The joint research group of team leader Yasuhiro Ishida, researcher Xiang Wang, and other researchers has developed a gel material that determines whether force applied from outside acts toward the right or toward the left and can change its form in only a single direction. They also demonstrated that this material can make materials, energy, and organisms move in only a single direction.

In this research, the joint research group prepared a composite material in which nanosheets of graphene oxide^*2 are diagonally oriented and embedded in a hydrogel,^*1 90% of which is water. This gel shows a polarity for force and behaves like a pendulum that swings either only to the right or only to the left. With this property, the material exerted the function of controlling directions of movement in various situations, such as converting disordered vibrations into unidirectional vibrations, transporting materials in a single direction, and forcing a group of nematode worms^*3 to move in a single direction (Fig. 1).

This research result indicates that this material has the ability to change a state of disorder into a state of order or to oppose the increase of entropy.^*4 The material can be expected to fill the large gap between organisms, which have the ability to keep themselves highly ordered, and artificial materials, which usually do not have such an ability, and can be applied in a wide variety of fields, such as separating substances, recovering energy, and controlling organism behavior.

*1 Gels and hydrogels
When a substance that is very conformable to a liquid is used to form a nano-sized three-dimensional network in the liquid, the liquid molecules entrapped in the network lose fluidity and the entire liquid becomes solid. Such a material is called a gel. A gel made of water is called a hydrogel.

*2 Nanosheet of graphene oxide
This is an ultra-thin, plate-shaped material made of oxidized graphite. It has a thickness of approx. 1 nanometer (nm, 1 nm is one billionth of a meter), which is equivalent to one carbon atom, and a width of approx. several micrometers (μm, 1 μm is one millionth of a meter).

*3 Nematode worms
A type of aschelminth with a body length of approx. 0.3 to 1 mm that is intermediate between protozoans, such as amebae and paramecia, and annelids, such as earthworms and leeches.

*4 Increases in entropy
Entropy is the degree of the state of disorder expressed as a numerical value. The more disordered a state is, the higher the entropy is (the larger the value is), and the more organized and ordered a state is, the lower the entropy is (the smaller the value is). The entropy of all things continues to increase so long as they are left as they are, and cannot be decreased unless work is applied intentionally from outside. This is called the law of increasing entropy.

There are many materials that show a polarity for electric fields, magnetic fields, or light. The discovery of rectifying diodes^*5 and optical isolators^*6 had an enormous impact that changed how people live. However, materials that show a polarity for force have heretofore never even been imagined.

Tools that show a polarity for force and have the function of conveying force in only a single direction exist. For example, banding bands, ratchet wrenches, and bicycles’ rear wheels are this type of tool. Such tools are made of a solid material processed to have a saw-blade-like shape where an asymmetric pattern is repeated, which enables them to have a profile that shows a polarity for force. However, these processed tools can only exert their function on objects of saw-blade size.

If a material can be developed that inherently has a mechanical polarity without relying on its profile, it would be a very useful material that can convey force in only a single direction for materials regardless of their sizes. In this research, the researchers took up the challenge of developing such an undiscovered material.

*5 Rectifying diode
An electronic device that make electric current flow in a single direction. Since it allows current to flow in only a single direction when a voltage is applied, it can convert AC into DC.

*6 Optical isolator
An optical device that allows light to pass through it in only a single direction and prevents light from passing through it in the opposite direction. Part of the light from a light source may be reflected on an optical device and return to the light source (return light). Return light may damage the light source or cause it to become unstable. Optical isolators can prevent such problems.

The joint research group synthesized this gel by diagonally orienting nanosheets of graphene oxide dispersed in water with a magnetic field and then allowing them to polymerize^*9 with monomers^*7 and a cross-linker^*8 dissolved in the water (Fig. 2-a).

To examine how this gel responds to force, the bottom surface of a cube of the gel was fixed to the floor, and the top surface was sheared^*10 to the left or to the right (Fig. 2-b). When sheared to the left, the nanosheets are bent due to in-plane compression (the left side of Fig. 2-b). When sheared to the right, the nanosheets are not bent due to in-plane tension (the right side of Fig. 2-b). As a result, this gel is easily deformed when it is sheared to the left, while it strongly resists the force and behaves like a pendulum that swings only either to the right or to the left from the center when it is sheared to the right. It was found that the modulus of elasticity of the gel when it is sheared to the right is 67 times as high as that when it is sheared to the left (the bottom of Fig. 2-b).

*7 Monomer *9 Polymerize
A monomer is a molecule with a low molecular weight that constitutes a linear chain-like molecule with a high molecular weight (polymer). Polymerization refers to a reaction in which monomers bond to each other to form a polymer.

*8 Cross-linker
A material that bonds polymers together.

*10 Shear
For example, when force is applied to the top surface of a set of playing cards in a lateral direction, each card moves in a lateral direction in proportion to the height. An inclined deformation such as this is called a shear.

The developed material with a mechanical polarity (hereinafter, the "mechanically polar gel") is considered to have a function of opposing the increase of entropy.

First, the behavior of the mechanically polar gel when force is applied evenly over the entire gel was examined.

When symmetric vibrations were applied from the bottom surface of the gel in the horizontal direction, the vibrations conveyed to the top surface were converted into unidirectional vibrations. These vibrations make materials move in a single direction (Fig. 3-a). When symmetric vibrations were applied to the gel from below with a water droplet on the top surface, the water droplet moved to the right at a constant speed (Fig. 3-b). When the vibrator was positioned vertically, the water droplet moved upward against gravity at a constant speed (Fig. 3-c). When six fan-shaped pieces of the gel were bonded together to form a gel disk in which the nanosheets were arranged in windmill form, unidirectional rotary motion could be observed (Fig. 3-d).

Next, the behavior of the mechanically polar gel when local force is applied to part of it was examined.

When a cylinder was laid down on the top surface of a flat plate of the gel and then pushed into the plate in the vertical direction, the gel deformed asymmetrically (the lower left part of Fig. 4-a). On the left side of the cylinder, where the gel could easily be deformed, the area of contact between the cylinder and the gel was much larger than on the right side, and the cylinder was subjected to a larger force from the gel. Therefore, it is expected that an object that comes into contact or collides with the gel surface will be subjected to a force that pushes it out to the right. In fact, when a small steel ball was allowed to fall freely onto a flat plate of the gel, the ball bounded highly toward the right (Fig. 4-b).

In addition, given the facts that more than 90% of this mechanically polar gel is water and that this gel is a biologically friendly material, the researchers surmised that this gel could be used to make organisms move in a single direction. When approx. 20 nematode worms were put on the center of a flat plate of the gel and their movements were tracked, all the individual worms moved toward the right (Fig. 5-c).

The functions of the mechanically polar gel developed in this research are expected to be used in a variety of applications. The function of converting disordered vibrations into unidirectional vibrations may be able to be employed in a system for recovering vibration energy, which has previously been discarded as energy that is less useful; the function of bounding a colliding object in an asymmetrical direction may be employed in highly functional sports goods that convey force in the desired direction; and the function of making a group of nematode worms move in a single direction may be employed for separating cells in chromatography, preserving stem cells undifferentiated, or polarizing cell tissues.

The gel that was developed in this research as the first material with a polarity for force and has a function of opposing increases in entropy can be expected to greatly impact both fundamental research and practical applications.