Supramolecular Science has become an important issue in constructing molecular architectures through intermolecular forces by mimicking molecular assemblies in nature. Polyrotaxane is defined as a molecular assembly in which many cyclic compounds are threaded onto a linear polymeric chain capped with bulky end-groups. In the last decade, a variety of polyrotaxanes has been extensively designed as to act as novel biomaterials in our group. In this article, enhanced multivalent binding with biological systems has been introduced throughout our systematic studies on ligand-immobilized polyrotaxanes. Our studies have revealed such an enhanced binding effect seen in the polyrotaxanes is due to the combination of introducing multiple copies of the ligands and their highly mobile nature due to the inter-locked structure. A new insight of designing functional biomaterials will emerge from suchstructural characteristics of the polyrotaxanes.
Structural regulation and functional development of the polymeric multilayers prepared by the recently developed layer-by-layer (LbL) assembly technique are reviewed. Stereocomplexes formed between chemical-structurally regulated synthetic polymers were used in the LbL assembly, and the resulting films have potential applications in biomedical and other technological fields. The addition of inorganic salts into aqueous solutions of bioactive polyelectrolytes made it possible to fabricate the multilayers with a variable surface charge density. The bioactivities such as blood coagulation, enzymatic degradation, and cell responses were strongly dependent on the charge density as well as the outermost polymer component. Novel ultrathin films were fabricated by repetitive physical adsorption/drying processes from salt-containing aqueous solutions of strongly intra- and inter-active polymers. Furthermore, ultrathin films fabricated by chemical cross-links between polymers with a hydrophilic neutral unit caused an increaseinthe film thickness, in water, producing ultrathin hydrogels on the surfaces. These results will open a new field of polymer surface science.
In May 2003, establishment of human embryonic stem (ES) cell lines was reported in Japan, demonstrating that a new era of research on regenerative medicine had begun in Japan. Considering that stem cells, including ES cells, can be differentiated into various types of cells, regeneration of organs is expected. The population of stem cells is not enough for use in clinical therapy, so it is necessary to expand them in vitro. Animal-derived growth-supporting cells are generally required for the expansion. However, these non-human animal cells are not suitable because of potential cross-contamination with pathogens such as viruses or prions derived from animals. Therefore, development of an artificial substrate mimicking the cell-growing environment in place of the supporting cells is desired for use in regenerative medicine. Here, the design of a substrate surface for culture of stem cells is discussed.
A porous matrix for 3-D cell culture was designed in terms of highly water intrusion property and complete dissolution. The important characteristics of the matrix involve (i) easy cell invasion for 3-D cell culture and (ii) complete degradation and dissolution after tissue regeneration. A bioinspired polymers were prepared by using enantiomeric oligo(L-(D-)lactic acid)macromonomers, 2-methacryroyloxyethyl phosphorylcholine (MPC), and n-butyl methacrylate. Then, the porous matrix was formed by stereocomplexation between L-form and D-form oligomers. The matrix that consisted of aggregation of micro particles and also inter-connecting structure were observed by scanning electron microscopy. The water intrusion property was estimated by measuring the static contact angle of water droplet. The angle gradually decreased within 2 min, and the droplet completely spreaded out due to water intrusion into the matrix. On the other hand, the contact angle did not change on the conventional porous poly(D, L-lactic acid-co-glycolic acid). These results indicated that the bioinspired polymer stereocomplex had a high affinity to the water intrusion. Further, cell invasion into the matrix was carried out by using mouse fibroblast cells. In 5 days, the cells could attach and invade into the matrix. The cell invasion was observed only with the porous matrix. In conclusion, the bioinspired polymer could form porous matrix by stereocomplexation, and the matrix was available to 3-D cell culture.
The aim of this study is to explain the interactions between two cell groups. The tissue consists of different types of cell groups. Interactions between cell groups are important for organ or tissue functionality. Using cell sheet technology, we have proposed models of tissue structure in vitro. These models are very useful for studying interactions between cell groups at gene expression level. We studied the interactions between rat hepatocytes and human endothelial cells (HEC). Functions of rat hepatocytes monolayer culture disappeared within ten days. On the other hand, rat hepatocytes and HEC bilayered cell sheet culture system maintained over forty-one days of differentiated cell shapes and functions. These results indicated the importance of interactions between hepatocyte and HEC cell groups. The layered co-culturing of hepatocytes and HEC sheets allows the former's continuous expression of differentiated functions, thereby offering a major advancement in surface modification of biomaterials.
Single molecular photocurrent generation was investigated using a novel light-illumination scanning tunneling microscope (LI-STM) equipped with an optical fiber probe. For this study, self-assembled monolayers (SAMs) of biological photosynthesis mimicking chimera proteins composed of cytochrome b562 mutant proteins fused with a green fluorescent protein (GFP) were prepared. With light illumination, the GFP was excited, resulting in photocurrent generation due to the energy transfer to the cytochrome part instead of a typical fluorescence. Two different types of measurements were carried out employing a conventional LI-STM with a metallic probe and far-field optics, and another LI-STM in which the metallic probe was replaced with a laboratory-built doubly metal-coated optical fiber probe and thus a local near-field illumination was possible. Comparison of these two techniques indicated that the new technique was superior in sensitivity to the conventional one. Evidence of the intramolecular energy transfer at a single molecular level was shown.
Self-organization of nanostructures on semiconductor surfaces is attractive to the researchers of future Si integration technology because it is free from the size and the writing-speed limitations in lithography. In the case of coherently grown Ge nanostructures on Si surfaces, control of the strain field will be a key issue in the integration of well-defined and well-arranged nanostructures. The strain effect has a variety of aspects for self-organized Ge nanostructures on Si. Its magnitude determines the size and its symmetry modifies the shape. The strain distribution influences the nanostructure nucleation sites. Since the coherently grown Ge nanostructures induce strain on the substrate surface, interaction between the nano-structures are generated through the substrate strain. In this review, we describe strain engineering on Si substrates including artificial control of the strain distribution using buried oxide inclusions. Strain symmetry effect and interaction between the nanostructures are well demonstrated in Ge nanowires grown on Si(113).