Christopher C. Striemer

Charge- and size-based separation of macromolecules using ultrathin silicon membranes

Commercial ultrafiltration and dialysis membranes have broad pore size distributions and are over 1,000 times thicker than the molecules they are designed to separate, leading to poor size cut-off properties, filtrate loss within the membranes, and low transport rates. Nanofabricated membranes have great potential in molecular separation applications by offering more precise structural control, yet transport is also limited by micrometre-scale thicknesses. This limitation can be addressed by a new class of ultrathin nanostructured membranes where the membrane is roughly as thick (∼10 nm) as the molecules being separated, but membrane fragility and complex fabrication have prevented the use of ultrathin membranes for molecular separations. Here we report the development of an ultrathin porous nanocrystalline silicon (pnc-Si) membrane using straightforward silicon fabrication techniques that provide control over average pore sizes from approximately 5 nm to 25 nm. Our pnc-Si membranes can retain proteins while permitting the transport of small molecules at rates an order of magnitude faster than existing materials, separate differently sized proteins under physiological conditions, and separate similarly sized molecules carrying different charges. Despite being only 15 nm thick, pnc-Si membranes that are free-standing over 40,000 μm2 can support a full atmosphere of differential pressure without plastic deformation or fracture. By providing efficient, low-loss macromolecule separations, pnc-Si membranes are expected to enable a variety of new devices, including membrane-based chromatography systems and both analytical and preparative microfluidic systems that require highly efficient separations.

Dynamic etching of silicon for broadband antireflection applications

An electrochemical etching technique has been developed that provides continuous control over the porosity of a porous silicon layer as a function of etching depth. Thin films with engineered porosity gradients, and thus a controllable gradient in the index of refraction, have been used to demonstrate broadband antireflection properties on silicon wafer and solar cell substrates. A simulation was also developed to examine the effects of specific porosity profiles on film reflectivity.

Quantitative analysis of the sensitivity of porous silicon optical biosensors

The optical response of porous silicon affinity biosensors depends strongly on their nanomorphology because the sensing species does not completely fill the pores but is instead attached to the pore walls. The performance and sensitivity of porous silicon microcavity biosensors are calculated as a function of pore size using a simplified effective medium approximation. Excellent agreement is obtained between the model predictions and the experimental results after binding of aminopropyltriethoxysilane and glutaraldehyde in mesoporous and macroporous silicon microcavities. Detection of layers thinner than 0.01nm should be possible.

High-Performance Separation of Nanoparticles with Ultrathin Porous Nanocrystalline Silicon Membranes

Porous nanocrystalline silicon (pnc-Si) is a 15 nm thin free-standing membrane material with applications in small-scale separations, biosensors, cell culture, and lab-on-a-chip devices. Pnc-Si has already been shown to exhibit high permeability to diffusing species and selectivity based on molecular size or charge. In this report, we characterize properties of pnc-Si in pressurized flows. We compare results to long-standing theories for transport through short pores using actual pore distributions obtained directly from electron micrographs. The measured water permeability is in agreement with theory over a wide range of pore sizes and porosities and orders of magnitude higher than those exhibited by commercial ultrafiltration and experimental carbon nanotube membranes. We also show that pnc-Si membranes can be used in dead-end filtration to fractionate gold nanoparticles and protein size ladders with better than 5 nm resolution, insignificant sample loss, and little dilution of the filtrate. These performance characteristics, combined with scalable manufacturing, make pnc-Si filtration a straightforward solution to many nanoparticle and biological separation problems.

Ion-Selective Permeability of an Ultrathin Nanoporous Silicon Membrane as Probed by Scanning Electrochemical Microscopy Using Micropipet-Supported ITIES Tips

We report on the application of scanning electrochemical microscopy (SECM) to the measurement of the ion-selective permeability of porous nanocrystalline silicon membrane as a new type of nanoporous material with potential applications in analytical, biomedical, and biotechnology device development. The reliable measurement of high permeability in the molecularly thin nanoporous membrane to various ions is important for greater understanding of its structure-permeability relationship and also for its successful applications. In this work, this challenging measurement is enabled by introducing two novel features into amperometric SECM tips based on the micropipet-supported interface between two immiscible electrolyte solutions (ITIES) to reveal the important ion-transport properties of the ultrathin nanopore membrane. The tip of a conventional heat-pulled micropipet is milled using the focused ion beam (FIB) technique to be smoother, better aligned, and subsequently, approach closer to the membrane surface, which allows for more precise and accurate permeability measurement. The high membrane permeability to small monovalent ions is determined using FIB-milled micropipet tips to establish a theoretical formula for the membrane permeability that is controlled by free ion diffusion across water-filled nanopores. Moreover, the ITIES tips are rendered selective for larger polyions with biomedical importance, i.e., polyanionic pentasaccharide Arixtra and polycationic peptide protamine, to yield the membrane permeability that is lower than the corresponding diffusion-limited permeability. The hindered transport of the respective polyions is unequivocally ascribed to electrostatic and steric repulsions from the wall of the nanopores, i.e., the charge and size effects.

Porous nanocrystalline silicon membranes as highly permeable and molecularly thin substrates for cell culture.

Porous nanocrystalline silicon (pnc-Si) is new type of silicon nanomaterial with potential uses in lab-on-a-chip devices, cell culture, and tissue engineering. The pnc-Si material is a 15 nm thick, freestanding, nanoporous membrane made with scalable silicon manufacturing. Because pnc-Si membranes are approximately 1000 times thinner than any polymeric membrane, their permeability to small solutes is orders-of-magnitude greater than conventional membranes. As cell culture substrates, pnc-Si membranes can overcome the shortcomings of membranes used in commercial transwell devices and enable new devices for the control of cellular microenvironments. The current study investigates the feasibility of pnc-Si as a cell culture substrate by measuring cell adhesion, morphology, growth and viability on pnc-Si compared to conventional culture substrates. Results for immortalized fibroblasts and primary vascular endothelial cells are highly similar on pnc-Si, polystyrene and glass. Significantly, pnc-Si dissolves in cell culture media over several days without cytotoxic effects and stability is tunable by modifying the density of a superficial oxide. The results establish pnc-Si as a viable substrate for cell culture and a degradable biomaterial. Pnc-Si membranes should find use in the study of molecular transport through cell monolayers, in studies of cell-cell communication, and as biodegradable scaffolds for three-dimensional tissue constructs.

High-performance, low-voltage electroosmotic pumps with molecularly thin silicon nanomembranes

Significance Electroosmotic pumps (EOPs) are a class of pumps in which fluid is driven through a capillary or porous media within an electric field. Current research on EOPs concerns the development of new materials in which high electroosmotic flow rates can be achieved for low voltages. Such pumps could be used for portable microfluidic devices. Porous nanocrystalline silicon (pnc-Si) is a material that is formed into a 15-nm-thick nanomembrane. pnc-Si membranes are shown here to have high electroosmotic flow rates at low applied voltages due to the high electric fields achieved over the ultrathin membrane. A prototype EOP was designed using pnc-Si membranes and shown to pressurize fluid through capillary tubing at voltages as low as 250 mV. We have developed electroosmotic pumps (EOPs) fabricated from 15-nm-thick porous nanocrystalline silicon (pnc-Si) membranes. Ultrathin pnc-Si membranes enable high electroosmotic flow per unit voltage. We demonstrate that electroosmosis theory compares well with the observed pnc-Si flow rates. We attribute the high flow rates to high electrical fields present across the 15-nm span of the membrane. Surface modifications, such as plasma oxidation or silanization, can influence the electroosmotic flow rates through pnc-Si membranes by alteration of the zeta potential of the material. A prototype EOP that uses pnc-Si membranes and Ag/AgCl electrodes was shown to pump microliter per minute-range flow through a 0.5-mm-diameter capillary tubing with as low as 250 mV of applied voltage. This silicon-based platform enables straightforward integration of low-voltage, on-chip EOPs into portable microfluidic devices with low back pressures.

Methods for controlling the pore properties of ultra-thin nanocrystalline silicon membranes

Porous nanocrystalline silicon (pnc-Si) membranes are a new class of solid-state ultra-thin membranes with promising applications ranging from biological separations to use as a platform for electron imaging and spectroscopy. Because the thickness of the membrane is only 15-30 nm, on the order of that of the molecules to be separated, mass transport through the membrane is greatly enhanced. For applications involving molecular separations, it is crucial that the membrane is highly permeable to some species while being nearly impermeable to others. An important approach to adjusting the permeability of a membrane is by changing the size and density of the pores. With pnc-Si, a rapid thermal treatment is used to induce nanopore formation in a thin film of nanocrystalline silicon, which is then released over a silicon scaffold using an anisotropic etchant. In this study, we examine the influence of thin film deposition and thermal treatment parameters on pore size and density.

Ultrathin Silicon Membranes for Wearable Dialysis

The development of wearable or implantable technologies that replace center-based hemodialysis (HD) hold promise to improve outcomes and quality of life for patients with ESRD. A prerequisite for these technologies is the development of highly efficient membranes that can achieve high toxin clearance in small-device formats. Here we examine the application of the porous nanocrystalline silicon (pnc-Si) to HD. pnc-Si is a molecularly thin nanoporous membrane material that is orders of magnitude more permeable than conventional HD membranes. Material developments have allowed us to dramatically increase the amount of active membrane available for dialysis on pnc-Si chips. By controlling pore sizes during manufacturing, pnc-Si membranes can be engineered to pass middle-molecular-weight protein toxins while retaining albumin, mimicking the healthy kidney. A microfluidic dialysis device developed with pnc-Si achieves urea clearance rates that confirm that the membrane offers no resistance to urea passage. Finally, surface modifications with thin hydrophilic coatings are shown to block cell and protein adhesion.

Pore size control of ultrathin silicon membranes by rapid thermal carbonization.

Rapid thermal carbonization in a dilute acetylene (C(2)H(2)) atmosphere has been used to chemically modify and precisely tune the pore size of ultrathin porous nanocrystalline silicon (pnc-Si). The magnitude of size reduction was controlled by varying the process temperature and time. Under certain conditions, the carbon coating displayed atomic ordering indicative of graphene layer formation conformal to the pore walls. Initial experiments show that carbonized membranes follow theoretical predictions for hydraulic permeability and retain the precise separation capabilities of untreated membranes.