Concentration-driven molecular diffusion is a fundamental phenomenon essential for the transport of nutrients in cells, for oxygen exchange in the lungs, and mating of chemicals in industrial reactors and the food industry. Thus, diffusion plays a key role in a variety of disciplines. The concentration-driven diffusive transport is commonly described by Fick’s laws of diffusion. It is most often approximated by the Stokes-Einstein equation, which assumes a rigid solute sphere diffusing in a continuum of solvent at a low Reynolds number and infinite dilution.
Numerous technologies such as controlled- release systems for drug delivery, molecular sieving, and single-molecule detection are now employing micro and nano structures. In these cases of fluid nano-constraint, the prediction of the diffusivity is no longer accurate. Experimental investigations have observed a significant difference between the “measured” diffusion coefficient and the predicted values at increasing confinement. In particular, significant deviations occur as the size of channels or pores approaches the size of the single molecules, increasing the importance of molecular boundary interactions.
This work is related to a novel device and method for rapid, inexpensive, and accurate diffusion testing and diffusivity measurement through multiscale channeled or porous media. The invention uses a silicon nanochanneled membrane for drug delivery (nDSl). The device comes in three embodiments, each with its own advantages.
The first embodiment is composed of two stainless steel bodies, two silicon rubber O-rings, two silicon rubber caps, and two stainless steel screws and nuts. The device bodies are hollow, housing the solvent and solution chamber (approximately 350 μL each). They present a groove that precisely fits the nDSl membrane. A smaller inner groove is also machined to house the sealing O-rings. The membrane is then clamped in between the two bodies, which are pressed together by tightening the screws. The measurement of the amount of molecules diffused during the time is performed by sampling fluids from the sink solution and performing the appropriate concentration measurement for the molecule in analysis.
The second embodiment, developed as a modification of the first, is composed of two stainless steel bodies that house the drug, and the sink reservoirs separated by the nanochannel membrane. The membrane is sealed between the metal bodies through two silicon rubber O-rings. The drug solution reservoir presents a volume of 150 μL, which is capped through a silicon rubber cap. A 4.45-mL sink reservoir is obtained by bonding the lower metal body to a UV macro-cuvette through an UV-curing epoxy resin. This device was designed to allow the measurement of the amount of molecules diffused throughout a membrane or a porous media by means of spectroscopy techniques. In particular, this modification was invented to avoid the fluid sampling during the test. The avoidance of fluid sampling reduced the experimental error, and simplified the measurement protocol.
The third embodiment represents a modification of the above two embodiments for the rapid, inexpensive, agile, and accurate measurement of the diffusivity through multiscale channeled or porous media. The device is composed of PDMS (polydimethylsiloxane) and stainless steel bodies, one silicon rubber O-ring, and two stainless steel screws. The device is simple to operate, can be used with various standard instruments, is inexpensive, and diffusivity measurement is rapid.
This work was done by Arturas Ziemys, Jaskaran Gill, and Alessandro Grattoni of the University of Texas Health Science Center for Johnson Space Center. For further information, contact the JSC Technology Transfer Office at (281) 483-3809. MSC-24821-1