demonstration of the instruments' capabilities include high throughput measurements of the micelle to unimer transition for poly(styrene-b-isoprene) in mixed toluene/hexadecane solvent, obtained by systematically blending toluene-rich and hexadecane-rich polymer solutions. The critical solvent composition was quickly identified with less than 20 mg of polymer. Further capabilities include temperature control, demonstrated by identification of a critical micelle temperature of poly(ethylene oxide-b-propylene oxide-b-ethylene oxide), as well as multiangle DLS measurements.

Raman spectroscopic monitoring of droplet polymerization in a microfluidic device.

Microfluidic methodologies are becoming increasingly important for rapid formulation and screening of materials, and development of analytical tools for multiple sample screening is a critical step in achieving a combinatorial 'lab on a chip' approach. This work demonstrates the application of Raman spectroscopy for analysis of monomer composition and degree of conversion of methacrylate-based droplets in a microfluidic device. Droplet formation was conducted by flow focusing on the devices, and a gradient of component composition was created by varying the flow rates of the droplet-phase fluids into the microchannels. Raman data were collected using a fiber optic probe from a stationary array of the droplets/particles on the device, followed by partial least squares (PLS) calibration of the first derivative (1600 cm(-1) to 1550 cm(-1)) allowing successful measurement of monomer composition with a standard error of calibration (SEC) of +/-1.95% by volume. Following photopolymerization, the percentage of double bond conversion of the individual particles was calculated from the depletion of the normalized intensity of the C[double bond, length as m-dash]C stretching vibration at 1605 cm(-1). Raman data allowed accurate measurement of the decrease in double bond conversion as a function of increasing crosslinker concentration. The results from the research demonstrate that Raman spectroscopy is an effective, on-chip analytical tool for screening polymeric materials on the micrometre scale.

Gradient substrate assembly for quantifying cellular response to biomaterials.

Using quantitative fluorescence microscopy in conjunction with a method of gradient substrate assembly established in their group, the authors were able to introduce and measure reproducible changes in cellular morphology and cell density by manipulating polymer grafting density. The mechanism behind this change in cellular behavior was explained by a semiempirical, geometric model that describes the effect of the spatial distribution of the polymer on protein attachment. A 10-fold increase in graft density of poly(2-hydroxyethyl methacrylate) [PHEMA] along the surface of a gradient sample, preexposed to bovine fibronectin, caused a change in the size of fibroblasts on the surface (i.e., cell spreading) from (1238 +/- 704) to (377 +/- 216) microm(2). The results were in quantitative agreement with those obtained on three separate gradient samples. Both cellular response and fibronectin adsorption (as measured via ellipsometry) were found to vary sigmoidally with graft density of PHEMA, demonstrating the high degree of correlation between the two phenomena. A simple, rigid-disk model accounting for the surface coverage of PHEMA was able to predict the amount of adsorbed fibronectin with a correlation coefficient of 0.97. Maximal cell adhesion and cell spreading were found to occur at fibronectin surface densities of 50 and 100 ng/cm(2), respectively. The results demonstrate the role of gradient substrate assembly as a method for quantifying the relationship between protein and cellular response to technologically relevant polymeric materials.

Combinatorial screen of the effect of surface energy on fibronectin-mediated osteoblast adhesion, spreading and proliferation.

In order to accelerate tissue-engineering research, a combinatorial approach for investigating the effect of surface energy on cell response has been developed. Surface energy is a fundamental material property that can influence cell behavior. Gradients in surface energy were created by using an automated stage to decelerate a glass slide coated with a self-assembled monolayer (SAM, n-octyldimethylchlorosilane) beneath a UV lamp such that the SAM is exposed to the UV-light in a graded fashion. UV exposure causes oxidation of the SAM such that a longer exposure correlates with increased hydrophilicity. This approach yielded substrates having a linear gradient in surface energy ranging from 23 to 62 mN/m (water contact angles ranging from 25 degrees to 95 degrees ). Using the gradient specimen approach enables all surface energies from 23 to 60 mN/m to be screened on each slide. Before cell culture, surface energy gradients were coated with fibronectin to allow a study of the effect of surface energy on fibronectin-mediated cell response. Cells were seeded on the fibronectin-coated gradients and adhesion, spreading and proliferation were assessed with automated fluorescence microscopy. Surface energy did not affect initial cell adhesion at 8h. However, the rate of proliferation was linearly dependent on surface energy and increased with increasing hydrophobicity. Cell spread area was unaffected by changes in surface energy over the majority of the gradient although cells were significantly smaller on the most hydrophilic region. These results show that fibronectin-mediated cell spreading and proliferation are dependent on surface energy and establish a new combinatorial approach for screening cell response to changes in surface energy.

Microfluidic light scattering as a tool to study the structure of aqueous polymer solutions.

A small-angle light scattering (SALS) apparatus, coupled with a specially designed microfluidic device is shown to monitor the formation and subsequent size distribution of giant multilamellar vesicles of a diblock copolymer in aqueous solution. The closed-face design, fabricated between glass slides using a UV-curable optical adhesive, incorporates multiple inlets, a mixing system, and a viewing window to perform on-line SALS. The mixing of each component is tested using polystyrene latex microspheres. Vesicles of the block copolymer, EO6BO11 in aqueous solution are formed on the SALS chip and the pair distance distribution function determined using an inverse Fourier transformation of the scattered intensity to quantify the population and distribution for a range of vesicle sizes. These experiments provide demonstrations of how SALS on a microfluidic device can be used as a rapid screening tool to optimize processing conditions for a range of polymer solutions.

Cellular response to phase-separated blends of tyrosine-derived polycarbonates.

Two-dimensional thin films consisting of homopolymer and discrete compositional blends of tyrosine-derived polycarbonates were prepared and characterized in an effort to elucidate the nature of different cell responses that were measured in vitro. The structurally similar blends were found to phase separate after annealing with domain sizes dependent on the overall composition. The thin polymer films were characterized with the use of atomic force microscopy (AFM), water contact angles, and time-of-flight secondary ion mass spectrometry (TOF-SIMS) and significant changes in roughness were measured following the annealing process. Genetic expression profiles of interleukin-1beta and fibronectin in MC3T3-E1 osteoblasts and RAW 264.7 murine macrophages were measured at several time points, demonstrating the time and composition-dependent nature of the cell responses. Real-time reverse transcriptase polymerase chain reaction (RT-PCR) depicted upregulation of the fibronectin gene copy numbers in each of the blends relative to the homopolymers. Moreover, the interleukin-1beta expression profile was found to be compositionally dependent. The data suggest strongly that optimal composition and processing conditions can significantly affect the acute inflammatory and extracellular matrix production responses.

Tuning cell adhesion on gradient poly(2-hydroxyethyl methacrylate)-grafted surfaces.

A simple yet versatile method was developed to prepare a low-density polymerization initiator gradient, which was combined with surface-initiated atom transfer radical polymerization (ATRP) to produce a well-defined poly(2-hydroxyethyl methacrylate) (HEMA) gradient substrate. A smooth variation in film thickness was measured across the gradient, ranging from 20 A to over 80 A, but we observed a nonmonotonic variation in water contact angle. Fits of X-ray reflectivity profiles suggested that at the low graft density end, the polymer chain structure was in a "mushroom" regime, while the polymer chains at high graft density were in a "brush" regime. It was found that the "mushroom" region of the gradient could be made adhesive to cells by adsorbing adhesion proteins, and cell adhesion could be tuned by controlling the density of the polymer grafts. Fibroblasts were seeded on gradients precoated with fibronectin to test cellular responses to this novel substrate, but it was found that cell adhesion did not follow the expected trend; instead, saturated cell adhesion and spreading was found at the low grafting density region.

Gradient chemical micro patterns: a reference substrate for surface nanometrology.

We present fabrication routes for a new type of surface specimen that exhibits a micro pattern with a gradient in chemical contrast between the pattern domains. Design elements in the specimen allow chemical contrast in the micro pattern to be related to well-established surface characterization data, such as contact angle measurements. These gradient specimens represent a reference tool for calibrating image contrast in chemically sensitive scanning probe microscopy techniques and a platform for the high-throughput analysis of polymer thin film behavior.

Combinatorial approach to study enzyme/surface interactions.

A fast combinatorial approach to access information about the immobilization behavior and kinetics of enzymes on a variation of surfaces is presented. As a test system, Candida Antarctica Lipase B was immobilized on a self-assembled monolayer bearing a gradient of surface energy. The respective immobilization behavior was monitored by Fourier transform infrared micro-spectroscopy. In addition, the activity of the immobilized enzyme was monitored over the entire film in real time with a specially developed fluorescence activity assay embedded into a siloxane gel. It was found that the highest amount of active protein was immobilized on the hydrophilic end of the gradient surface. This effect is associated with a higher surface roughness of this area resulting in hydrophobic micro-environments in which the enzyme gets immobilized.

Phase behavior of block co-poly(ethylene oxide-butylene oxide), E18B9 in water, by small angle neutron scattering.

We present a small angle neutron scattering (SANS) study into the micellar structures of diblock copolymer E18B9 (where E denotes a ethylene oxide unit and B denotes a butylene oxide unit, 18 and 9 being the number of repeat units respectively) in aqueous solution over a range of five different concentrations (0.2, 1.0, 10.0, 20.0, and 40.0% (by mass fraction)) and eight temperatures (10 to 90 degrees C). The NG7 30 m SANS instrument provides a q range of 0.0009 to 0.5548 A(-1), thus probing the structure over a very broad length scale. At low temperature and low concentration, spherical micelles exist, elongating into worm-like structures at higher temperatures. This transition is observed by the scaling of the scattered intensity at low q and confirmed upon fitting to an appropriate model. Upon increasing concentration, the micelles pack into ordered arrays of either hexagonally packed rod-like micelles or lamellar sheets, again dependent on temperature. Both concentration and temperature effects of this block copolymer have been discussed.

Microfluidic platform for the generation of organic-phase microreactors.

Rapid prototyping photolithography of a thiolene-based resin was used to fabricate microfluidic devices stable to aliphatic and aromatic organic solvents. The swelling of the cross-linked polymer matrix in various organic solvents was quantified, and the solvent resistance properties of these microfluidic devices are described. Discrete droplets of hexanes and toluene of uniform size were generated in microfluidic devices inside a water matrix containing SDS surfactant (SDS = sodium dodecyl sulfate). Variation of water and organic flow rates in the fluidic channels was used to control droplet size and separation. Droplet composition could be controlled by varying flow rates of two joined organic streams. Organic-phase synthetic reactions within the droplets were demonstrated with the bromination of alkenes inside benzene droplets.

Counterion associative behavior with flexible polyelectrolytes.

At low ionic strength, organic counterions dress a flexible charged polymer as measured directly by small-angle neutron scattering and neutron spin-echo spectroscopy. This dressed state, quantified by the concentration dependence of the static correlation length, illustrates the polymer-counterion coupled nature on the nanometer length scale. The counterions, made visible by selective hydrogen and deuterium labeling, undress from the polymeric template by addition of sodium chloride. The addition of this electrolyte leads to two effects: increased Debye electrostatic screening and decoupled organic counterion-polymer correlations. Neutron spin-echo spectroscopy measures a slowing down of the effective diffusion coefficient of the labeled counterions at the length scale of 8 nm, the static correlation length, indicating the nanosecond counterion dynamics mimics the polymer. These experiments, performed with semidilute solutions of tetramethylammonium poly(styrene sulfonate) [(h-TMA(+)) d-PSS], apply to relevant biopolymers including single and double stranded DNA and unfolded proteins, which undergo orchestrated dynamics of counterions and chain segments to fold, unfold, and assemble.

A buckling-based metrology for measuring the elastic moduli of polymeric thin films.

As technology continues towards smaller, thinner and lighter devices, more stringent demands are placed on thin polymer films as diffusion barriers, dielectric coatings, electronic packaging and so on. Therefore, there is a growing need for testing platforms to rapidly determine the mechanical properties of thin polymer films and coatings. We introduce here an elegant, efficient measurement method that yields the elastic moduli of nanoscale polymer films in a rapid and quantitative manner without the need for expensive equipment or material-specific modelling. The technique exploits a buckling instability that occurs in bilayers consisting of a stiff, thin film coated onto a relatively soft, thick substrate. Using the spacing of these highly periodic wrinkles, we calculate the film's elastic modulus by applying well-established buckling mechanics. We successfully apply this new measurement platform to several systems displaying a wide range of thicknessess (nanometre to micrometre) and moduli (MPa to GPa).

Biocompatibility of sorbitol-containing polyesters. Part I: Synthesis, surface analysis and cell response in vitro.

A series of sorbitol-containing polyesters were synthesized via a one-pot lipase-catalyzed condensation polymerization. Thin films were prepared by spin coating on silicon wafers and surfaces were analyzed by tapping mode atomic force microscopy and contact angle measurements. Surface morphologies and surface energies across the series of polyester films, including a poly(epsilon-caprolactone) (PCL) control were nearly indistinguishable. Biocompatibility of the sorbitol-containing polyester series was evaluated against a PCL control by measuring cell spreading and proliferation of a mouse fibroblast 3T3 cell line in vitro. Results confirmed that the sorbitol-containing polyester surfaces elicited cell behavior similar to the PCL control. These results establish the sorbitol-containing polyester series as a promising material for tissue engineering research and development.

High-throughput investigation of osteoblast response to polymer crystallinity: influence of nanometer-scale roughness on proliferation.

A high-throughput method for analyzing cellular response to crystallinity in a polymer material is presented. Variations in crystallinity lead to changes in surface roughness on nanometer length scales, and it is shown that cells are exquisitely sensitive to these changes. Gradients of polymer crystallinity were fabricated on films of poly(L-lactic acid) using a gradient in annealing temperature. The resultant morphologies were characterized using an atomic force microscope. Root-mean-square (rms) roughness values ranging from 0.5 to 13 nm were created on a single sample. MC3T3-E1 osteoblastic cells were cultured for 1, 3 and 5 d, and the number of cells was measured using automated fluorescence microscopy. It is shown that the rate of proliferation on the smooth regions of the films is much greater than that on the rough regions, and a monotonic variation in rate is observed as a function of roughness. The critical rms roughness, above which a statistically significant reduction in rate of proliferation occurs, was approximately 1.1 nm. Fluorescence microscopy measurements on immunostained cells indicate there is no significant change in cell area, the number or type of adhesions formed, or the degree of actin polymerization. Results from enzyme-linked immunofluorescence assays indicated that there was no detectable change in adhesion protein accessibility, suggesting the cells directly respond to substrate topography. The use of the gradient library approach yielded the functional dependence of cell proliferation on nanometer-scale roughness and gave a sensitive estimate of the critical roughness for which a decrease in proliferation is observed.

Combinatorial characterization of cell interactions with polymer surfaces.

We report a novel combinatorial methodology for characterizing the effects of polymer surface features on cell function. Libraries containing hundreds to thousands of distinct chemistries, microstructures, and roughnesses are prepared using composition spread and temperature gradient techniques. The method enables orders of magnitude increases in discovery rate, decreases variance, and allows for the first time high-throughput assays of cell response to physical and chemical surface features. The technique overcomes complex variable spaces that limit development of biomaterial surfaces for control of cell function. This report demonstrates these advantages by investigating the sensitivity of osteoblasts to the chemistry, microstructure, and roughness of poly(D,L-lactide) and poly(epsilon-caprolactone) blends. In particular, we use the phenomenon of heat-induced phase separation in these polymer mixtures to generate libraries with diverse surface features, followed by culture of UMR-106 and MC3T3-E1 osteoblasts on the libraries. Surface features produced at a specific composition and process temperature range were discovered to enhance dramatically alkaline phosphatase expression in both cell lines, not previously observed for osteoblasts on polymer blends.

Co-extrusion of biocompatible polymers for scaffolds with co-continuous morphology.

A methodology for the preparation of porous scaffolds for tissue engineering using co-extrusion is presented. Poly(epsilon-caprolactone) is blended with poly(ethylene oxide) in a twinscrew extruder to form a two-phase material with micron-sized domains. Selective dissolution of the poly(ethylene oxide) with water results in a porous material. A range of blend volume fractions results in co-continuous networks of polymer and void spaces. Annealing studies demonstrate that the characteristic pore size may be increased to larger than 100 microm. The mechanical properties of the scaffolds are characterized by a compressive modulus on the order of 1 MPa at low strains but displaying a marked strain-dependence. The results of osteoblast seeding suggest it is possible to use co-extrusion to prepare polymer scaffolds without the introduction of toxic contaminants. Polymer co-extrusion is amenable to both laboratory- and industrial-scale production of scaffolds for tissue engineering and only requires rheological characterization of the blend components. This method leads to scaffolds that have continuous void space and controlled characteristic length scales without the use of potentially toxic organic solvents.