2D-optical feedback system for microbes in micro channels and Euglena-based neural network computing (in progress)

After a long history of evolution, flagellate microbes have acquired excellent naturally tailored sensors and actuators for survival, as well as heterotrophy, autotrophy, and reproduction systems, unified in single cells within several tens of micrometers. Euglena gracilis cells exhibit phototaxis reactions: sensing light intensities and turning towards moderate lights that are more favorable for photosynthesis. These sophisticated sensing/actuating systems have been intensively investigated at molecular levels, but still cannot be reproduced artificially, even with well-developed nanofabrication/micromachine technologies. In general, flagellate microbes exhibit more complex or advanced reactions and behavior to external stimuli, such as adaptation to the stimuli (learning), alternative reactions to survive (exploiting), variation in the reaction threshold, self-sacrificing to preserve its own species, or even the evolution of the species. All those reactions derive from the survival strategies of the microbes, and some are memorized in individual cells as a change in metabolic chemicals in the body.

We can utilize the various survival strategies of microbes by constructing a bio-physical feedback system, which observes the reactions of the target microbes and imposes flexible and dynamic stimuli on the target microbes. When the stimulus is generated through a specific algorithm, the bio-physical system will evolute spontaneously, and exhibit a complex/sophisticated behavior in its temporal evolution, which no one ever examined. We are investigating on the bio-physical feedback system with living Euglena cells to elucidate its feasibilities for various researches and applications. This work is now in progress and of collaboration with prof. Song at Hanyang University.

We developed two-dimensional (2D) optical-feedback control system for phototaxis flagellate Euglena cells confined in closed-type microfluidic channels (micro-aquariums), and demonstrated that the 2D optical feedback enables the control of the density and position of Euglena cells in micro-aquariums externally, flexibly, and dynamically. Using three types of feedback algorithms, the density of Euglena cells in a specified area can be controlled arbitrarily and dynamically, and more than 70% of the cells can be concentrated into a specified area. Separation of photo-sensitive/insensitive Euglena cells was also demonstrated. The study proves that 2D optical feedback control of photoreactive flagellate microbes is promising for microbial biology studies as well as applications for chemical/environmental biosensors, microbe-based micro-robots, and cell separation based on photosensitivity or chemical sensitivity.
paper: [63]

Using real Euglena cells in a micro-aquarium as photoreactive biomaterials, we demonstrated Euglena-based neurocomputing with 2D optical feedback using the modified Hopfield-Tank algorithm. The blue light intensity required to evoke the photophobic reactions of Euglena cells was experimentally determined, and the empirically derived auto-adjustment of parameters was incorporated in the algorithm. The Euglena-based neurocomputing of 4-city traveling salesman problem possessed two fundamental characteristics: (1) attaining one of the best solutions of the problem and (2) searching for a number of solutions via dynamic transition among the solutions (multi-solution search). The spontaneous reduction in cell number in illuminated areas and the existence of photo-insensitive robust cells are the essential mechanisms responsible for the two characteristics of the Euglena-based neurocomputing.
paper: [56],[57],[65]
see also: Materials page

Anodization and hydrothermal treatment to prepare TiO2-based nanostructures (in progress)

We investigated the TiO2-based nanostructure fabrication through anodization in deep eutectic solvents (DES) and through hydro-thermal anodization, and found the following so far. This work is of collaboration with prof. Matsushita at Tokyo Institute of Technology.
(1) Spontaneous oscillation of anodization current was observed in DES, and the oscillation corresponds to the nodes of bamboo structures of TiO2 fabricated.
(2) Higher growth rates and improved crystallinity in TiO2 nanotubes are obtained by simultaneous hydro-thermal anodization of Ti.
(3) TiO2 nanotubes fabricated by hydro-thermal anodization shows a better biocompatibility as in faster formation of hydroxyapatite on its surface.
(4) Simultaneous hydro-thermal anodization can produce the nanostubes even for those hard alloys such as Ti-Nb-Zr-Ta.
paper: [67],[68],[78]

Surface potential of Alq3 and SNOM-KFM

We investigated the characteristics and mechanisms of photoinduced reduction in giant surface potential (gSP) on tris (8-hydroxyquinolinato) aluminum (III) (Alq3) thin films, based on the results of the reduction dependences on exposure time/intensity and the preservation of photopatterned gSP. Uniform gSP, formed spontaneously by vacuum evaporation of Alq3 in the dark, were patterned by contact-mask photo-exposure. The resulting surface-potential patterns were observed by scanning-probe Kelvin-force microscope (KFM). The reduction dependences are explained well with numerical-model calculation of surface potential reduction due to the drift of photoexcited carriers assuming the Poole-Frenkel formula for electron mobility. The preservation of patterned gSPs in the dark at -20oC for more than a year suggests the existence of deep traps involved in the carrier-drift mechanism. Two-dimensional (2D) transfer functions of KFM probes are discussed from KFM potential profiles obtained for a step pattern of surface potential.
paper: [51],[53],[54]

We developed a scanning probe near-field optical microscope (SNOM) combined with KFM that uses a slim and bent optical fiber probe (S/B) fiber probe. The developed SNOM-KFM system enables near-field photoexcitation through an apex of the S/B fiber probe during KFM measurement, so that the photoexcitation effects on SP can be measured with submicron spatial resolution. By measuring the photo-patterned SP of Alq3 thin films, we found that the S/B fiber probes have large negative values in the KFM transfer function. Near-field photoexcitation was performed on Alq3 thin films through the S/B fiber probes, and the spatial pattern of photoinduced reduction in SP was visualized by KFM measurement with the same probe. Micron-scale arbitrary patterned photoexciatation was realized by scanning the S/B fiber probe with SNOM-KFM.
paper: [51],[58]

We also investigated SP undulation on Alq3 thin films with KFM with intermittent photoexposure. SP undulation with a cloudlike morphology of 200-300 nm in lateral size was observed for Alq3 films of 10-200 nm in thickness. A short photoexposure increased the SP undulation approximately twice as that of the unexposed values, while the SP average decreased monotonically. We analyzed the origin of the SP undulation and the mechanism of its photoenhancement based on its morphology, film thickness dependence, and photoexposure dependence. Nonuniform distributions of mobility and charged traps were suggested from the experimental results.
paper: [60],[62],[64]