Hierarchical materials present particular structure or architecture at different length scales. Understanding how the structure and resultant properties couple across length scales is challenging, but we try anyway! Creating materials with controlled structure across length scales allows us to access a wide range of optical, mechanical, electronic and biological properties.
Creating hierarchyThere are many bottom-up and top-down ways of creating a hierarchical system, varying physico-chemical parameters to control structure on many length scales. Some approaches we use:
- controlling crystal growth to achieve a particular structure/phase
- nanofabrication to create and pattern thin films
- organic-inorganic composites to direct crystal habit, protein aggregation, carbohydrate polymerization
- protein or carbohydrate aggregate morphology through process parameters such as pH
- crystal/protein layering for mesoscale composite structure
- standard microfabrication and soft lithography to impose mesoscale structure
Stimulating hierarchical growthWe see indications of hierarchical process as well as structure in our materials preparation methods. In developing methods to create hierarchy, we have come to understand the ways in which different length scales interact. As a result, we can now stimulate structure on one length scale by controlling the structure on another length scale! An example of atomic-scale structure determining mesoscale patterning (essentially epitaxy across several orders of magnitude):
|Protein (collagen) aligns to mineral (mica) crystallographic orientation.|
Sun, M.; Stetco, A.; Merschrod S.; E.F. "Surface-Templated Formation of Protein Microfibril Arrays" Langmuir 24 5418–5421 (2008). DOI: 10.1021/la703292h
Other examples include control of crystal structure with a hierarchically-organized organic matrix (large-scale structure determines small-scale structure), and control of film roughness by controlling fibril morphology (small-scale structure determines large-scale structure).
Measuring hierarchySignatures of hierarchy can show up in many properties, as well as in structure. Some ways that we identify coupling or complexity of structure across length scales:
- XRD: crystal orientation and polycrystalline alignment, not just crystal structure
- NMR: local interactions in non-crystalline materials
- AFM: structure but also mechanical response
- Optical methods: polarized light microscopy, particle tracking
Elasticity at minimum indentation depth
Young's modulus vs. fibril size: indicators of hierarchy/fibril maturity
Applications of hierarchical materialsThere are many applications for hierarchical materials, thanks to their interesting properties. Many of our preparation and analysis methods involve a substrate, so we incorporate that substrate into our materials design (using the surface as another "reagent").
- Surfaces and interfaces are ubiquitous in biological systems. Interfaces provide chemical and
topological cues for:
- cell activation (e.g. immune response),
- tissue growth (e.g. scarring), etc.
- Interfaces can also provide unique properties due to their anisotropy, which we can exploit
for sensors by enhancing both binding and signal transduction:
- cell chips and arrays - tailored topography and chemistry to control cell adhesion
- microfluidic sensors - higher surface area means greater sensitivity
- optical coatings - enhanced signal (e.g. surface-enhanced Raman), suppressed noise (e.g. coatings for DNA microarrays to reduce background fluorescence)
This page is a work in progress, created and maintained by Erika Merschrod.
Copyright © 2017 Erika Merschrod