(29f) Micro and Nano Scale 3D Structure of Biomaterials and Their Relationship to Material Properties

Authors: 
Ramaswamy, S., University of Minnesota
Ramanna, S., U of Minnesota
Ramarao, B., SUNY ESF

Biomaterials such as paper, board, bio-based composites and pigmented paper coatings have complex internal structures. Internal structures of porous materials have a significant influence on the efficiency of the manufacturing process, the properties of the materials and their end-use applications. With the advent of novel materials characterization techniques it is now possible to non-intrusively visualize the internal three dimensional structure of porous materials and attempt to better understand structure property functional relationships. X-ray micro computed tomography (X-μCT) is a technique that can be used to non-intrusively visualize micron and sub-micron scale structure of materials. Both absorptive contrast and phase contrast imaging and image reconstruction techniques can be used to obtain the 3D structure.  The basic principle of 3D computed tomography is based on capturing the images of the sample by scanning at different angles and reconstructing the three dimensional image using image transformation methodologies.

Using X-ray micro CT three dimensional images of porous biomaterials, we have developed computational methods to characterize the structural features including material porosity, fiber-pore interfacial area, structural tortuosity, 3D pore size distribution, pore-fiber network characteristics such as node density distribution, bond length distribution, and morphological maps of the structure such as pore and bond skeletons. In addition, we have used the actual 3D micro structure of porous materials to predict their liquid and vapor transport properties.  Using random walk simulations in 3D structures, we were able to predict the anisotropic simultaneous pore-fiber diffusion characteristics of fibrous porous media. In addition to transport properties, using cell wall mechanical properties and actual 3D structures and a meshed finite element analysis, we continue to develop methods to predict the elastic and viscoelastic mechanical properties of the network structure. Using this approach, effect of fiber properties and processing conditions on material properties can be better discerned.

Even though micron scale structural details may be sufficient in many applications of biomaterials, in order to better understand the ultrastructure of plant cell walls and their role in biomass conversion processes, it is necessary to probe the ultra-structural features at the nanoscale. 3D computed tomography technique mentioned above is now being extended to other imaging techniques such as Transmission Electron Microsopy (TEM).  The spatial resolution of the 3D TEM is reported to be of the order of 5 nm. Using TEM nano computed tomography (TEM-nCT), we have been exploring the ultrastructure of biomass cell walls and attempting to characterize their structural features.  This technique has immense potential in not only probing the nano scale ultrastructure but also superimposing topo chemical distributions using correlative imaging. These advanced materials characterization tools will enable us to truly “engineer” material structures tailored for specific end-use applications.