In the system proposed by the Cambridge-Bath team, laser heating of gold, polymer-covered 60-nm spheres drives off water from the polymer, causing the dispersed spheres to collapse into a central cluster and storing elastic energy. Subsequent cooling and reabsorption of water by the polymer releases the stored elastic energy. The resulting light-activated “nano-piston” could, according to the team, serve as an actuator to drive nanomachines. [Image: Yi Ju/University of Cambridge NanoPhotonics]
Engineers have dreamed up many a scheme for nanomachines that would deliver drugs to diseased tissues, perform environmental sensing, and generally go to work where the rest of us can’t. But building efficient nano-engines to power such diminutive servants has proved a bit of a stumbling block. A group of scientists at the University of Cambridge and the University of Bath, U.K., has now devised a nano-transducer, built of gold and temperature-responsive polymers and actuated by laser light, that they argue could deliver a “step change” in the performance of nanodevices (Proc. Natl. Acad. Sci. USA, doi: 10.1073/pnas.1524209113).
Questions of force and speed
The biological world affords many examples of molecular motors, for systems such as bacterial flagella, cilia, and cargo transport along microtubules within the cell, that researchers have attempted to press into service for use in nanomachines. But such biological motors tend to operate relatively slowly, and often deliver forces only on the order of piconewtons. Moreover, efforts to engineer molecular motors in the lab have often ended up in schemes that are awkward to actuate and control, delicate, and expensive to fabricate—disadvantages in many envisioned applications for nanomachines, which require nano-engines that are biocompatible, robust, cheap to manufacture and fast in operation.
The Cambridge-Bath researchers, led by OSA Fellow Jeremy Baumberg of Cambridge’s Cavendish Laboratory, believe they’ve found a way around these quandaries in a device they call an actuating nanotransducer (ANT). The transducer consists of a colloidal collection of 60-nm-diameter gold spheres, each one coated with an amino-terminated, temperature responsive polymer, poly(N-isopropylacrylamide), or pNIPAM.
Spring-loaded ANTs
An important characteristic of pNIPAM—and one at the heart of the Cambridge-Bath team’s new nanotransducer—is that, as it rises above a critical temperature of 32 °C, it rapidly expels water, in a reaction that reverses as the temperature falls below the critical temperature. That reversible reaction offered a route to a nano-engine very different from the molecular motors of biological systems, and that could be activated by laser pulses.
The team found that, when the polymer-coated nanospheres are “cold” (below the critical temperature), the pNIPAM shell is filled with water, and the spheres bounce off of one another in the colloidal solution. But when laser light is applied to the system, plasmonic effects at the gold sphere surfaces increase the temperature of the spheres above the critical temperature, rapidly driving off the water from the polymer.
With the polymer in the collapsed state, the van der Waals attractions between the gold cores take over, and the system collapses into a tight cluster much smaller in volume—storing a great deal of elastic energy within the polymer in the process, like a coiled spring. When the laser light is removed and the spheres cool to below the critical temperature, the polymer quickly reabsorbs water, the van der Waals forces are broken, and the stored elastic energy is released.
A powerful “nano-piston”
The whole process takes place on microsecond timescales—and is likened by one of the study’s coauthors, OSA Member Ventsislav Valev, to using light “to power a piston engine at the nanoscale.” That piston packs a substantial amount of force; the team’s measurements suggest that the system can provide nanoscale forces of several nanonewtons—“a force per unit weight,” according to the scientists, “nearly a hundred times better than any motor or muscle.” The research team also stresses that the material is biocompatible, should be easy and cheap to manufacture, and is potentially quite energy efficient.
Team leader Baumberg acknowledges that there’s still some work to be done in adapting the system for actual machinery, and the paper particularly highlights the need to configure the scheme for applying directional, rather than isotropic, forces. But team is optimistic that the general system can “underpin a plethora of new designs” in the micromachine realm. According to a press release, the scientists are now working with Cambridge Enterprise, the tech-transfer arm of Cambridge University, as well as with several other companies to find first-stage biological/microfluidic applications.