Both plants and animals possess tissues that are assembled through a hierarchical network of pores. Such pores are tuned and designed to adapt to their external environment and have evolved to maximise mass transport processes and rates of reaction.
The physical principles of these networks are embodied in Murray’s law, but the synthetic production of man-made Murray materials has always proved problematic due to the challenges of creating vascularized structures.
An international collaboration of researchers has now managed to produce biologically inspired materials that emulate natural systems following Murray's law through a bottom-up, layer-by-layer (LbL) self-assembly approach.
Many natural systems have adopted hierarchical networks based around Murray’s law to achieve a high efficiency and low power consumption for the transfer and exchange of substances. Evolution has allowed nature to optimize these networks by regularly decreasing and branching the pores, which terminates in size-invariant units i.e. the leaf, stem etc.
Murray’s Law is commonly referred to as the connection of a whole natural porous network, within a finite volume, that minimizes the transport resistance for all the pores and ensures a fluent transfer throughout the network. Such natural structures can be further enhanced through further branching and space-filling to maximize the exchange surface and increase the pore sizes to macro-dimensions.
These natural organisms, also referred to as natural Murray networks, can sustain life, obey Murray's law can and regulate pore diameters from the macroscopic to microscopic dimensions.
Such regulations have allowed for the connection of multi-scaled pores. Connections between the stem and leaf, and in tracheal pores, are two areas where the mass transfer, photosynthesis rates and gaseous diffusion process are enhanced through these networks.
Despite their importance in natural systems, materials based around Murray's law have been seriously overlooked in all branches of science. This is because Murray's law is only applicable to processes where there is no mass variation, and there are bottlenecking issues in interconnected pores.
Various synthetic methods have recently been proposed and developed for hierarchically porous materials, but many suffer from a limited material choice, lack of control, harsh chemical environments, poorly degrading scaffolds and clogged pores.
Synthesising Murray Materials
The researchers have produced a bio-inspired, self-assembled material that contains space-filling macro–meso–micropores (M–M–M) based around Murray’s Law. As Murray’s law is based around a fixed volume, the researchers revisited the principle and developed a generalized Murray’s Law which is applicable to optimised mass transfer processes involving mass variations.
The material contains a hierarchical network of pores and was produced using a bottom-up, layer-by-layer (LbL) evaporation-driven self-assembly process using zinc oxide (ZnO) nanoparticles as the primary building blocks.
The spontaneous release of gas molecules in the formation process leads to the formation of nanochannels in the ZnO nanoparticles. The optimised nanoparticles are 30 nm in size and contain up to 8 nanochannels each.
The researchers also implemented micro and mesoporous channels, through hexane evaporation methods, that introduced holes by pushing the nanoparticles further apart from the centre of the channel and causing them to aggregate.
The researchers characterised and confirmed their structures through scanning electron microscoscopy (SEM), mercury porosimetry, argon adsorption measurements, small-angle x-ray scattering and density functional theory (DFT) calculations.
The materials are composed of interconnected channels with precise dimensions; the produced materials show a highly-enhanced mass exchange and transfer in liquid–solid, gas–solid and electrochemical reactions.
This has given these Murray materials great tunability, which can be fabricated for a wide range of applications, especially those where mass diffusion or ion transfer is the dominant transport process. The researchers have documented and demonstrated three specific applications where these materials have provided an enhanced performance- photocatalysis, gas sensing and as Li-ion battery electrodes.
The researchers have also stipulated that Murray materials revolutionize sustainable industrial processes, namely in the design of industrial-scale chemical reactors with an enhanced efficiency, lower power output, reduced reaction timescales and a more efficient raw material consumption.
Zheng X., Shen G., Wang C. Li Y., Dunphy D., Hasan T., Brinker J. C., Su B-L., Bio-inspired Murray materials for mass transfer and activity, Nature Communications, 2017, 8, 14921
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