R&D Profile: Stimuli-Responsive Polymers in bioMEMS Devices: F. Montagne, Swiss Center for Electronics and Microtechnology, CH
Dr Franck Montagne is senior R&D engineer in the department of Nanotechnology and Life Science at the Swiss Center for Electronics and Microtechnology (CSEM), a privately held research and development company which has headquarters in Neuchâtel, Switzerland.
Stimuli-responsive polymers, also referred to as “smart” polymers, are a very interesting class of materials since they exhibit marked and rapid conformational changes in response to external stimuli such as temperature, pH, electric field or ionic strength. When coated onto a surface, they confer to the resulting materials some unique properties and offer the possibility to achieve control over biocompatibility, wettability or permeability, for instance. Due to their outstanding properties, stimuli-responsive polymers have been attracting considerable attention in biotechnologies and successful applications have already been demonstrated in sensing, intelligent textiles, bio-separation and drug delivery systems. Integration of smart polymers in the fabrication process or in the post-modification of Micro/Nano Electro-Mechanical Systems (MEMS/NEMS) is considered to be a real cornerstone since it allows the introduction of new functionalities to micron/nanoscale silicon-based devices.
In order to illustrate CSEM’s activities in this field, we presented at Nanotech 2008 in Boston two examples of real case application of stimuli-responsive polymers in bioMEMS devices. First, we described the fabrication of silicon chips whose surface is modified with poly(N-isopropylacrylamide) (PNIPAM), a thermally-responsive polymer exhibiting a lower critical solubility temperature (LCST) at about 32°C in pure water. Below the LCST, PNIPAM chains are hydrated and form expanded structures in water, whereas they are dehydrated and collapse when temperature is raised above the LCST. In the latter case, it is known that cells adhere and even proliferate on PNIPAM surface, whereas they are progressively released when the temperature is decreased below the LCST. The reversibility of this process was demonstrated in-house with mouse fibroblast 3T3. Based on these results, we produced small silicon chips presenting a pattern of PNIPAM micro-domains of tuneable sizes, typically ranging from 10 μm to 200 μm, for the reversible capture and release of individual cells. These intelligent chips are now integrated into a fully automated cell injection platform equipped with micromanipulator and vision system for high throughput cell transfection.
As a second example, we reported the wafer-scale fabrication of free-standing nanoporous silicon-based membrane having thickness < 100 nm and pore size of about 35 nm (see SEM pictures). These membranes are produced using a CSEM proprietary process combining block copolymer lithography and standard microfabrication techniques. Here, the use of responsive block copolymers permits to precisely control the size and size distribution of the nanopores (typically from few nm to few tens of nm), as well as the final thickness of the membrane. These membranes show remarkable mechanical properties since they withstand a differential pressure of a few bars. The use of these nanoporous membranes for highly selective filtration of biological species and for sensing is currently evaluated at CSEM. First results already show great promises and indicate that nanoporous membranes made from block copolymers self-assembly clearly surpass existing membrane technology (track etched and ultrafiltration membranes) in terms of selectivity and separation rate.