Creation of microscale flow circuits using fluid walls for cell biology

Developing new drugs or drug-delivery systems involves coordinated study of several variables. These include precise analysis of toxicity, cellular internalization of drug molecules, and many others. Despite strict regulatory requirements and the decreasing number of new drugs approved annually, adverse effects of new drugs still remains significant. The gold standard for preclinical trials is animal in vivo studies, but inter-species differences limit their reliability in reflecting human responses. Against this background, it is important to complement these in vivo approaches with in vitro ones.

In traditional cultures, cells are isolated from organs and grown in a favourable environment that should resemble the native one. Culturing is usually performed in glass or plastic vessels where cells grow as two-dimensional adherent monolayers. This method benefits from simplicity and low-cost, but it does not mimic the three-dimensional (3D) structures of native tissues. Nowadays, the two main methods for 3D culture involve hydrogel matrices and growth of self-assembled structures (e.g., spheroids) in vessels that prevent cells adhering to the substrate. Results obtained from such 3D cultures were immediately promising. However, the lack of supply of nutrients and poor gas exchange often results in necrotic cores that develop within ~200 of microns from the surface.

Microfluidics is the branch of physics and engineering that enables the control and manipulation of small volumes of fluids within confined environments with dimensions of tens to hundreds of micrometres. At such a scale, fluid flows are not turbulent, and so can be modelled more simply and predictably. These characteristics made microfluidics suitable for cell culture where perfusion rates, stresses, and solute and gas diffusion can be finely tuned to recreate, in vitro, conditions more reflecting those found in vivo. Despite promising results obtained with microfluidic devices by engineers, uptake by biomedical scientists has been poor. One of the main reasons given for this is the use of solid plastic walls to confine fluids which prevents bioscientists from directly accessing cells growing in the devices with their pipets.

Fluids at the microscale, unlike rivers at the macroscale, do not require solid confining banks; they can be confined by other fluids. A counter-intuitive approach, called 'freestyle fluidics', exploits this realization. When volumes are in the order of millilitre or smaller, fluid behaviour is governed by interfacial forces (or surface tensions) as the effects of gravity and inertial forces become negligible. Then, an interface between two immiscible liquids can act as a confining wall (much as an air/water interface confines rain drops on a window pane). In 'freestyle fluidics', chambers and conduits made of cell media are bounded by an overlaid immiscible fluorocarbon (FC40).

The main goal of this research is to develop a microfluidic platform for in vitro 3D cell culture using freestyle fluidics. The conventional equations governing flows in microfluidic circuits generally involve invariant boundary conditions. Uniquely fluid-walled conduits morph in response to flow induced by any pressure change. Consequently, the challenge is to develop new sets of governing equations that include the flexibility of the walls in a conduit and changes in cross sectional area. A second step will include the analysis of the pressures developed inside microenvironments with different shapes, and to exploit these results to model and develop a passive pumping system (i.e., one that does not use external powered pumps) able to automatically displace liquids from a source to a sink.

This project falls within the EPSRC Engineering Microsystem research area and it is partially funded by iotaSciences Ltd.