We integrated ultra-porous (99% porous) elements composed of nanoporous forests of vertically aligned carbon nanotubes (VACNTs) in MEMS, demonstrating their use in microfluidic applications for bioparticle isolation and health diagnostics.  Distinct from prior works where the effects of fluids on VACNT elements would result in either structural deformation or catastrophic forest collapse ^[1] , the approach here enables creation of high aspect ratio (~ 1-mm in height) nanoporous elements and preserves their shape even under flow-through conditions. An example device, shown in Figure 1, consists of a patterned and (wet) functionalized VACNT forest integrated into a PDMS microfluidic channel.

Compared to state-of-the-art designs that exploit solid materials (e.g., silicon, PDMS) for the structural features, our nanoporous elements enables fluid flow both around and through the VACNT elements, thus enhancing physical interaction between the particles in the flow and the functional elements. A ~7X increase in specific bioparticle capture when transitioning to VACNT porous designs was demonstrated for multiple device layouts ^[1] . The large surface-to-volume ratio of nanoporous materials yields also a significant increase in functional surface area (~250-500X for the layouts analyzed in our works ^{[2] [3]} ), thus further promoting bioparticle capture.

Specific isolation of bioparticles ranging over 4 orders of magnitude in size (from cells to viruses) was experimentally demonstrated (Figure 1), including the ability to perform simultaneous multiphysics, multiscale isolation on a single chip. Particles smaller than the average distance between single nanotubes in the VACNT elements (~80 nm) can penetrate the elements and can be isolated using chemical affinity; simultaneously, particles larger than 80 nm cannot enter the nanoporous elements and can be isolated on the elements’ outer surfaces using both mechanical filtration and biomolecular recognition. The nanoporous elements are versatile and could provide access to underexplored sub-micron particles (e.g., proteins, exosomes).

1. D. N. Futaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura, and S. Iijima. “Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as super-capacitor electrodes,” Nature Materials, vol. 5, pp. 987-994, 2006.
2. G. D. Chen, F. Fachin, M. Fernandez-Suarez, B. L. Wardle, and M. Toner, “Nanoporous elements in microfluidics for multiscale nanipulation of bioparticles,” Small, vol. 7, pp. 1061-1067, 2011.
3. F. Fachin, G. D. Chen, M. Toner, and B. L. Wardle, “Integration of vertically-aligned carbon nanotube forests in microfluidic devices for multiscale isolation of bioparticles,” Proc. IEEE Sensors 2010, Kona, HI, pp. 47-51.

Characterization of thin film layered materials is critical for many MEMS devices. Residual stresses from production determine both final shape and performance of microdevices and should therefore be accurately determined. Stresses are typically extracted using simple test structures (clamped beams and cantilevers, see Figures 1a-b) that allow for mean and gradient residual stress estimation ^[1] . However, current approaches to material characterization have two major limitations. First, their accuracy is directly proportional to their cost. This is especially true for mean compressive stress, where more accurate estimates require a larger number of different test structures. Second, they oversimplify test-structure boundary conditions by considering them to be ideal (e.g., perfectly clamped in the case of buckled beams for mean compressive stress determination ^[1] ). To overcome these issues, we have developed a new methodology for characterizing the complete stress state (effective mean and gradient stresses) in CMOS layered materials that also assesses non-ideality of clamped boundaries ^[2] . The approach uses a closed-form solution of the postbuckling problem of micromachined beams including non-ideal boundaries (Figure 1). In Table 1 we show the results relative to the characterization of four different CMOS material combinations. The outcomes show mean compressive stresses ranging between -15 and -105MPa, thus demonstrating the method’s capability to characterize structures subjected to both large and small compressive stresses. This capability is in contrast with traditional critical length methods that encounter difficulties in quantifying small compressive stresses due to their inability to distinguish between mean stress and gradient stress effects ^[2] . For the CMOS materials examined here, the accuracy was ± 2MPa for mean stresses and ±3MPa/µm for gradients. Boundary non-ideality is found to be 90% of perfectly clamped for the CMOS-released films, having such a significant effect on the extracted stresses that it must be considered.

1. M. J. Madou, Fundamentals of Microfabrication: The Science of Miniaturization, 2nd Edition, CRC Press, New York, 1997.
2. F. Fachin, M. Varghese, S. A. Nikles, and B. L. Wardle, “Characterization of the complete stress state in thin-film CMOS layered materials,” in Proc. Hilton Head, Hilton Head, SC, pp. 312-315.