Gel materials against entropy increase | RIKEN

April 14, 2023

RIKEN
Science and Technology Promotion Agency

－Distinguish left and right of force and move matter, energy, and living things in one direction－

Researcher Xiang Wang, Team Leader Yasuhiro Ishida, RIKEN Center for Emergent Matter ScienceJoint research groupcan distinguish between left and right directions of force applied from the outside, and can deform only in one direction.gel^[1]We have developed a material and demonstrated that it has the ability to move matter, energy and living things in one direction.

The result of this research is that the material creates an ordered state from a disordered state, that is, “entropy increase^[2]”, and it is expected to be applied in a wide range of fields such as material separation, energy recovery, and biological behavior control.

This time, the joint research groupNanosheets of graphene oxide^[3]was embedded in the gel.Lateral to this gelshear^[4]When , the nanosheets bend under left shear and the gel is easily deformed, while under right shear the nanosheets do not bend and the gel strongly resists. There is a 67-fold difference in stiffness in this left-right shear, and the gel behaves like a “pendulum that can only swing left or right from the center”. As a result, this gel converts random vibrations into unidirectional vibrations, transports objects in one direction,Nematode^[5]The function to control the direction was demonstrated in various situations, such as making a group of people run in one direction. These are all functions that oppose increasing entropy, bridge the wide gap between living organisms that have the ability to increase order on their own and artificial materials that do not normally have that ability, as well as new technologies for material separation and energy conversion. It may be related to technology.

This research was published in the scientific journalSciencewas published in the online version (April 13th: April 14th Japan time).

Materials that deform only in one direction perform various functions that enhance the order of things

background

Matter systems in this world usually undergo a natural transition from a highly ordered state to a less ordered state, and this change is called “entropy increase.” On the other hand, in biological tissues and functional materials, there is a mechanism that uses energy from the outside world to increase the order of the system, in other words, counteract the increase in entropy (Fig. 1a-c). These mechanisms are the root of life (motor protein^[6]etc.) and fascinate scientists even if they are fantastical (Maxwell’s demon^[7]etc.), the realization of which has transformed the way of life of mankind (rectifier diode^[8]、optical isolator^[9]Such).

They resist entropy increase because they show different responses, or “polarity”, depending on the direction of stimulation when stimulated from the left or right. If the stimulus is a wave,Non-reciprocity^[10]It can also be said to indicate Materials exhibiting polarity in response to electrical, magnetic, and optical stimuli have been studied for many years, but materials exhibiting polarity in response to “force,” that is, “mechanical polarity,” have not even been assumed to exist.

Here is a familiar example that shows the importance of dynamical polarity. Even ordinary solid materials such as plastics and metals can be processed into repetitive, asymmetrical shapes (such as saw teeth, fish spines, etc.), and the entire workpiece will exhibit a polar force to the force. Typical examples of such workpieces are cable ties, ratchet wrenches and rear axles of bicycles. They exhibit the important function of transmitting force in only one direction, which cannot be achieved with other tools. However, these workpieces exhibit mechanical polarity only when faced with objects of similar size to their geometrical units (one saw tooth, one fish bone, etc.). It means “tool” instead of “.

What would happen if a material were developed that exhibits mechanical polarity as an inherent property of the material without relying on its external shape as a work piece? It should be an extremely useful material capable of transmitting unidirectional force to matter of all sizes (Fig. 1d). In this way, it is expected that materials that exhibit polarity with respect to force will exert a function that opposes the increase in entropy through a mechanism different from that of materials that exhibit polarity with respect to electricity, magnetism, and light. In this research, we challenged the development of this unexplored material.

Fig. 1 Example of a mechanism that opposes entropy increase

• (a)“Motor protein” is a typical example that exists in nature.
• (b)“Maxwell’s Demon” is a representative example of a scientist’s thought experiment.
• (c)The rectifier diode is a representative example of a material that has completely changed human life.
• (d)The newly developed “material that shows polarity once morest force”.

Research method and results

The joint research group oriented obliquely graphene oxide nanosheets, which are extremely small and ultrathin plate-like substances, and embedded them in the gel (Fig. 2a).Specifically, a magnetic field was applied to the graphene oxide nanosheets dispersed in water, and all the nanosheets were oriented obliquely, and then dissolved in water in advance.monomer^[11]andcross-linking agent^[12]ofcoincide^[11]By doing so, we synthesized this gel.

To investigate the force response of this gel, the bottom surface of the cube-shaped gel was fixed to the floor and the top surface was sheared left or right (Fig. 2b). An important point here is that the internal structure of the gel is left-right asymmetric, resulting in geometric inequality between left and right shear. In other words, in the left shear, the nanosheets are compressed in-plane and flexed, making it impossible to reinforce the gel (Fig. 2b, left). (Fig. 2b right). As a result, it was found that this gel deforms easily under left shear, but strongly resists right shear, with a 67-fold difference in elastic modulus between the two (Fig. 2b, bottom).

Fig. 2 Synthesis and shear response of mechanically polar gels

• (a)Synthesis of mechanically polar gels.
• (b)Relationship between stress and strain when shear is applied to a mechanically polar gel. Gels with right shear have an elastic modulus approximately 67 times higher than those with left shear.

In this way, we succeeded in developing a material that exhibits mechanical polarity as an inherent property of matter. Henceforth, this gel will be called a “mechanical polar gel”. Its behavior is likened to a “pendulum that can only swing left or right from the center”, and it is thought that it exerts a function that opposes the increase in entropy.

Therefore, we first investigated the case where force is uniformly applied to the entire mechanically polar gel. When a plate-type gel is placed on a vibrator and symmetrical vibration is applied horizontally from the bottom surface of the gel, the vibration transmitted to the top surface of the gel is significantly asymmetric (Fig. 3a). The resulting asymmetric vibration can move an object in one direction. When a Teflon plate was placed on the upper surface of the gel and a water drop was placed on it and symmetrical vibration was applied from the lower surface of the gel, the water drop was subjected to left-right asymmetrical vibration and moved to the right at a constant speed (Fig. 3b). The efficiency of this unidirectional movement was extremely high, and even when the vibrator was placed vertically as shown in Fig. 3c, the water droplets climbed the Teflon plate at a constant speed once morest gravity.

Furthermore, by pasting six fan-shaped gels together and using a gel disk with nanosheets arranged like a pinwheel, we were able to achieve unidirectional rotational motion instead of simple unidirectional linear motion (Fig. 3d). ). This rotational motion occurs efficiently even when random vibrations generated by motors and speakers are applied to the gel, so it is possible to apply it as a system to recover vibrational energy, which has been discarded as energy of low utility value. increase.

Fig. 3 Generation of asymmetric vibration by mechanically polar gel and its application to unidirectional motion

• (a)Asymmetry of vibration output from the top surface (red arrow) when symmetric vibration is input to the gel from the bottom surface (black arrow). The graph on the right is a comparison of the movement distance between input and output.
• (b)Unidirectional movement of red droplets using gel-generated asymmetric vibrations.
• (c)Unidirectional movement of red water droplets defying gravity.
• (d)One-way rotation of a Teflon wheel using asymmetric vibration generated by disk-shaped gel. The red line of the mark rotates clockwise.

Next, we investigated the case where a local force is applied to a part of the mechanically polar gel. When a cylinder was placed on top of a plate-shaped gel and pushed vertically, the gel deformed asymmetrically (bottom left of Fig. 4a). This asymmetric deformation isfinite element method^[13]It was found that the strain distribution inside the gel was left-right asymmetric (Fig. 4a, lower right). It should be noted that on the left side of the cylinder where the gel can be flexibly deformed, the contact area between the cylinder and the gel is much larger than on the right side, and the force that the cylinder receives from the gel is also large.

Therefore, an object that contacts or collides with the gel surface is expected to receive a force that pushes it toward the right side. In fact, when the iron globules were allowed to fall freely onto the gel slab, the globules rebounded in a direction greatly tilted to the right from the incident angle (Fig. 4b). In this way, mechanically polar gels that bounce objects in asymmetric directions have interesting applications, such as high-performance sporting goods that transmit forces in desired directions.

Fig. 4 Asymmetric deformation in local indentation of mechanically polar gel and its application to unidirectional motion

• (a)Left-right asymmetric deformation of the gel when a cylinder is pushed vertically into the gel (lower left) and its finite element analysis (lower right). From finite element analysis, it was found that the strain distribution inside the gel was also left-right asymmetric.
• (b)Bouncing to the right of a free-falling globule when colliding with a gel.

As mentioned so far, mechanical polar gels are materials that have the ability to move objects of various sizes and shapes in one direction, but at the same time, they are composed of more than 90% water.hydrogel^[1]It is also a bio-friendly material. Focusing on this point, I thought that it might be possible to move living organisms in one direction using a mechanically polar gel.

The target is a type of nematode with a length of regarding 1 mm, which has been widely studied as a typical model organism.C. elegans^[5]We chose (Fig. 5a). In fact, when a nematode is placed on the upper surface of a plate-type gel and observed with a special optical microscope (polarized microscope), the gel adheres to the nematode on the left side of the nematode and deforms, while the gel on the right side is deformed, just like the cylinder mentioned earlier. At , the deformation is shallow and wide (Fig. 5b). We placed regarding 20 individuals in the center of the gel plate and tracked their running direction. Surprisingly, all individuals moved to the right and reached the right end of the gel following 40 minutes (Fig. 5c).

This result indicates that the mechanopolar gel can induce unidirectional movement not only of substances but also of living organisms.In the future, similar running direction control will be applied not only to nematodes but also tocell migration^[14]If it is also induced, it may lead to various applications such as chromatographic separation of cells, maintenance of undifferentiated stem cells, and polarization of cell tissues.

Fig. 5 C. elegans induced by mechanically polar gel (C. elegans) in one direction

• (a)C. elegans running experiments on the top surface of a mechanically polar gel.
• (b)Deformation of the gel around one nematode was visualized by polarized light microscopy. The left side of the nematode body appears dark and the right side appears bright. This indicates that the gel on the left side of the nematode deformed in close contact with the nematode, and the gel on the right side deformed shallowly and broadly. In other words, the nematode receives a force that pushes it toward the right side.
• (c)Net migration of each individual during 40 min running of a swarm of nematodes on top of a mechanically polar gel. All individuals ran to the right.

future expectations

In this study, we have developed a gel containing diagonally oriented nanosheets as the first material that exhibits polarity with respect to force. This mechanically polar gel exerts the function of controlling the direction, such as converting random vibrations into unidirectional vibrations, transporting objects in one direction, and moving a group of nematodes in one direction. Each of these functions includes a device that collects random vibrational energy that was previously wasted, high-performance sports equipment that transmits force in a desired direction, and next-generation media that control cell migration and differentiation. Some applications can be expected.

All of the above functions create an ordered state from a disordered state, in other words, they are functions that oppose entropy increase. As typified by rectifier diodes and optical isolators, materials that have the ability to resist entropy increase have potential ripple effects that can change people’s lifestyles. Considering that the gel this time exhibits this function through an unprecedented physical property of “polarity once morest force” rather than “polarity once morest electricity, magnetism, and light,” it has a great impact on both basics and practical applications. hope it will bring.

supplementary explanation

• 1.gel, hydrogel
  When a nano-sized three-dimensional network structure is formed using a substance that is well compatible with a certain liquid, the liquid molecules confined within the network lose their fluidity, and the entire system becomes solid. Such substances are called gels. A gel made of water is called a hydrogel, and familiar examples include agar, jelly, tofu, and konnyaku.
• 2.entropy increase
  Entropy is a numerical value that expresses the degree of disorder. The more chaotic the state, the higher the entropy (larger value), while the more ordered the state, the lower the entropy (smaller value). As long as all things are left as they are, entropy will continue to increase, and entropy cannot be reduced unless work is intentionally added from the outside. This is called the law of increasing entropy.
• 3.Nanosheets of graphene oxide
  Ultra-thin plate-like material obtained by oxidizing graphite and exfoliating the two-dimensional network of carbon into a single layer. The thickness is regarding 1 nanometer (nm, 1 nm is one billionth of a meter), which is equivalent to one carbon atom, and the width is regarding several micrometers (μm, 1 μm is one millionth of a meter).
• 4.shear
  For example, applying a lateral force to the top surface of a stack of playing cards causes each layer to move laterally in proportion to its height. Such tilted deformation is called shear.
• 5.Nematode,C. elegans
  Nematodes are a kind of bag-shaped animals that are intermediate between protozoans such as amoeba and paramecium and annelids such as earthworms and leeches. They usually have a small body length of 0.3 to 1 mm, and have a body shape similar to that of an earthworm, but they do not have a segmented structure like that of an earthworm.is a type of nematodeC. elegansIt has been studied as a model organism for a long time because it is easy to breed, has a simple and easy-to-observe body structure, and is easy to elucidate and analyze all genetic information.
• 6.motor protein
  A protein that hydrolyzes adenosine triphosphate (ATP) to generate motility in cells. It converts the chemical energy of ATP into work and performs unidirectional rotational motion in a thermally fluctuating environment.
• 7.Maxwell’s demon
  A fictitious mechanism assumed in a thought experiment proposed by physicist James Clark Maxwell. A box containing a gas of uniform temperature (however, the velocity of individual molecules is not constant) is partitioned by walls to divide it into chambers A and B. The walls that separate the rooms have small holes that can be opened and closed. Here is a “devil” who can discern each molecule and opens and closes the partition so that only fast molecules move from room A to room B, and only slow molecules move from room B to room A. This “devil” can then create a temperature difference between rooms A and B, thus reducing entropy, without doing any work.
• 8.rectifier diode
  An electronic component that allows the flow of electricity to flow in one direction. It is usually made by joining a p-type semiconductor and an n-type semiconductor. It can convert alternating current to direct current because it only allows current to flow in one direction when voltage is applied.
• 9.optical isolator
  An optical element that allows light to pass through in one direction and blocks light in the opposite direction. A ferromagnetic crystal is often used in the central part. Part of the light emitted from the light source may be reflected from some optical component and return to the light source (return light). Returned light can cause instability and damage to the light source, which the optical isolator can prevent.
• 10.Non-reciprocity
  A wave propagating in one direction and a wave propagating 180 degrees in the opposite direction have different properties.
• 11.monomer, polymerization
  A monomer is a molecule with a small molecular weight that becomes a structural unit of a one-dimensionally elongated molecule (polymer) with a large molecular weight. Polymerization is the reaction in which monomers link together to form a polymer.
• 12.cross-linking agent
  A substance that connects polymers to each other. Normal monomers form two bonds at one end and the other during polymerization, but special monomers that can form bonds at three or more positions can be used as cross-linking agents. Polymerization of this special monomer together with normal monomers results in a network structure in which the polymers are three-dimensionally linked.
• 13.finite element method
  When numerically analyzing the behavior of an object, instead of mathematically solving an equation that must be satisfied by the entire object, we divide the object into small regions, or finite number of elements, which average out the equation. An approximation technique that finds a satisfactory solution to . When analyzing the deformation of an object under load, the deformation of the entire object can be reproduced by individually deforming many elements.
• 14.cell migration
  The movement of cells from one location to another. It is a process that plays a central role in the development and maintenance of multicellular organisms. Embryonic development, wound healing, and tissue formation during immune responses all require the organized migration of cells to specific locations and directions.

Joint research group

Institute of Physics and Chemistry
RIKEN Center for Emergent Matter Science
Emergent Organism-Related Soft Matter Research Team
Team Leader Yasuhiro Ishida
Researcher Xiang Wang
Postdoctoral Researcher in Basic Sciences (at the time of research) Koki Sano
Special researcher (at the time of research) Zhifang Sun
Emergent Soft Matter Function Research Group
Group Director Takuzo Aida
RIKEN Center for Brain Science Neural circuit for multisensory integration RIKEN Hakubi Research Team
RIKEN Hakubi Research Team Leader Asuka Takeishi
(Pioneering Research Division Takeishi Multisensory Integration Neural Circuit RIKEN Hakubi Research Team)
Special Researcher Zhenhua Shao

Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo
Master program student Zhihao Li
PhD student Shuxu Wang

Solid Mechanics Research Group, Department of Mechanical Systems Engineering, Graduate School of Engineering, Nagoya University
Professor Dai Okumura
Assistant Professor Seishiro Matsubara

National Institute for Materials Science Nanoarchitectonic Materials Research Center
Soft Chemistry Group
Fellow Takayoshi Sasaki
Chief Researcher Nobuyuki Sakai

research support

This research was conducted by the Japan Science and Technology Agency (JST) Strategic Basic Research Promotion Project CREST “Development and application of dynamic photonic crystals composed mostly of water (research representative: Yasuhiro Ishida, JPMJCR17N1)” “Innovation once morest increasing entropy” Rectification of matter, energy, and organisms by the material “mechanical polar gel” (research representative: Yasuhiro Ishida, JPMJCR22B1)”.

original paper information

• Xiang Wang, Zhihao Li, Shuxu Wang, Koki Sano, Zhifang Sun, Zhenhua Shao, Asuka Takeishi, Seishiro Matsubara, Dai Okumura, Nobuyuki Sakai, Takayoshi Sasaki, Takuzo Aida, and Yasuhiro Ishida, “Mechanical nonreciprocity in a uniform composite material”, Science, 10.1126/science.adf1206

Presenter

RIKEN
Center for Emergent Matter Science Emergent Organism-Related Soft Matter Research Team
Team Leader Yasuhiro Ishida
Researcher Wang Xiang

Yasuhiro Ishida

Wang Xiang

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