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Posted: May 22, 2007

Trying to copy the greatest nanotechnologist of all

(Nanowerk Spotlight) There are quite a number of terms such as bionics, biomimetics, biognosis, biomimicry, or even 'bionical creativity engineering' that refer to more or less the same thing: the application of methods and systems found in nature to the study and design of engineering systems and modern technology. A relatively new entry in this list is 'nanomimetics', an area of biomimetic nanotechnology that tries to duplicate what nature has been doing for billions of years on this planet - creating and manipulating complex nanoscale structures. Nanoscientists and nanotechnology researchers use the terms 'self-assembly' and 'bottom-up fabrication' in their efforts to copy the best nanotechnologist around - Mother Nature. Managing another small step in this direction, researchers now have reported the accurate replication of fragile biological nanoscale shapes normally associated with self-assembly by using a robust top-down lithographic technology. The ability to replicate biological shapes with nanoscale precision could have profound implications in tissue engineering, cell scaffolding, drug delivery, sensors, imaging, and immunology.
We have written a number of Nanowerk Spotlights on this fascinating topic of nanotechnologists playing catch-up with nature: "Nature's bottom-up nanofabrication of armor ", "Nanotechnology inspired by mussels and seashells" or "Algae shells: one example how nanotechnology is trying to copy Mother Nature", to name just a few.
Current micro- and nanofabrication approaches can reproducibly fabricate precise, regular shapes out of a variety of robust materials, but the morphologies are typically limited to shapes such as pillars, lines, pyramids or other geometric shapes. These shapes do not mimic the structural complexity that can be created by self-assembled organic and biological structures such as viruses.
Now, Dr. Joseph M. DeSimone tells us that his research group is able to accurately replicate shapes of naturally occurring nanoscale objects into other materials. DeSimone is a Professor of Chemistry and Chemical Engineering at the University of North Carolina at Chapel Hill as well as Director, NSF Science & Technology Center for Environmentally Responsible Solvents and Processes and Director, Institute for Advanced Materials, Nanoscience and Technology.
"Naturally occurring supramolecular objects, such as proteins, micelles, and viruses, exhibit sophisticated morphological shapes or surface motifs that conventional synthetic and fabrication techniques cannot replicate" DeSimone explains to Nanowerk. "These structures owe their interesting shapes and shape-related properties largely to noncovalent chemical interactions that can produce unique, 'evolutionarily designed' shapes with nanometer precision."
So far, it has been impossible for scientists to replicate nature's self-assembly process when trying to control the complex nanoscale shape of organic or inorganic materials. On one hand, the chemical structure of each component has to be carefully designed and precisely synthesized to ensure that the desired morphology is obtained.
On the other hand, "even if the chemical design of the material is sufficient to promote the desired assembly process, factors such as temperature, solution purity, and kinetic limitations can significantly, and often detrimentally, affect the resulting morphology of bottom-up self-assembled materials" says DeSimone. "Alternatively, traditional micro- and nanofabrication approaches can reproducibly fabricate precise, regular shapes out of a variety of robust materials, but the morphologies are typically limited to geometric shapes and therefore cannot easily mimic the structural complexity that can be created by self-assembled organic and biological structures, such as viruses."
Molding and replication of adenovirus particles
Transmission electron microtomography (TEMT) images depicting molding and replication of adenovirus particles. A) AFM image of an adenovirus master, prepared by depositing adenovirus particles onto a silicon surface; vertical scale=100 nm. B) AFM image of a PFPE mold formed from an adenovirus master; vertical scale=50 nm. C) AFM image of a triacrylate/bisphenol A dimethacryate adenovirus replica; vertical scale=100 nm. D) TEMT reconstruction of a triacrylate/bisphenol A dimethacrylate adenovirus replica. E) Cryo-electron microscopy reconstruction of adenovirus. (Reprinted with permission from Wiley-VCH Verlag)
In their recent work, DeSimone's group reports a nanofabrication method that is able to reproduce shapes normally associated with self-assembly using robust nanoscale replication methods, thereby combining the morphological sophistication of the natural world with the scalable processing technologies associated with lithography.
"We use extremely low surface energy, minimally adhesive fluoroelastomers – photocurable perfluoropolyether, PFPE – to replicate naturally-occurring objects ('master templates')" explains DeSimone. "The naturally-occurring object is replicated in the fluoropolymer by pouring the curable fluoropolymer resin over the master template and photopolymerizing the resin into a flexible 'mold' that transfers the details of the master morphology into the mold. The mold precursor resin will spontaneously spread on almost all substrates found in nature, including organic materials. After spreading on the self-assembled materials to 'mimic' the precise shape of the self-assembled material, the resin is polymerized to capture the shape in the mold."
The mold is then released from the self-assembled master templates and can be used to replicate these shapes into other materials. The fluoropolymer resin is inherently non-interacting with the biological materials, so it does not perturb the fragile self-assembled structure. Therefore, one can get a very accurate replica of the self-assembled master template, in some cases down to below 2 nm in height.
Being able to reproduce the sophisticated shapes of the natural world could open the door to biomedical applications where they potentially can be used in place of for instance a live virus.
As for next steps, DeSimone's group wants to explore the impact of shape on biological recognition.
"For example, it would be a real breakthrough to show that a replicated bio-interface has some of the same recognition properties as a natural bio-interface" says DeSimone. "Also, the ability to use attach chemical ligands (targeting agents, signaling molecules) and to use molecular imprinting in conjunction with shape specific nanoreplication could lead to truly biomimetic materials such as artificial viruses."
These findings have been reported in a recent paper in Small, titled "Supramolecular Nanomimetics: Replication of Micelles, Viruses, and Other Naturally Occurring Nanoscale Objects".
By , Copyright Nanowerk LLC

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