Microscale Patterning of Thermoplastic Polymer Surfaces by Selective Solvent Swelling

A new method for the fabrication of microscale features in thermoplastic substrates is presented. Unlike traditional thermoplastic microfabrication techniques, in which bulk polymer is displaced from the substrate by machining or embossing, a unique process termed orogenic microfabrication has been developed in which selected regions of a thermoplastic surface are raised from the substrate by an irreversible solvent swelling mechanism. The orogenic technique allows thermoplastic surfaces to be patterned using a variety of masking methods, resulting in three-dimensional features that would be difficult to achieve through traditional microfabrication methods. Using cyclic olefin copolymer as a model thermoplastic material, several variations of this process are described to realize growth heights ranging from several nanometers to tens of micrometers, with patterning techniques include direct photoresist masking, patterned UV/ozone surface passivation, elastomeric stamping, and noncontact spotting. Orogenic microfabrication is also demonstrated by direct inkjet printing as a facile photolithography-free masking method for rapid desktop thermoplastic microfabrication

Lack of statistically significant correlation between the percentage of conductance recovery following EDTA treatment and the prior time of exposure to LaCl₃.

<p>Each curve is an independent experiment. The averages of the relative percentage of conductance recovery of different experiments in each group are shown in the inset. The results were grouped and normalized to the values of the “<10 min” group. No statistically significant difference was observed.</p

Dynamics of Ceramide Channels Detected Using a Microfluidic System

<div><p>Ceramide, a proapoptotic sphingolipid, has been shown to form channels, in mitochondrial outer membranes, large enough to translocate proteins. In phospholipid membranes, electrophysiological studies and electron microscopic visualization both report that these channels form in a range of sizes with a modal value of 10 nm in diameter. A hydrogen bonded barrel-like structure consisting of hundreds of ceramide molecules has been proposed for the structure of the channel and this is supported by electrophysiological studies and molecular dynamic simulations. To our knowledge, the mechanical strength and deformability of such a large diameter but extremely thin cylindrical structure has never been reported. Here we present evidence for a reversible mechanical distortion of the cylinder following the addition of La³⁺. A microfluidic system was used to repeatedly lower and then restore the conductance by alternatively perfusing La³⁺ and EDTA. Although aspects of the kinetics of conductance drop and recovery are consistent with a disassembly/diffusion/reassembly model, others are inconsistent with the expected time scale of lateral diffusion of disassembled channel fragments in the membrane. The presence of a residual conductance following La³⁺ treatment and the relationship between the residual conductance and the initial conductance were both indicative of a distortion/recovery process in analogy with a pressure-induced distortion of a flexible cylinder.</p> </div

Working model of a ceramide channel.

<p>(A) A channel composed of forty-eight columns arranged in an antiparallel fashion. (B) One of these columns is shown is some detail. It is composed of 6 ceramides stacked so as to span the apolar portion of the membrane. Hydrogen bonds (green) between the amide linkages connect adjacent ceramides. The two hydroxyl groups at the polar end of each ceramide also hydrogen bond with neighboring groups. (C) At the interface between the ceramide channel and the phospholipid bilayer some curvature is needed to obtain a continuous polar surface. The interfacial phospholipids must curve to meet the channel and the interfacial ceramides must curve outward resulting in a channel with a somewhat hourglass shape.</p

Cyclic changes in ceramide channel conductance following alternative perfusions with La³⁺ (50 µM) and EDTA (50 µM)-containing medium.

<p>Downward arrowheads indicate the start of perfusion with La³⁺ medium whereas upward arrowheads indicate the start of perfusion with EDTA-containing medium. The inset shows the overlap of conductance increases following EDTA treatment for short (upper curve) and long (lower curve) pretreatments with La³⁺. The short treatment was the second La³⁺ treatment in the record and the long was the third treatment.</p

The delay between the delivery of EDTA and the initiation of conductance increase correlates with the time of exposure to La³⁺ prior to EDTA treatment.

<p>Each curve is from an independent experiment. Inset: For each experiment, the delay times between the start of EDTA perfusion and the start of conductance increase were grouped as follows: <10 min, 10–30 min, and >30 min based on the length of time of LaCl₃ treatment and then normalized to the result of the “>30 min” group. The averages ± SD of the relative delay times of the different experiments in each group is shown. The “*” indicates that the “<10 min” group was significantly different from the “>30 min” group at the 95% confidence level.</p

Lanthanum chloride addition increases the transmembrane pressure needed to increase the area of a planar phospholipid membrane.

<p>The data of the pressure/area curve was first collected in the absence of LaCl₃. Then 50 µM LaCl₃ was added to one side of the same membrane and the data was collected again. Finally 50 µM LaCl₃ was added also to the other side of the same membrane and the final data set was collected. For the experiments illustrated, the La³⁺ buffer was used (see Methods). Pressure was applied by increasing the level of the solution on one side of the membrane. Inset: Empirical equations were fit to the data in each curve in the main figure. Using these expressions, the difference in pressure needed to achieve the same membrane area with and without LaCl₃, was calculated. As the subtraction involved the difference between two empirical equations, the full function was plotted in the inset. The results shown are typical of two independent experiments.</p

Correlations between the rates of conductance increases and decreases with channel size.

<p>The initial rate of conductance decrease (nS/min) is proportional to the starting conductance (nS) (A) (r = 0.96) and the calculated initial rate of column loss (columns/min) is proportional to the starting circumference of columns (inset in (A) (r = 0.94)). The initial rate of conductance increase (nS/min) is proportional to the conductance (nS) just before EDTA perfusion (B) (r = 0.98), and the calculated initial rate of column reassembly (columns/min) is proportional to the circumference of columns before EDTA perfusion (inset in (B) (r = 0.96)).</p

There is a linear relationship between the conductance (nS) of a ceramide channel after LaCl₃ treatment and the conductance (nS) before LaCl₃ perfusion (r = 0.94).

<p>The circles are results of treatments with 50 µM LaCl₃ whereas the triangle is an experiment with 500 µM LaCl₃.</p

Theoretical calculations of cross-sectional area changes resulting from the formation of a concave structure with positive and negative radii of curvature.

<p>The perimeter of the structure was maintained constant as the axial ratio was increased while maintaining the absolute value of the ratio of the two curvatures (negative/positive) at 0.5, 1 and 2 as shown. Calculations were performed at the plotted points. The cross-sections shown are the results of calculations and are to scale relative to each other. The “distance from center referred to on the label of the x-axis is the distance from the center of the structure to the center of the radius of positive curvature, as illustrated.</p