A living, breathing lung-on-a-chip has been developed that can mimic the boundary between the lung's air sacs and its capillaries.
It's at this boundary that inhaled pathogens and potentially harmful nanoparticles pass into the bloodstream. Reproducing those processes on a chip could one day provide an alternative to animal testing for drug development and toxicity screening.
The coin-sized lung-on-a-chip consists of a simple network of microfluidic channels etched into a rubbery, transparent polymer called polydimethylsiloxane (PDMS). The central channel contains two layers of human cells, separated by a porous membrane (see image).
In the upper layer the cells come from alveoli, the cavities deep inside the lung where gases pass between the lungs and the bloodstream. The lower layer contains endothelium cells from the capillaries that carry oxygen-rich blood away.
As well as mimicking the cellular structure of the lung, the chip copies its behaviour too: it can "breathe". As air pressure in two channels flanking the main channel is periodically reduced and increased, the central membrane is widened, stretching the cells as it does to, before they contract once more as the pressure is increased, says Donald Ingber, director of the Wyss Institute for Biologically Inspired Engineering at Harvard University, and leader of the lung-on-a-chip team.
Because the device is transparent, it's possible to make real-time measurements of the inflammatory response that occurs when pathogens or silica nanoparticles are introduced into the airflow chamber. The measurements are made using high-resolution fluorescence microscopy. The extent to which these particles pass into the simulated bloodstream can also be recorded, Ingber says.
These measurements show that the "breathing" mechanism appears to encourage the uptake of silica nanoparticles – a result that the team found also occurs when they introduced the same nanoparticles into a mouse lung connected to a ventilator.
The fact that the lung-on-a-chip behaves so much like the real mouse lung is an encouraging sign that ethically acceptable and cheaper alternatives to animal testing may be on the way. Cell-culture techniques, which are also being investigated as an option, cannot take into account important mechanical influences that help regulate the organs, such as the stretching of lung tissue caused by breathing. "This is something that has been missing from almost all in vitro models," Ingber says.
Anthony Holmes, of the UK National Centre for the Replacement, Refinement and Reduction of Animals in Research in London, agrees. "There's lot of evidence that the normal functions of organs require certain physical stimulations," he says. The lungs are one example but it applies equally to bone, cartilage and other tissues. "It's a nice model and an interesting approach."
"It's wonderful that it breathes, and definitely a step in the right direction," says Kelly BéruBé, a cell biologist at Cardiff University, UK, who acts as scientific adviser to the UK's Safer Medicines Trust. But she warns that the immortalised cell lines used in the lung-on-a-chip tend not to have the same properties as "primary" cells taken from patients. "Unless they can get primary cells, they are not going to be able to replace animal tests."