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Ultra-robust GO-CNC/GO-SF functional bio-nanocomposites

Nanocomposites in particular are widely used under heavy loading circumstances owing to their favorable mechanical properties. For example, graphene–based nanocomposite membranes such as graphene oxide (GO)-cellulose nanocrystal (CNC) and GO-silk fibroin (SF)  have low density yet ultra-robust mechanical properties, and have outstanding flexibility with greater toughness. Although numerous nanocomposites have been developed and widely used, knowledge of the fundamental mechanical properties of bio-nanocomposites is still lacking. We have worked both in fabricating various nanocomposites and characterizing their mechanical properties (e.g., elastic modulus, flexural rigidity, toughness) using AFM, micro/nanoindenter, triboscope, and micro/nanobulging tests. We have focused on manufacturing various types of nanocomposites using self-assembly techniques such as the spin assisted Layer-by-Layer (SA-LbL) assembly technique. We also plan to establish models of the relevant micro/nanoscale mechanics, which can inform and improve the design and manufacturing of nanocomposites, with the goals of building and analyzing micro/nanoscale engineering systems so as to make them flexible, robust, scalable, and relatively low density.

Interfacial engineering and mechanical behavior of bio-nanocomposites

The interfacial interaction between two different components in bio-nanocomposites is a critical property which should be determined to ensure the robustness of the bio-nanocomposites. Delamination can occur at the interface between components with weaker load transfer capability, causing degradation of the overall mechanical properties of bio-nanocomposites. While the influence of interfacial interaction on the robustness of a bio-nanocomposite is very significant, there has been insufficient fundamental research on determining interfacial interaction of bio-nanocomposites. To address this issue, We have worked to propose further insight of interfacial interaction as a crucial design factor to build strong and robust bio-nanocomposites. We also extend our knowledge to employ specific methodologies (using atomic force microscopy (AFM), triboscope, etc.) in order to systematically study interfacial interaction of any bio-nanocomposites. This work will develop additional understanding of the critical factors of interfacial strength besides chemical bonding, and provide a framework to build various bio-nanocomposites that can be readily integrated with biological materials such as SF and CNC.

Dynamic AFM relaxation time test can be conducted to characterize the interfacial interaction of the bio- nanocomposites. Force-distance AFM measurements can be performed to analyze the relaxation time of the bio-nanocomposites, which are freely suspended on an aperture. Relaxation time can be characterized by the obtained force-distance curves (FDCs) under selected ramp rates. The dominated molecular density of a material can be identified at different dynamic probing regions.

The mechanical properties of the bio-nanocomposites can be determined by analyzing the collected FDCs with theoretical models of contact mechanics. Furthermore, physical surface properties such as adhesion and friction of surface, which are directly corresponded by the surface topography, can be mapped by FDCs-based imaging analysis through quantitative nanoscale mechanical analysis.

Encapsulation and delivery of active ingredients such as drugs, proteins, or living cells is becoming increasingly important for a wide variety of  applications and technologies for drug delivery system. Thin shell microcapsules with responsive properties have potential applications in the field of drug deliver. One important thing is that the potential use of microcapsules requires the capability of releasing active compounds at certain condition.  The microcapsule should have switchable permeability.  LbL technique is one of the most promising  fabrication method to fabricate functional microcapsules. Low interfacial strength, however, between microcapsule layers may cause microcapsule corruption and rupture. Thus, understanding interface strength is very important to design robust LbL microcapsule. It is possible to identify the interfacial shear strength between two different layers of LBL microcapsule by using AFM.



LbL silk ionomers microcapsule 

(z-scale: 400 nm)

S. Kim et al. Biomacromolecules, 2017

S. Kim et al. Langmuir, 2016

Y. Wang, R. Ma, K. Hu, S. Kim et al. ACS Appl. Mater. Interfaces, 2016

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