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Posted: Dec 13, 2007

Light-controlled smart material exploits billions of years of evolutionary performance tuning

(Nanowerk Spotlight) Can you imagine living in an environment with temperatures as high as 120 degrees C - an environment so toxic that it would peel the flesh from your body or cause instant asphyxiation? A horrifying prospect indeed for human habit, yet, these are the typical conditions that the hardy protein, bacteriorhodopsin, calls home. Most proteins, chemical compounds that provide basic units of function in living entities, require physiological conditions to survive and function – conditions that can be equated to a breezy summer’s day in sharp contrast to the fiery hell-storm that bacteriorhodopsin can withstand. This unique durability of the protein is due to the genetic evolution of tiny microbes called Archaea, in which the protein is found. These tiny microbes, also called extremophiles, have learnt to live in extremely harsh conditions such as hot sulfur springs, salt marshes and oil wells. Millions of years of living in such conditions have made bacteriorhodopsin, a distant cousin of the delicate rhodopsin protein found in the human eye, the Spartanesque molecule it is today. The typical function of this light-sensitive protein is to provide chemical energy to these microbes—known as Halobacteria in scientific lingo. When nutrients get scarce, it is bacteriorhodopsin that kicks in to provide the life-saving electric spark. Specifically, in response to light bacteriorhodopsin "pumps" protons across the membrane, transporting charged ions into and out of the microbe. In this way, bacteriorhodopsin is a protein powerhouse that turns on in times of famine, changing color from purple to yellow as it absorbs light. It is this very perfection achieved by billions of years of evolution that draws scientists, who have realized that the protein’s functional performance is better than anything human-designed materials could ever hope to achieve. Scientists have started applying bacteriorhodopsin to numerous branches of science ranging from biochemisty to optics. This research could result in numerous applications, ranging from clinical drug-delivery technologies, nanofluidic valves, cell culture nanotopography, to even stealth surfaces.
Past studies of photo-responsive proteins have generally dwelled on applications for energy-saving computer displays, light-based computing or computer memory. The general theme in the research tends to rely on the controllable and quick state change of the protein. But, the ability of the protein to actually incite a physical change in a composite macroscopic system has been largely unexplored.
Harnessing this ability of the protein to incite change in a polymeric material could have broad implications in the field of biomedical and materials engineering. Getting bacteriorhodopsin to "play ball" with other bulk materials could render those materials sensitive to a light. As a stimulus, light is often much easier to control than those typically explored by researchers such as bulk pH, temperature or electric fields.
To truly leverage the power of bacteriorhodopsin it needs to be tied in to a material platform that can then be fabricated at the requisite length scale of application, whether that be nano, micro or beyond.
In our recent work ("e-Beam Nanopatterned Photoresponsive Bacteriorhodopsin-Containing Hydrogels"), we combine polymer chemistry with the photochemistry of bacteriorhodopsin. Using light as a stimulus, we are then able to controllably shrink and swell polymer structures on the order of 200 nm, a size thousands of times smaller than the thickness of a human hair.
carbon nanopipette probing a carcinoma cell
Tapping-mode AFM images obtained at room temperature in water at pH 9. Samples were placed in a fluid cell and imaged before (DARK) and after (LIGHT) illumination by a 500 mW and 514 nm light source (where light source was turned on as the scan was initiated). Hydrogel dot arrays (4 x 4) were formed by EBL using (Image: Saaem and Tian/Duke University)
What is shown in the above figure is the response of a polyelectrolyte, a type of ionic polymer, to the local pH change that is being created by bacteriorhodopsin when we hit it with green light. The system is almost plug-and-play as you can swap out the commonly found bacteriorhodopsin that we used with genetically engineered mutants responsive to different wavelengths of lights. You might even be able to use multiple versions of this protein to have a system be responsive to multiple stumuli.
We want to stress the generic and adaptable nature of the system presented, considering it to be a smart material for use in constructing increasingly complex designs. One can imagine it akin to stainless steel – a building material that today is used everywhere from skyscrapers to cutlery and to even stents implanted in the human heart. Our lab at Duke University looks to develop new bionanotechnologies using the guiding principle of synthetic biology to engineer reusable biologically-inspired parts that can then be used in more and more complicated systems.
Though there could be many applications possible, the most attractive and lucrative ones in the field of biomedical engineering would be clinical drug-delivery technologies, nanofluidic valves, stealth surfaces, and cell culture nanotopography to name a few.
This work could hold milestone implications in the nascent field of biologically-derived smart materials. Further characterization is required to fully understand what is being observed and to realize the full potential of the research. What has been presented in the Advanced Materials paper could potentially be applied to the numerous polyelectrolytes that are commercially available in order to enrich them with tunable properties. Additionally, such systems could provide high utility when used with other proteins or even DNA.
By Dr. Jingdong Tian and Ishtiaq Saaem, Department of Biomedical Engineering & Institute for Genome Sciences and Policy, Duke University. Copyright Nanowerk LLC

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