Research

Our goals

1. Physically understand life phenomena of living cells using artificial cells
2. Describe the behavior of soft matter such as polymer confined in cell size space
3. Construct useful micro-material with artificial cells

These subjects are interconnected. The knowledge from subject 2, the difference of molecular diffusion between in bulk and cell-size space, is essential to understand the complex behaviors of living cells. In addition, we try to fabricate functional micro-materials from biopolymers confined in artificial cells by applying the knowledge from these basic researches to material science.

Our team is constituted of members from various backgrounds such as physics, chemistry, biology, and engineering. Each member is actively studying his/her subject making the most of their strong point, discussing with other members.

1. Physical understanding of life phenomena by using model cells

Phospholipid membrane capsules, called vesicles (or liposomes), are used for many daily necessities such as medicines and cosmetics because of their high biocompatibility. In recent years, they are also widely used as cell models. For example, spherical liposomes deform to various shapes through dehydration under high osmotic pressure in a similar manner to actual cells such as red blood cells. The deformations have been actively studied not only for cell imitation but also for physics since they can be described as thermodynamic equilibrium states (see details).

However, such cell models consisting only of lipid membranes are fragile and cannot reproduce all cell shapes. One of the reason is the lack of cytoplasm and cytoskeleton. To solve these problems, we developed model cells that contain high concentration polymer solution as a model cytoplasm inside [1] and have artificial cytoskeleton by using DNA nanotechnology [2].

Our current interest is to elucidate how such the mechanical properties of internal structures affect the membrane deformation mechanism. We also try to understand the shape of cell tissues by using membrane-adhering multiple liposomes.

• Understand cell morphology control mechanism using liposome
• Elucidate correlation between liposome shape and internal viscoelasticity

2. Investigation of molecular diffusion and phase transition in cell-size space

The cytoplasm contains high concentration of various biopolymers such as proteins, nucleic acids, and polysaccharides. This condition is called (macromolecular) crowding. According to reports, anomalous diffusion and ergodicity breaking take place in such crowding condition in cells.

Recently, we mimick such crowding condition by using cell size droplets of polymers and found that molecular diffusion becomes anomalous when a crowding polymer solution is in cell-size confinement that covered with a lipid membrane [3]. This result implies that the crowding solution and the cell-size confinement by the lipid membrane synergistically induce anomalous diffusion in cells. Based on this idea, we are now conducting research to elucidate the physical factors that induce anomalous diffusion in the cell mimicking systems. If the factors become clear, it may be possible to explain the phase transition of polymers [4] and accelerarated gene expression in cell sized droplets, which is different from bulk systems [5].

• Molecular diffusion of biopolymers in cell models
• Phase transition (phase separation, gelation) of biopolymers in cell models

3. Fabrication of biomimetic micro-materials

The cells contain liquid and also gel-like phase, for example cytoskeleton composed of actin. By mimicking such cell structure in coexistence phase of liquid and solid (gel) in such a cell-sized space, we have succeeded to change shape of the microgel only by temperature [6, 7], and the elasticity of the microgel by confinement space size [8]. These changes are caused by physical parameters such as the interaction (wettability or affinity) of the gelled polymer with the membrane covering the confinement space, and the progress rate of gelation and phase separation. Now we try to establish a control method of gel shape and mechanical properties through the gelation space and also a mechanical measurement method of a small microgel.

• Control of shape and mechanical properties of polymer gel via gelation space
• New viscoelasticity measurement method of a single microgel