A Microfluidic Device for Long-Term Maintenance of Organotypic Liver Cultures.

Liver cultures may be used for disease modeling, testing therapies and predicting drug-induced injury. The complexity of the liver cultures has evolved from hepatocyte monocultures to co-cultures with non-parenchymal cells and finally to precision-cut liver slices. The latter culture format retains liver's native biomolecular and cellular complexity and therefore holds considerable promise for in vitro testing. However, liver slices remain functional for ~72 h in vitro and display limited utility for some disease modeling and therapy testing applications that require longer culture times. This paper describes a microfluidic device for longer-term maintenance of functional organotypic liver cultures. Our microfluidic culture system was designed to enable direct injection of liver tissue into a culture chamber through a valve-enabled side port. Liver tissue was embedded in collagen and remained functional for up to 31 days, highlighted by continued production of albumin and urea. These organotypic cultures also expressed several enzymes involved in xenobiotic metabolism. Conversely, matched liver tissue embedded in collagen in a 96-well plate lost its phenotype and function within 3-5 days. The microfluidic organotypic liver cultures described here represent a significant advance in liver cultivation and may be used for future modeling of liver diseases or for individualized liver-directed therapies.

Microfluidic confinement enhances phenotype and function of hepatocyte spheroids.

A number of cell culture approaches have been described for maintenance of primary hepatocytes. Forming hepatocytes into three-dimensional (3-D) spheroids is one well-accepted method for extending epithelial phenotype of these cells. Our laboratory has previously observed enhanced function of two-dimensional (2-D, monolayer) hepatocyte cultures in microfluidic devices due to increased production of several hepato-inductive growth factors, including hepatocyte growth factor (HGF). In the present study, we wanted to test a hypothesis that culturing hepatocyte spheroids (3-D) in microfluidic devices will also result in enhanced phenotype and function. To test this hypothesis, we fabricated devices with small and large volumes. Both types of devices included a microstructured floor containing arrays of pyramidal wells to promote assembly of hepatocytes into spheroids with individual diameters of ~100 µm. The hepatocyte spheroids were found to be more functional, as evidenced by higher level of albumin synthesis, bile acid production, and hepatic enzyme expression, in low-volume compared with large-volume devices. Importantly, high functionality of spheroid cultures correlated with elevated levels of HGF secretion. Although decay of hepatic function (albumin secretion) was observed over the course 3 wk, this behavior could be abrogated by inhibiting TGF-ß1 signaling. With TGF-ß1 inhibitor, microfluidic hepatocyte spheroid cultures maintained high and stable levels of albumin synthesis over the course of 4 wk. To further highlight utility of this culture platform for liver disease modeling, we carried out alcohol injury experiments in microfluidic devices and tested protective effects of interleukin-22: a potential therapy for alcoholic hepatitis.

Anisotropic permeability in deterministic lateral displacement arrays.

We uncover anisotropic permeability in microfluidic deterministic lateral displacement (DLD) arrays. A DLD array can achieve high-resolution bimodal size-based separation of microparticles, including bioparticles, such as cells. For an application with a given separation size, correct device operation requires that the flow remains at a fixed angle to the obstacle array. We demonstrate via experiments and lattice-Boltzmann simulations that subtle array design features cause anisotropic permeability. Anisotropic permeability indicates the microfluidic array's intrinsic tendency to induce an undesired lateral pressure gradient. This can cause an inclined flow and therefore local changes in the critical separation size. Thus, particle trajectories can become unpredictable and the device useless for the desired separation task. Anisotropy becomes severe for arrays with unequal axial and lateral gaps between obstacle posts and highly asymmetric post shapes. Furthermore, of the two equivalent array layouts employed with the DLD, the rotated-square layout does not display intrinsic anisotropy. We therefore recommend this layout over the easier-to-implement parallelogram layout. We provide additional guidelines for avoiding adverse effects of anisotropy on the DLD.

Microfluidic approaches to synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectral microscopy of living biosystems.

A long-standing desire in biological and biomedical sciences is to be able to probe cellular chemistry as biological processes are happening inside living cells. Synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectral microscopy is a label-free and nondestructive analytical technique that can provide spatiotemporal distributions and relative abundances of biomolecules of a specimen by their characteristic vibrational modes. Despite great progress in recent years, SR-FTIR imaging of living biological systems remains challenging because of the demanding requirements on environmental control and strong infrared absorption of water. To meet this challenge, microfluidic devices have emerged as a method to control the water thickness while providing a hospitable environment to measure cellular processes and responses over many hours or days. This paper will provide an overview of microfluidic device development for SR-FTIR imaging of living biological systems, provide contrast between the various techniques including closed and open-channel designs, and discuss future directions of development within this area. Even as the fundamental science and technological demonstrations develop, other ongoing issues must be addressed; for example, choosing applications whose experimental requirements closely match device capabilities, and developing strategies to efficiently complete the cycle of development. These will require imagination, ingenuity and collaboration.

Open-channel microfluidic membrane device for long-term FT-IR spectromicroscopy of live adherent cells.

Spatially resolved infrared spectroscopy is a label-free and nondestructive analytical technique that can provide spatiotemporal information on functional groups in biomolecules of a sample by their characteristic vibrational modes. One difficulty in performing long-term FT-IR measurements on live cells is the competition between the strong IR absorption from water and the need to supply nutrients and remove waste. In this proof of principle study, we developed an open-channel membrane device that allows long-term continuous IR measurement of live, adherent mammalian cells. Composed of a gold-coated porous membrane between a feeding channel and a viewing chamber, it allows cells to be maintained on the upper membrane surface in a thin layer of fluid while media is replenished from the feeding channel below. Using this device, we monitored the spatiotemporal chemical changes in living colonies of PC12 cells under nerve growth factor (NGF) stimulation for up to 7 days using both conventional globar and high-resolution synchrotron radiation-based IR sources. We identified the primary chemical change cells undergo is an increase in glycogen that may be associated with secretion of glycoprotein to protect the cells from evaporative stress at the air-liquid interface. Analyzing the spectral maps with multivariate methods of hierarchical cluster analysis (HCA) and principal component analysis (PCA), we found that the cells at the boundary of the colony and in a localized region in the center of the colony tend to produce more glycogen and glycoprotein than cells located elsewhere in the colony and that the degree of spatial heterogeneity decreases with time. This method provides a promising approach for long-term live-cell spectromicroscopy on mammalian cell systems.

Cell motility and drug gradients in the emergence of resistance to chemotherapy.

The emergence of resistance to chemotherapy by cancer cells, when combined with metastasis, is the primary driver of mortality in cancer and has proven to be refractory to many efforts. Theory and computer modeling suggest that the rate of emergence of resistance is driven by the strong selective pressure of mutagenic chemotherapy and enhanced by the motility of mutant cells in a chemotherapy gradient to areas of higher drug concentration and lower population competition. To test these models, we constructed a synthetic microecology which superposed a mutagenic doxorubicin gradient across a population of motile, metastatic breast cancer cells (MDA-MB-231). We observed the emergence of MDA-MB-231 cancer cells capable of proliferation at 200 nM doxorubicin in this complex microecology. Individual cell tracking showed both movement of the MDA-MB-231 cancer cells toward higher drug concentrations and proliferation of the cells at the highest doxorubicin concentrations within 72 h, showing the importance of both motility and drug gradients in the emergence of resistance.

Deterministic separation of cancer cells from blood at 10 mL/min.

Circulating tumor cells (CTCs) and circulating clusters of cancer and stromal cells have been identified in the blood of patients with malignant cancer and can be used as a diagnostic for disease severity, assess the efficacy of different treatment strategies and possibly determine the eventual location of metastatic invasions for possible treatment. There is thus a critical need to isolate, propagate and characterize viable CTCs and clusters of cancer cells with their associated stroma cells. Here, we present a microfluidic device for mL/min flow rate, continuous-flow capture of viable CTCs from blood using deterministic lateral displacement (DLD) arrays. We show here that a DLD array device can isolate CTCs from blood with capture efficiency greater than 85% CTCs at volumetric flow rates of up to 10 mL/min with no effect on cell viability.

A microfluidic device for continuous cancer cell culture and passage with hydrodynamic forces.

We demonstrate a novel and robust microfluidic chip with combined functions of continuous culture and output of PC-3 prostate cancer cells. With digital controls, polydimethylsiloxane (PDMS) flexible diaphragms are able to apply hydrodynamic shear forces on cultures, detaching a fraction of attached cancer cells from the surface for output while leaving others for reuse in subsequent cultures. The fractions of detached cells and remaining cells can be precisely controlled. The system has not only the advantages of small size, high cell culture efficiency, and digital control, but also of simple fabrication at low cost, easy operation and robust performance. The chip performs 9 passages during 30 days of continuous culture and shows promise as a durable design suitable for long-term cell output.

Deterministic microfluidic ratchet.

We present a deterministic, nonthermal ratchet where the trajectory of particles in a certain size range is not reversible when the sign of the pressure gradient is reversed at a low Reynolds number. This effect is produced by employing triangular rather than the conventional circular posts in an array that selectively displaces particles transported through the array. The ratchet irreversibly moves particles of a certain size range in a direction orthogonal to an oscillating flow, with no net displacement of the fluid itself. The underlying mechanism of this ratchet is shown to be connected to irreversible particle-post interactions and the asymmetric fluid velocity distribution through the gap between the triangular posts. Diffusion plays no role in this ratchet, and hence the device parameters presented here can be scaled up to high rates of flow, of clear importance in separation technologies.

Crossing microfluidic streamlines to lyse, label and wash cells.

We present a versatile method for continuous-flow, on-chip biological processing of cells, large bio-particles, and functional beads. Using an asymmetric post array in pressure-driven microfluidic flow, we can move particles of interest across multiple, independent chemical streams, enabling sequential chemical operations. With this method, we demonstrate on-chip cell treatments such as labeling and washing, and bacterial lysis and chromosomal extraction. The washing capabilities of this method are particularly valuable because they allow many analytical or treatment procedures to be cascaded on a single device while still effectively isolating their reagents from cross-contamination.

Hydrodynamic metamaterials: microfabricated arrays to steer, refract, and focus streams of biomaterials.

We show that it is possible to direct particles entrained in a fluid along trajectories much like rays of light in classical optics. A microstructured, asymmetric post array forms the core hydrodynamic element and is used as a building block to construct microfluidic metamaterials and to demonstrate refractive, focusing, and dispersive pathways for flowing beads and cells. The core element is based on the concept of deterministic lateral displacement where particles choose different paths through the asymmetric array based on their size: Particles larger than a critical size are displaced laterally at each row by a post and move along the asymmetric axis at an angle to the flow, while smaller particles move along streamline paths. We create compound elements with complex particle handling modes by tiling this core element using multiple transformation operations; we show that particle trajectories can be bent at an interface between two elements and that particles can be focused into hydrodynamic jets by using a single inlet port. Although particles propagate through these elements in a way that strongly resembles light rays propagating through optical elements, there are unique differences in the paths of our particles as compared with photons. The unusual aspects of these modular, microfluidic metamaterials form a rich design toolkit for mixing, separating, and analyzing cells and functional beads on-chip.