Soft Matter Blog

What does softness do for life? According to Zhigang Suo, Havard University, for life the important feature of soft materials is that they are easily deformable. In the human body the deformation of soft materials enables the heart to beat, the vocal folds to produce sound and the eye to focus. In all these examples, a stimulus is applied to the soft material and a large reversible deformation occurs. This deformation in turns provides a function.

Soft active materials are not only important in life, but also have technological relevance for example in adaptive optics, self-regulated fluidics and soft robotics. Suo is interested in how mechanics, chemistry and electrostatics all work together to generate large deformations in soft materials. In soft dielectrics actuation can be readily observed by applying a voltage across the membrane, causing the thickness to reduce and the area to expand. In polymers, strains of up to 30% are easily achieved using this method.

To get higher deformations two limitations need to be overcome; electrical breakdown and electromechanical instability. An electromechanical instability is observed in compliant dielectrics such as elastomers, when the applied voltage causes the material to thin down excessively, amplifying the electric field. This is known as a snap-though instability and often occurs prior to electrical breakdown. Suo has shown, however, that if this electromechanical instability can be overcome, and the elastomer reaches a stable state without breakdown occurring, giant voltage-induced deformations of over 1000% are achievable.

Suo’s theory shows that the instability can be eliminated when pre-stretched or short-chain polymers, where the elastic strain is sufficiently low, are used. The elastomer is compliant at low deformations and stiffens rapidly as it is slowly stretched. This stiffening averts the rapid excessive deformation that would otherwise cause the material to fail. The elastomer survives the instability and reaches a steady state without breakdown occurring. Suo and his collaborators have used these ideas to carefully design and tailor the properties of soft materials realising area expansions of 1692%.

For more information see:

Keplinger et al., Harnessing snap-through instability in soft dielectrics to achieve giant voltage-triggered deformation, Soft Matter, 8,  285-288, 2012.

Lu et al., Dielectric elastomer actuators under equal-biaxial forces, uniaxial forces, and uniaxial constraint of stiff fibers, Soft Matter, 8, 6167-6173, 2012.

Zhao and Suo, Theory of Dielectric Elastomers Capable of Giant Deformation of Actuation, Phys. Rev. Lett., 104, 178302, 2010.

Schematic of DNA translocation in a glass capillary. Image taken from Soft Matter, 2012, doi:10.1039/C2SM25346A

Detecting single strands of DNA is a tricky business. One way in which it can be done is by creating a pore and sensing the DNA strand as it passes through the pore. This is what Ullrich Keyser from the University of Cambridge, UK and his group have been doing.

Keyser forms nanopores with diameters of less than 100nm from glass capillary tubes (the diameters can be as small as 20nm). These nanocapillaries act as single molecule sensors. Using electrophoresis, the negatively charged DNA is pulled towards a positively charged electrode inside the capillary, in a process known as DNA translocation. As the strand enters the capillary the resistance across the capillary pore changes, allowing the DNA to be detected. The change in ionic current is dependent not only on the presence of a DNA strand, but also on its folded state. This method offers a simple and cost-effective method for the detection of single molecules and for DNA sequencing.

Full control over DNA translocation can be achieved using optical trapping, where the DNA is attached to a colloidal particle, held in place by an optical trap. The DNA can then be moved in and out of the capillary pore at will. Using this method, Keyser and his group have measured the capture force due to the electric field acting on the DNA. Their results show that the DNA capture force is linearly dependent on the number of strands captured in the capillary.

Whether using glass capillaries, or pores formed in silicon nitride membranes via focussed ion beam milling, control over the exact shape and functionality of the nanocavity can be problematic. Keyser has taken DNA detection yet another step further by using the DNA itself to create the nanopore. The shape into which a DNA strand folds can be controlled in a a process known as DNA origami; the DNA is synthesised such that it will self-assemble into a pre-designed three-dimensional shape. Using this origami, it is possible to design and fabricate virtually any nanosized shape that you want.

Keyser designed the DNA so that it folded into a funnel like shape with a long tail. This structure was then pulled through a pore in a  silicon nitride membrane to form a hybrid nanopore with a diameter of 7.5nm. The assembly of the hybrid pore is robust and easily reversible. These DNA/silicon nitride pores have been successfully used to detect single strands of DNA. The hybrid nanopores offer a novel way to change the size, shape and functionality of pores.

Relevant papers in SoftMatter:

Chen, Q. et al., How does a supercoiled DNA chain pass through a small conical glass pore? Soft Matter, 2012, Advanced Article.

Geerts, N., Eiser, E., DNA functionalized colloids: Physical properties and applications. Soft Matter, 2010, 6, 4647-4660.

Kim, K. N., et al., Comparison of methods for orienting and aligning DNA origami. Soft Matter, 2011, 7, 4636-4643.

A recently published Soft Matter paper has been featured on the BBC website: ‘Avalanche research aids search for tastier ice cream’.

Avalanche experts were consulted for a study on how ice cream’s structure changes when it is stored in a household freezer. The structure within the ice cream is important for the taste of the food.

Read the original research here:
3D-characterization of three-phase systems using X-ray tomography: tracking the microstructural evolution in ice cream
B. R. Pinzer ,  A. Medebach ,  H. J. Limbach ,  C. Dubois ,  M. Stampanoni and M. Schneebeli
Soft Matter, 2012, Advance Article
DOI: 10.1039/C2SM00034B

Read the most popular Review Articles of 2011 for free today

Self-assembly of amphiphilic peptides
I. W. Hamley
Soft Matter, 2011, 7, 4122-4138
DOI: 10.1039/C0SM01218A

Stimulus responsive core-shell nanoparticles: synthesis and applications of polymer based aqueous systems
Olivier J. Cayre , Nelly Chagneux and Simon Biggs
Soft Matter, 2011, 7, 2211-2234
DOI: 10.1039/C0SM01072C

Cellulose nanowhiskers: promising materials for advanced applications
Stephen J. Eichhorn
Soft Matter, 2011, 7, 303-315
DOI: 10.1039/C0SM00142B

Polylactide (PLA)-based amphiphilic block copolymers: synthesis, self-assembly, and biomedical applications
Jung Kwon Oh
Soft Matter, 2011, 7, 5096-5108
DOI: 10.1039/C0SM01539C

Morphology of polymer-based bulk heterojunction films for organic photovoltaics
Matthias A. Ruderer and Peter Müller-Buschbaum
Soft Matter, 2011, 7, 5482-5493
DOI: 10.1039/C0SM01502D

Stimulus responsive nanogels for drug delivery
Liusheng Zha , Brittany Banik and Frank Alexis
Soft Matter, 2011, 7, 5908-5916
DOI: 10.1039/C0SM01307B

Covalently cross-linked amphiphilic block copolymer micelles
Cornelus F. van Nostrum
Soft Matter, 2011, 7, 3246-3259
DOI: 10.1039/C0SM00999G

PNIPAM microgels for biomedical applications: from dispersed particles to 3D assemblies
Ying Guan and Yongjun Zhang
Soft Matter, 2011, 7, 6375-6384
DOI: 10.1039/C0SM01541E

Nanoparticles with targeting, triggered release, and imaging functionality for cancer applications
Kristin Loomis , Kathleen McNeeley and Ravi V. Bellamkonda
Soft Matter, 2011, 7, 839-856
DOI: 10.1039/C0SM00534G

Hydrophilic and superhydrophilic surfaces and materials
Jaroslaw Drelich , Emil Chibowski , Dennis Desheng Meng and Konrad Terpilowski
Soft Matter, 2011, 7, 9804-9828
DOI: 10.1039/C1SM05849E

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Growing up is a stressful process, particularly during embryonic development. The embryo starts out as a single symmetric cell. This cell can be considered as an active fluid bound by the cell membrane. Flow within this fluid leads to the build-up of stresses, the formation of patterns and asymmetries within the cell. Stephan Grill, at the Max-Planck Insitute for the Physics of complex systems and the Max-Planck Institute for Molecular Cell Biology and Genetics, Dresden, is interested in understanding how these flows lead to the polarisation observed in the Caenorhabditis elegans zygote.

Grill has shown that the changes in cell polarity are driven by myosin flow on the surface of the cell. The cortex can be considered as a dynamic self-contracting polymer gel surface, which lies underneath the cell membrane. This polymer gel layer behaves as a thin film of an active fluid. Passive advective transport of molecules – in this case myosin – embedded in the fluid can occur depending on the diffusivity and the flow velocities of the molecules.  In C. elegans the flows are fast enough that advection does play a role, influencing the distribution of the molecules as they diffuse on the cortex. Modelling shows that the passive advective transport by flow of such a mechanically active materials acts as a trigger for the segregation of the proteins, resulting in the polarisation of the zygote.

The movement of myosin across the surface of the cell also results in anisotropies in the cortical tension. These so called active stresses cause isolated sections of the cortex to self-contract. Grill has developed a novel method for locally determining the stresses, by cutting the cortex with a laser and measuring the recoil. The cortical tension is found to be greatest in the direction orthogonal to the flow.

Grill suggests that advective transport in active fluids is a general mechanism for the formation of patterns in developmental biology.

For more information see:

Goehring, N.W. et al., Polarization of PAR Proteins by Advective Triggering of a Pattern-Forming System, Science, 2011.

Bois, S, et al., Pattern Formation in Active Fluids, Phys. Rev. Lett., 2011.

Mayer. M, et al., Anisotropies in cortical tension reveal the physical basis of polarizing cortical flows, Nature, 2010.

The image above is taken from: Cyclodextrin/dextran based drug carriers for a controlled release of hydrophobic drugs in zebrafish embryos, Soft Matter, 2011, and shows C. elegans embryo 30hrs after fertilisation.