HeLa cells internalize tiny pressure sensor chips. Reprinted from Nature Nanotechnology.

Cell biologists are growing to appreciate the importance of mechanical loads, forces and pressures on biological processes such as development or disease progression. Pressures in particular, however, have been difficult to measure directly or without damaging the cell membrane.

Cell mechanical measurements are one area where tools developed using nanotechnology principles appear poised to make a big impact. To date, though, most nanotechnology devices developed for cell biology applications have only permitted extracellular or invasive types of analyses.

With a new design for an intracellular pressure sensor, José Antonio Plaza's team at the Microelectronic Institute of Barcelona (IMB-CNM) in collaboration with Teresa Suárez's group at the Biological Research Center (CIB) (two Spanish National Research Council research centers) in Spain shrunk a sensor chip down until it was small enough to be internalized by living cells. Semiconductor and microelectronics technology now allows even chips with nanometer-sized structures to be made, which are much smaller than living cells, notes Plaza. “So why not produce chips small enough to be internalized in living cells?” he says.

The 4 μm by 6 μm pressure sensor chip designed by Plaza's team is based on a Fabry-Pérot resonator, which consists of two silicon-based membranes separated by a vacuum gap. These membranes act as parallel reflecting mirrors; external pressure deflects the membranes, changing the size of the vacuum gap and modulating the intensity of reflected light. Confocal laser scanning microscopy is used to illuminate the sensors and read out the intensity of the light that is reflected back.

The sensors can be internalized by HeLa cells by liposome transfection. Because of the highly reflective sensor surface, they are easily detected. The chip volume represented just 0.2% of the total cell volume. Cells containing the sensors divided normally and remained healthy for over a week, and the sensors did not degrade over this time. Some of the sensors remained in vacuoles, which actually worked to the researchers' advantage, as vacuoles insulate the sensor from organelles and cytoskeletal filaments that can induce small forces, and the uniform refractive index of the vacuole environment produces better confocal microscopy images.

The researchers used the sensor chip to monitor pressure changes in response to osmotic shock. Though osmotic shock is predicted to increase the hydrostatic pressure, the sensors did not measure substantial pressure changes. Such pressure changes might be too small to measure with the current sensor design, because HeLa cells are able to adapt when subjected to osmotic shock, avoiding a large increase in intracellular pressure.

Currently, the sensors can detect pressures of a few hundred millibars. “Extracellular mechanical loads can be on this order,” notes Plaza, “however, many biological processes will take place with lower pressures.” Plaza's team is working to optimize the sensor design, which will increase sensitivity and expand the biological applicability of the these devices.

They are also exploring how they can apply nanotechnology to develop other types of tiny intracellular sensors. “We think that in the near future, intracellular chips will produce unprecedented knowledge in cell biology,” Plaza says.