Characterization and Neutral Atom Beam Surface Modiﬁcation of a Clear Castable Polyurethane for Biomicroﬂuidic Applications

: Polyurethanes (PU) are a broad class of polymers that offer good solvent compatibility and a wide range of properties that can be used to generate microﬂuidic layers. Here, we report the ﬁrst characterization of a commercially available Shore 80D polyurethane (Ultraclear™ 480N) for biomicroﬂuidic applications. Studies included comparing optical clarity with Polydimethylsiloxane (PDMS) and using high-ﬁdelity replica molding to produce solid PU structures from the millimeter to nanometer scales. Additionally, we report the ﬁrst use of NanoAccel™ treatment in Accelerated Neutral Atom Beam (ANAB) mode to permanently roughen the surface of PU and improve the adhesion of breast cancer cells (MDA-MB-231) on PU. Surface energy measurements using Owens-Wendt equations indicate an increase in polar and total surface energy due to ANAB treatment. Fourier-transform infrared (FTIR) spectroscopy in attenuated total reﬂectance (ATR) mode was used to demonstrate that the treatment does not introduce any new types of functional groups on the surface of Ultraclear™ PU. Finally, applicability in rapid prototyping for biomicroﬂuidics was demonstrated by utilizing a 3D-printing-based replica molding strategy to create PU microﬂuidic layers. These layers were sealed to polystyrene (PS) bases to produce PU-PS microﬂuidic chips. Ultraclear™ PU can serve as a clear and castable alternative to PDMS in biomicroﬂuidic studies.


Introduction
Microfluidics involves the precise manipulation of fluids at submillimeter scales leveraging fabrication technologies developed by the semiconductor and microelectromechanical system (MEMS) industries [1]. A device built using microfluidic principles is commonly referred to as a micro total analysis system (µTAS) [2] or a Lab-on-a-Chip (LoC) [1]. There are certain distinct advantages to conducting experiments in a microfluidic setting rather than on the macroscale [1]. Microfluidic chips need substantially smaller sample volumes that reduce the cost of reagents. They allow simplified analysis of multiple samples in parallel to generate maximum data per batch, while also providing greater control of spatiotemporal fluid dynamics. Additionally, multiple targets can be analyzed on the

Feature Range Characterization
SU-8 (MicroChem Corp., Westborough, MA, USA) patterns on Si wafers were used to create masters. To maintain a rigid-flexible-rigid replica molding strategy, masters with features from the millimeter to the nanometer scale were used to develop flexible PDMS stamps. 1 cm layers of uncured PDMS were poured onto the masters enclosed in 150 mm (diameter) Petri dishes (Fisherbrand™, Thermo Fisher Scientific, Waltham, MA, USA) and cured. Cured PDMS stamps were peeled off the molds and used to generate PU replicas in a similar manner. Finally, PDMS layers were peeled off to leave PU blocks with the same features as the SU-8 patterned Si master.

Scanning Electron Microscopy
PU samples were cut into 2 cm wide squares for Scanning Electron Microscopy (SEM). PU samples were sputtered with Au/Pd (60:40) in a Denton Vacuum Desk IV ® (Denton Vacuum, LLC, Moorestown, NJ, USA) using 30 mA for 75 s to avoid excessive charging, and then mounted with carbon tape. SEM images were collected using a LEO 1550 (Carl Zeiss Microscopy, LLC, Thornwood, NY, USA) at accelerating voltages between 1 kV-3 kV and magnifications between 125 X-25 kX.

Surface Modification by Corona Treatment
5 mm layers of PU were cured in 60 mm (diameter) Petri dishes. A BD-20AC Laboratory Corona Treater (Electro-Technic Products, Inc., Chicago, IL, USA) with a field effect electrode was used to treat each sample for 60 s.

Surface Modification by NanoAccel™ Treatment in ANAB Mode
5 mm layers of PU were cured in 60 mm (diameter) Petri dishes. Briefly, large clusters (~1000-5000 atoms/cluster) of argon gas were ionized and accelerated to 30 keV in a vacuum (with base pressure of 6.5E-7 Torr). By promoting cluster dissociation and deflecting charged cluster-fragments away, a beam of neutral argon atoms impinged on the PU surfaces with average kinetic energies in the Surfaces 2019, 2, 9 3 of 16 10-100 eV range. To ensure that all samples were subjected to the same vacuum and the parameter to be measured was affected by ANAB treatment alone, untreated samples were also placed in the NanoAccel™ tool with the beam blocked by a Ni mask.

Water Contact Angle Measurement
Water contact angle was measured for three different surfaces-Untreated PU, ANAB-treated PU, and corona-treated PU. 5 mm layers of PU were cured in 60 mm (diameter) Petri dishes. Cured samples were cut into 20 mm wide squares. A Cam-Plus Micro contact angle meter (ChemInstruments, Fairfield, OH, USA) was used to measure the water contact angle for 2 µL drops at ten randomly chosen spots across each surface. Drops were allowed to stabilize on the surface for 90 s before measurement of contact angle by the Half-Angle method. Three independent experiments were conducted with measurement of ten drops for each sample.

Atomic Force Microscopy
Images for ANAB-treated PU and untreated PU were taken using a Dimension Icon (Bruker, Billerica, MA, USA) in tapping mode for three randomly chosen spots on each sample. 10 × 10 µm 2 and 5 × 5 µm 2 images were captured and RMS roughness (Rq) values were recorded. 2D isotropic power spectral density plots were generated using NanoScope Analysis 1.8 (Bruker).

Surface Energy Estimation
Surface energy of untreated PU and ANAB-treated PU was estimated by the Owens-Wendt method [40][41][42][43] using three test liquids-water, formamide, and diiodomethane. Surface free energy parameters were taken from a PU study by Krol et al. and are listed in Table S1 [44]. Contact angle measurement was conducted as in Section 2.7. Average contact angle values were used to calculate the polar and dispersive parts of surface energy from the Owens-Wendt equations. Three independent experiments were conducted with measurement of ten drops for each sample.

Fourier-Transform Infrared Spectroscopy in Attenuated Total Reflectance Mode
Attentuated total reflectance-Fourier-transform Infrared (ATR-FTIR) spectroscopy was used for chemical surface characterization of untreated PU and ANAB-treated PU. A Tensor 27 (Bruker, Billerica, MA, USA) with a PIKE MIRacle™ ATR accessory (PIKE Technologies, Madison, WI, USA) and a ZnSe crystal was used to collect spectra between 520-4000 cm −1 at a resolution of 4 cm −1 . Each spectrum collected was an average of 128 scans. PU samples were 2 mm thick and clamped down to the crystal using the accessory. Baseline correction was performed in SpectraGryph 1.2. Live-dead staining by 0.4% Trypan Blue Dye (Bio-Rad, Hercules, CA, USA) exclusion was used to quantify the percentage of cell death after 24 h. Cells were monitored over the next 3 days and passaged to verify trypsinization on PU surfaces. Cells on sample sets were imaged before and after passaging to visualize differences in cell adhesion. Four runs were conducted in duplicate.

Figure 1.
Comparison of the optical transmittance of PDMS and PU over the UV-Vis-NIR range. PU samples were prepared with two gel times (10 minutes for 480N-10 and 60 minutes for 480N-60). Both PU samples absorb in the UV range and transmit in the Vis-NIR range. The transmittance for all samples is similar above 380 nm. Thick lines represent averages of samples for each polymer. Thin lines above and below each thick line of the same color represent upper and lower first standard deviations for each polymer's dataset. Figure 2 illustrates the replica molding strategy used to fabricate solid Ultraclear™ PU structures. Rigid-flexible-rigid steps allowed easy separation of layers. Xia et al. [32] have previously demonstrated high-fidelity replication of Au masters to UV-curable PU replicas using intermediate PDMS stamps. Ultraclear™ PU performs similarly in high-fidelity replica molding. In general, PUs are known to yellow under prolonged UV irradiation [48]. The photodegradation mechanism for aromatic PUs is suspected to take place via a quinonoid route [49]. However, Surfaces 2019, 2, 9 5 of 16 manufacturers use proprietary additives to prevent UV degradation of PUs. Ultraclear™ PU is an example of a UV-resistant PU that absorbs completely in the UV range. PDMS is also known to degrade under UV radiation without protective additives [50,51].

SU-8 patterned
The spectra in Figure 1 are consistent with softer PU formulations (shore A), as characterized by Domansky et al. [6]. Ultraclear™ PU's similarity in optical transmittance with PDMS at higher wavelengths is useful as these wavelengths are relevant for most biological assays. PU 480N-10 samples gelled quickly and needed immediate degassing under a strong vacuum. This resulted in slight inconsistencies while preparing such samples. Since the optical properties of both PU formulations were identical, PU 480N-60 was chosen for all further experiments to ease sample preparation. Figure 2 illustrates the replica molding strategy used to fabricate solid Ultraclear™ PU structures. Rigid-flexible-rigid steps allowed easy separation of layers. Xia et al. [32] have previously demonstrated high-fidelity replication of Au masters to UV-curable PU replicas using intermediate PDMS stamps. Ultraclear™ PU performs similarly in high-fidelity replica molding.  Figure 2 illustrates the replica molding strategy used to fabricate solid Ultraclear™ PU structures. Rigid-flexible-rigid steps allowed easy separation of layers. Xia et al. [32] have previously demonstrated high-fidelity replication of Au masters to UV-curable PU replicas using intermediate PDMS stamps. Ultraclear™ PU performs similarly in high-fidelity replica molding. SEM images in Figure 3 depict identical replication of Si-based masters using intermediate PDMS stamps in the 130 nm-1.5 mm range. This demonstrates that Ultraclear™ PU can be used to make structures across the entire breadth of feature sizes used in microfluidics and can serve as an alternative to PDMS. Additionally, unlike PDMS, PU microfluidic channels are not prone to sagging [9,52]. Cell-based microfluidic chips have much larger feature sizes than chemistry-based microfluidic chips. In fact, microfluidic channels in cell-based chips can often be up to 1 mm wide [52,53]. A wide variety of shapes were chosen for characterizing the feature range achievable to encompass large microfluidic reservoirs and channels ( Figure 3A,B) and smaller features to help with sorting and alignment ( Figure 3C,F).

Microfluidic range achievable
Zhang et al. [33] have previously demonstrated that UV-curable PUs, being stiffer than PDMS, allow fabrication of structures with aspect ratios up to 12 without being susceptible to ground and lateral collapse. While a similar trend is expected for Ultraclear™ PU, its applicability in cell-based microfluidics with respect to feature sizes is adequately depicted by the moderate aspect ratio features in Figure 3.
Surfaces 2019, 2 106 SEM images in Figure 3 depict identical replication of Si-based masters using intermediate PDMS stamps in the 130 nm-1.5 mm range. This demonstrates that Ultraclear™ PU can be used to make structures across the entire breadth of feature sizes used in microfluidics and can serve as an alternative to PDMS. Additionally, unlike PDMS, PU microfluidic channels are not prone to sagging [9,52]. Cell-based microfluidic chips have much larger feature sizes than chemistry-based microfluidic chips. In fact, microfluidic channels in cell-based chips can often be up to 1 mm wide [52,53]. A wide variety of shapes were chosen for characterizing the feature range achievable to encompass large microfluidic reservoirs and channels ( Figure 3A and 3B) and smaller features to help with sorting and alignment ( Figure 3C through 3F).
Zhang et al. [33] have previously demonstrated that UV-curable PUs, being stiffer than PDMS, allow fabrication of structures with aspect ratios up to 12 without being susceptible to ground and lateral collapse. While a similar trend is expected for Ultraclear™ PU, its applicability in cell-based microfluidics with respect to feature sizes is adequately depicted by the moderate aspect ratio features in Figure 3.

Hydrophilic Surface Modification
Unlike shore A PUs [6], Ultraclear™ PU was found to be hydrophobic after curing. To demonstrate applicability in cell-based microfluidics, we utilized two methods to make Ultraclear™ PU hydrophilic-corona treatment and ANAB treatment. The water contact angle for untreated PU samples was 106 • . Figure S1 depicts the temporary hydrophilic gain and eventual hydrophobic recovery of PU samples after corona treatment. The contact angle decreased to 34.6 • immediately after treatment. As expected, the samples experienced hydrophobic recovery and the contact angle rose back to 100.4 • after 24 h.
While a temporary reduction of the contact angle can be useful for bonding steps in chip assembly, permanent reduction is desirable for cell adhesion and growth. Since cell adhesion is favored on surfaces with moderate hydrophilicity [52,54], permanent surface modification was needed to make Ultraclear™ PU suitable for biomicrofluidic applications. NanoAccel™ treatment [36,37] in ANAB mode was used to permanently modify the surface. Figure 4A depicts a permanent increase in hydrophilicity after ANAB treatment (samples tested up to six months later). An increase in beam flux gradually decreased the water contact angle. Figure 4B,C are representative AFM images for untreated and ANAB-treated PU samples, respectively. The average Rq value for surface roughness increased from 2.03 nm for untreated PU samples to 12.5 nm for ANAB-treated samples. This corroborates the data shown in Figure 4A as rougher surfaces (on the scale of ANAB treatment results) tend to have lower water contact angles. For the representative images shown in Figure 4B,C, 2D isotropic power spectral density plots were generated, as shown in Figure S2. chips. Fidelity of the replica molding procedure was high as features in PU were identical in size to those in the SU-patterned Si wafers. A) A 1.5 X 0.2 mm cross. Scale bar: 200 µm. B) Inverse of a microfluidic channel with a 0.5 mm inlet (diameter) and 0.25 mm wide channel. Scale bar: 100 µm. C) A staggered array of perpendicular bars (80 x 5 x 20 µm). Scale bar: 20 µm. D) Zoomed in image of the bars in C). Scale bar: 10 µm. E) An inline array of 10 µm wells (diameter). Scale bar: 10 µm. F) 130 nm wide parallel lines. Scale bar: 1 µm.

Hydrophilic surface modification
Unlike shore A PUs [6], Ultraclear™ PU was found to be hydrophobic after curing. To demonstrate applicability in cell-based microfluidics, we utilized two methods to make Ultraclear™ PU hydrophilic-corona treatment and ANAB treatment. The water contact angle for untreated PU samples was 106°. Figure S1 depicts the temporary hydrophilic gain and eventual hydrophobic recovery of PU samples after corona treatment. The contact angle decreased to 34.6° immediately after treatment. As expected, the samples experienced hydrophobic recovery and the contact angle rose back to 100.4° after 24 hours.
While a temporary reduction of the contact angle can be useful for bonding steps in chip assembly, permanent reduction is desirable for cell adhesion and growth. Since cell adhesion is favored on surfaces with moderate hydrophilicity [52,54], permanent surface modification was needed to make Ultraclear™ PU suitable for biomicrofluidic applications. NanoAccel™ treatment [36,37] in ANAB mode was used to permanently modify the surface. Figure 4A depicts a permanent increase in hydrophilicity after ANAB treatment (samples tested up to six months later). An increase in beam flux gradually decreased the water contact angle. Figure  4B and 4C are representative AFM images for untreated and ANAB-treated PU samples, respectively. The average Rq value for surface roughness increased from 2.03 nm for untreated PU samples to 12.5 nm for ANAB-treated samples. This corroborates the data shown in Figure 4A as rougher surfaces (on the scale of ANAB treatment results) tend to have lower water contact angles. For the representative images shown in Figure 4B and 4C, 2D isotropic power spectral density plots were generated, as shown in Figure S2.  The Owens-Wendt method is commonly used to estimate solid surface energy for polymers by assuming it to be made up of two interactions-polar and dispersive. While the dispersive component accounts for van der Waals and non-site specific interactions, the polar component accounts for dipole-dipole, hydrogen bonding, and site specific interactions [42].
To confirm an increase in hydrophilic character as shown by water contact angle data ( Figure 4A), we estimated the surface energy of Ultraclear™ PU using the Owens-Wendt equations. Table 1 contains values for surface energy calculated from contact angle data for three liquids (water, formamide and diiodomethane). As shown, the polar component of surface energy for PU samples increased with treatment flux. However, the dispersive component remained largely unchanged. Thus, the total surface energy of the PU samples increased. This indicates an increase in hydrophilic character. Testing liquids were chosen to include a range of polar surface free energy parameters (water: High; formamide: Moderate; and diiodomethane: Low). As expected, contact angles for treated samples decreased sharply for water, moderately for formamide and gradually for diiodomethane. Similar to the trend in data from Figure 4A, the effect of ANAB treatment on surface energy of Ultraclear™ PU tends to flatten out beyond an optimal flux value of 2.5 × 10 16 atoms/cm 2 (X). Finally, to investigate chemical bonding group changes due to ANAB treatment, ATR-FTIR spectra of Ultraclear™ PU samples were collected. The stacked spectra in Figure 5A clearly demonstrate that ANAB treatment did not introduce any new types of functional groups on the PU surfaces. Peaks in the 1700 cm −1 region, 2900 cm −1 region and 3400 cm −1 region were identified as C=O, CH 2 and NH groups, respectively [55,56]. The peak at 2270 cm −1 was identified as N=C=O from the H12MDI monomer [55,57]. While all the spectra are similar in Figure 5A, the NH group influence gets slightly stronger with increasing treatment flux. Figure 5B shows the NH group peaks in the 3180-3590 cm −1 range and the uptick in their absorbance. NH groups directly affect the availability for polar interactions.
Qualitatively, ruling out the presence of new types of functional groups due to ANAB treatment of the PU surface is straightforward. However, we must note some concerns with quantifying NH influence from the ATR-FTIR spectra. ATR-FTIR data are dependent, among other parameters, on the smoothness of the samples used, the pressure applied by the clamp to force the sample against the crystal and the aggressiveness of postprocessing spectra with baseline correction. Despite these limitations, we notice an uptick in NH peaks with treatment flux. At the same time, surface energy measurements using two-parameter (polar and dispersive) models like the Owens-Wendt equations are unavoidably dependent on the accuracy of the contact angles measured (manufacturer reported accuracy of 0.8 • ) and the surface free energy parameters chosen for the testing liquids. Thus, the trends in our data from surface energy calculations and ATR-FTIR spectra coupled with the increase in surface roughness from Figure 4B,C seem to agree even though they are collected from independent methods.
Unlike other methods that introduce OH group influence on the surface and add to the polar and total surface energy of PU samples [40], NanoAccel™ treatment of PU adds to the polar component of surface energy by increasing NH group influence on the surface. We attribute this to an increased roughness due to ANAB treatment. This leads to increased surface area and consequently more NH Qualitatively, ruling out the presence of new types of functional groups due to ANAB treatment of the PU surface is straightforward. However, we must note some concerns with quantifying NH influence from the ATR-FTIR spectra. ATR-FTIR data are dependent, among other parameters, on the smoothness of the samples used, the pressure applied by the clamp to force the sample against the crystal and the aggressiveness of postprocessing spectra with baseline correction. Despite these limitations, we notice an uptick in NH peaks with treatment flux. At the same time, surface energy measurements using two-parameter (polar and dispersive) models like the Owens-Wendt equations are unavoidably dependent on the accuracy of the contact angles measured (manufacturer reported accuracy of 0.8°) and the surface free energy parameters chosen for the testing liquids. Thus, the trends in our data from surface energy calculations and ATR-FTIR spectra coupled with the increase in surface roughness from Figure 4B and 4C seem to agree even though they are collected from independent methods.
Unlike other methods that introduce OH group influence on the surface and add to the polar and total surface energy of PU samples [40], NanoAccel™ treatment of PU adds to the polar component of surface energy by increasing NH group influence on the surface. We attribute this to an increased roughness due to ANAB treatment. This leads to increased surface area and consequently more NH groups on the surface available for polar interactions. The overall effect is an increase in polar surface energy (and thus, total surface energy).

Cell Viability
To demonstrate the applicability of Ultraclear™ PU in biomicrofluidics, cell viability was assessed. MDA-MB-231 cells were used because of their adhesive behavior and abundant use in

Cell Viability
To demonstrate the applicability of Ultraclear™ PU in biomicrofluidics, cell viability was assessed. MDA-MB-231 cells were used because of their adhesive behavior and abundant use in microfluidics [58][59][60][61]. The contact angle for polystyrene used in tissue culture ranges between 55.8 • to 63.5 • [62]. To tailor Ultraclear™ PU's surface for moderate hydrophilicity [54] and optimal cell adhesion, a beam flux of 2.5 × 10 16 atoms/cm 2 (X) was chosen to analyze cell viability by measuring percentage cell death using Trypan Blue staining. As shown in Figure 6, ANAB-treated samples displayed significantly reduced cell death after 24 h when compared to untreated PU. MDA-MB-231 cell morphology was also consistent with morphology when grown on traditionally used polystyrene.
Upon reaching confluence, cells were passaged using trypsin. Figure S3 demonstrates the persistence of a similar trend in cell adhesion after passaging, indicating that ANAB treatment led to a sustained improvement in cell adhesion properties of Ultraclear™ PU. displayed significantly reduced cell death after 24 hours when compared to untreated PU. MDA-MB-231 cell morphology was also consistent with morphology when grown on traditionally used polystyrene.
Upon reaching confluence, cells were passaged using trypsin. Figure S3 demonstrates the persistence of a similar trend in cell adhesion after passaging, indicating that ANAB treatment led to a sustained improvement in cell adhesion properties of Ultraclear™ PU.

Chip Fabrication
3D-printing-based replica molding strategies are useful for rapid prototyping and simple production of microfluidic layers. However, the prints generated are rough, and when compared to traditional microfluidics, necessitate the use of adhesive-assisted bonding for reliable sealing.
As shown in Figure 7, we used a simple 3D-printing-based replica molding strategy to fabricate PU microfluidic layers. Microfluidic channels in traditional cell-based chips can often be up to 1 mm wide [52,53]. With the Form 1+ 3D printer, we were able to reliably print microfluidic features well under this range with the smallest channel widths down to 250 µm. Taking advantage of the relatively large feature sizes, we were able to avoid stamping methods for adhesive bonding [45,46].

Chip Fabrication
3D-printing-based replica molding strategies are useful for rapid prototyping and simple production of microfluidic layers. However, the prints generated are rough, and when compared to traditional microfluidics, necessitate the use of adhesive-assisted bonding for reliable sealing.
As shown in Figure 7, we used a simple 3D-printing-based replica molding strategy to fabricate PU microfluidic layers. Microfluidic channels in traditional cell-based chips can often be up to 1 mm wide [52,53]. With the Form 1+ 3D printer, we were able to reliably print microfluidic features well under this range with the smallest channel widths down to 250 µm. Taking advantage of the relatively large feature sizes, we were able to avoid stamping methods for adhesive bonding [45,46].
To demonstrate the applicability of Ultraclear™ PU across cell-based microfluidics, we used PS as the bottom layer for our chips bonded to a microfluidic PU layer using NOA-63. As shown in Figure 8, two versions of PU-PS chips were fabricated to showcase the rapid prototyping potential of this method-a smaller chip with a single reservoir encompassed by a sacrificial channel and a larger chip with three interconnected reservoirs. A sacrificial channel [47] was used on the smaller chip to prevent clogging the microfluidic channels with the adhesive. The larger chip did not need a sacrificial channel.
Overall, our chip fabrication method completely avoids the disadvantages of PDMS by using PU, eliminates the use of stamping methods during assembly, eliminates the need to use photolithography by keeping feature sizes attainable via 3D-printing-based replica molding, and incorporates the most logical material for cell-based microfluidics (PS) as a bottom layer.

Conclusions
A declining interest in using PDMS for microfluidics has led to the characterization of novel materials for rapid prototyping. This trend has continued in biomicrofluidic research. PU-based polymers are often used in biomicrofluidics because they do not absorb hydrophobic molecules and have better solvent compatibility and stiffness than PDMS. We report the first characterization of a commercially available Shore 80D PU for biomicrofluidic applications.

Conclusions
A declining interest in using PDMS for microfluidics has led to the characterization of novel materials for rapid prototyping. This trend has continued in biomicrofluidic research. PU-based polymers are often used in biomicrofluidics because they do not absorb hydrophobic molecules and have better solvent compatibility and stiffness than PDMS. We report the first characterization of a commercially available Shore 80D PU for biomicrofluidic applications.
Ultraclear™ PU has an optical transmittance similar to PDMS in the Vis-NIR range. It can be used reliably, with replica molding strategies, to fabricate solid structures across the breadth of the microfluidic range. Unlike other PUs, Ultraclear™ PU is hydrophobic after curing. Corona treatment causes a temporary gain in hydrophilicity. To demonstrate applicability for biomicrofluidic studies, we report the first use of NanoAccel™ neutral atom beam surface modification of PU surfaces to permanently roughen the surface of Ultraclear™ PU and reduce its water contact angle. Surface energy measurements using Owens-Wendt equations demonstrate an increase in polar surface energy with increasing treatment flux. ATR-FTIR spectra prove that no new functional groups are introduced on the surface due to treatment. The improved surface roughness and hydrophilic behavior also favors MDA-MB-231 cell adhesion. Lastly, to demonstrate applicability in rapid prototyping, a 3D-printing-based replica molding strategy is utilized to create PU microfluidic layers that are sealed to PS using adhesive bonding. As a proof of concept, two versions of PU-PS chips were made. Overall, we demonstrate that Ultraclear™ PU is a clear, castable alternative to PDMS for use in rapid prototyping and biomicrofluidics. Future directions include testing the potential of NanoAccel™ treatment to pattern Ultraclear™ PU surfaces with specific hydrophilic and hydrophobic regions and incorporating such strategies into microfluidic chip usage.
Supplementary Materials: The following are available online at http://www.mdpi.com/2571-9637/2/1/9/s1. Figure S1: Corona treatment of Ultraclear™ PU results in a temporary gain of hydrophilicity and eventual hydrophobic recovery, Figure S2: Power spectral density analysis for representative AFM images of Ultraclear™ PU with and without NanoAccel™ treatment in ANAB mode, Table S1: Parameters used to estimate surface energy by the Owens-Wendt method, Figure S3: Improved cell adhesion of ANAB-treated PU persists even after passaging cells.  (Figures 7 and 8D). S.S. helped image samples to corroborate surface roughness data ( Figure 4B,C). N.T., J.K. and S.P.R. helped design experiments to increase PU hydrophilicity using NanoAccel™ treatment in ANAB mode ( Figure 4A). N.C. and N.T. helped structure the manuscript. J.A.M. and J.C. supervised the project.
Funding: This research was funded by SUNY Polytechnic Institute, Albany, NY.