We demonstrate a new method for joining thermoplastic surfaces to produce microfluidic devices. The method takes advantage of the sharply defined permeation boundary of case-II diffusion to generate dimensionally controlled, activated bonding layers at the surfaces being joined. The technique is capable of producing bonds that exhibit cohesive failure, while preserving the fidelity of fine features in the bonding interface. This approach is uniquely suited to production of layered microfluidic structures, as it allows the bond-forming interface between plastic parts to be precisely manipulated at micrometre length scales. Distortions in microfluidic device channels are limited to the size scale of the permeant-swollen layer; 6 microm deep channels are routinely produced with no detectable cross-sectional distortions. Conventional thermal diffusion bonding of identical parts yields less strongly bonded microfluidic structures with increasingly severe dimensional compressions as bonding temperatures approach the thermoplastic glass-transition temperature: a preliminary rheological analysis is consistent with the observed compressions. The bond-enhancing procedure is easily integrated in standard process flows, uses inexpensive reagents, and requires no specialized equipment.
The interaction of water with a common commercial glass cloth silane finish is explored by neutron reflection. The silane coating is applied to the oxide surfaces of polished silicon wafers. Detailed profiles of D20 within the -80 A silane finish layers are measured after exposure for 48 hours to a saturated D2O atmosphere at either 22°C or 80°C. The nature of the interaction of D 2 0 with the finish layer is probed by exposing the samples to vacuum following adsorption. In both samples, the profile of adsorbed D20 is composed of at least two distinct layers: a thin ( < 30 A) D20 -rich layer adjacent to the interface, and the bulk of the film in which only a low level of DzO is present. The amount of adsorbed D20 is greater for the sample conditioned at 80°C than for the sample conditioned at 22°C. In addition, adsorbed D20 within the interfacial layer is removed more slowly during evacuation for the sample conditioned at 80°C than for the sample conditioned at 22°C. These latter two results are interpreted as indicating increased hydrolysis of siloxane bonds for the samples conditioned at 80°C. Surprisingly, after several months in vacuum the remaining D2O redistributes within the layer, accumulating within a very thin layer a t the interface in both samples. The nature of this redistribution is not understood.
Host molecules have been designed and synthesized to selectively complex and lipophilize guest molecules. Examples of the use of the following binding interactions are given: hydrogen bonding, ion pairing, cation to n-electrons, carbonyl to n-electrons and pi-pi bonding. Multiheteromacrocycles have been prepared whose association constants with tert-butylammonium salts in chloroform range from <50 to 106 M . Host molecules with built-in counterions have been prepared that selectively complex and lipophilize metal and alkylammonium cations. Locations of complementary binding sites and noncomplementary steric barriers provide for selective binding by host molecules of candidate guest molecules. Locations of appropriate chiral barriers and multiple complexing sites in guest compounds have led to the complete optical resolution of host compounds by optically active amino acids, and of amino acid esters by optically active host compounds. Ratios of association constants for diastereomeric complexes in excess of ten have been obtained. A molecular basis for designing an amino acid resolving machine has been developed.Central to nature's enzyme, transport and regulatory systems are highly structured molecular complexes. Large host molecules bind smaller guest molecules, and the chemical and physical properties of each are vastly altered. Nature's complexes are characterized by a high degree of structure, very high rates of formation and decomposition, and mutual structural recognition of host and guest. In enzymic catalysis, the rate-limiting transition state energies are lowered by complexation and orientation. In transport mechanisms, selection and lipophilization of polar entities are frequently accomplished by complexation. In regulatory systems, competitive complexation between inhibitor and substrate for sites of host molecules control *
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