Biological materials naturally display an astonishing variety of sophisticated nanostructures that are difficult to obtain even with the most technologically advanced synthetic methodologies. Inspired from nature materials with hierarchical structures, many functional materials are developed based on the templating synthesis method. This review will introduce the way to fabricate novel functional materials based on nature bio-structures with a great diversity of morphologies, in State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University in near five years. We focused on replicating the morphological characteristics and the functionality of a biological species (e.g. wood, agriculture castoff, butterfly wings). We change their original components into our desired materials with original morphologies faithfully kept. Properties of the obtained materials are studied in details. Based on these results, we discuss the possibility of using these materials in photonic control, solar cells, electromagnetic shielding, energy harvesting, and gas sensitive devices, et al. In addition, the fabrication method could be applied to other nature substrate template and inorganic systems that could eventually lead to the production of optical, magnetic. or electric devices or components as building blocks for nanoelectronic, magnetic, or photonic integrated systems. These bioinspired functional materials with improved performance characteristics are becoming increasing important, which will have great values on the development on structural function materials in the near future.

The National Aeronautics and Space Administration (NASA) is currently evaluating composite materials for primary and secondary structures. Composite materials are of great interest to NASA due to the potential significant mass savings they would bring, resulting in more efficient deep space exploration. A principal concern is that of lack of damage tolerance, where nearly imperceptible cracks can have drastic effects on structural integrity. To compensate, composites structures are currently designed to be thicker and heavier than would otherwise be required, thus negating the weight savings they promise. Self-healing composites could help improve damage tolerance and in addition can reduce the complexity of composite maintenance while increasing materials performance lifetime and improving reliability. Current work at the Johnson Space Center investigates the use of carbon nanotubes in a thermally activated re-polymerizing matrix in order to develop a self-healing composite.
Composite panels consisting of carbon nanotubes, carbon fibers, and a thermally reversible polymer resin were fabricated and tested. These multifunctional composites exhibit improvements in the thermal, electrical, and mechanical properties. Nanotube additions influence thermal properties through a decrease in the glass transition and increases in the heat capacity, thermal expansion coefficient, and thermal conductivity of the composite. The electrical conductivity is drastically lowered, while storage modulus and toughness are increased. Carbon nanotubes are known to exhibit enhanced microwave heating, and this property is exploited for the heat generation required for healing of the matrix. Incorporation of nanotubes results in better heating efficiency and thermal transport, allowing the composite to be uniformly heated to the target temperature for self healing. Furthermore, microwave irradiation of impacted composites demonstrates the ability to provide visual inspection of the damaged area. Microcracks produced by impaction generate localized heating which are clearly evident in the thermal images of microwave irradiated composites. The ability to use microwaves to observe barely visual damage provides the capacity for damage detection and real-time health monitoring of the healing process. Thus, carbon nanotubes embedded in a thermally reversible polymer matrix can result in a multifunctional nanocomposite with healing and health monitoring capability.

The use of biological materials as templates for functional molecular assemblies is an active research field at the interface between chemistry, biology, and materials science. We demonstrate formation of gold nanofiber films on beta-sheet peptide domains assembled at the air/water interface. The gold deposition scheme employed a recently-discovered versatile chemical process involving spontaneous crystallization and reduction of water-soluble gold-thiocyanate complex upon anchoring to surface-displayed amine moieties (Morag et al, Advanced Functional Materials, 2013, DOI: 10.1002/adfm.201300881). We show that an interlinked network of crystalline Au nanofibers is readily formed upon incubation of the Au(III) thiocyanate complex with the peptide monolayers. Intriguingly, the resultant films were optically transparent, enabled electrical conductivity, and displayed pronounced surface enhanced Raman spectroscopy (SERS) activity, making the approach a promising avenue for construction of nano-structured films exhibiting varied practical applications and functionalities.

In this study, biodegradable nanofoams were produced using cellulose nanofibrils (CNFs) and starch (S). The availability of high volumes of CNFs at lower costs is rapidly progressing with advances in pilot-scale and commercial facilities. The composite foams were produced using a freeze-drying process with CNF/S water suspensions ranging from 1 to 7.5 wt. % solids content. Microscopic evaluation showed that the foams have microcellular structure includes foam walls covered with CNF`s. Microscopic evaluation of the nanofibrils and pore sizes at the foam walls showed no difference among the composites (CNF reinforced starch foams), with average values from 20 nm to 100 nm for pore size diameters and 30 nm to 100 nm for fibril diameters. The physical, mechanical and thermal properties of the CNF/S composite foams were determined according to ASTM standards. The resulting CNF/S composite foam were determined to have an open-cell structure with densities ranging from 0.012 to 0.082 g/cm3 with corresponding porosities between 93.46% and 99.10%. Thermal conductivity ranged from 0.041 to 0.054 W/m-K. Changes in the solids content and CNF/S ratio had little impact on thermal conductivity of the foams. The mechanical performance of the foams produced from the unreinforced starch control was extremely low and the material was very friable. The addition of CNF to starch produced foams, which exhibited structural integrity. The 1.5% CNF-6% starch combination showed significantly better mechanical properties (average of 2530 kPa modulus of elasticity, 6590 kPa modulus of rupture, 907 kPa compression modulus and 3330 kPa compressive strength) when compared with the other composite formulations. The mechanical properties of materials increased with an increase in solids content and CNF/S ratios.

Synthetic nanocomposites using carbon nanostructures such as carbon nanotubes, graphene, or graphene oxide as the load-bearing units within a polymer matrix are promising structural materials due to their potential for high strength and stiffness. However, the mechanical properties of these macroscale composites are presently limited by many types of flaws and relatively weak interfaces, preventing them from achieving high strengths comparable to their parent nanostructures. Because these carbon-based nanofillers comprise of particles that are non-uniform in composition, structures, and morphologies, and their distribution within a composite are difficult to control, we choose to fabricate nanocomposites that are flaw-tolerant, rather than flawless. In this talk, we describe the fabrication, mechanical measurements, and structure-property relationships of multilayer graphene oxide-polymer composite films with alternating soft, crack-hindering polymer layers and stiff, load-bearing graphene oxide layers. This structure is inspired by that of sponge spicules[1], where periodic lamellae of soft protein and hard mineral hinder crack propagation, increasing strength and toughness[2,3]. We find multilayer films fabricated in this fashion have double the strength of pure graphene oxide films without sacrificing stiffness, and that employing an optimal polymer layer thickness can maximize both stiffness and strength. Finite-element analysis of the multilayer film reveals the stress redistribution that occurs around a broken graphene oxide layer, providing insights into the statistical nature of the strengths of these multilayer films. This work was supported by the ARO MURI (Award # W991NF-09-1-0541).
1. Woesz, A., et al., J. Mater. Res. 2006, 21, 2068-2078.
2. Fratzl, P., et al., Adv. Mater. 2007, 19, 2657-2661.
3. Kolednik, O., et al., Adv. Funct. Mater. 2011, 21, 3634-3641.