We have developed a new technology for producing three-dimensional (3D) biological structures composed of living cells and hydrogel in vitro, via the direct and accurate printing of cells with an inkjet printing system. Various hydrogel structures were constructed with our custom-made inkjet printer, which we termed 3D bioprinter. In the present study, we used an alginate hydrogel that was obtained through the reaction of a sodium alginate solution with a calcium chloride solution. For the construction of the gel structure, sodium alginate solution was ejected from the inkjet nozzle (SEA-Jet, Seiko Epson Corp., Suwa, Japan) and was mixed with a substrate composed of a calcium chloride solution. In our 3D bioprinter, the nozzle head can be moved in three dimensions. Owing to the development of the 3D bioprinter, an innovative fabrication method that enables the gentle and precise fixation of 3D gel structures was established using living cells as a material. To date, several 3D structures that include living cells have been fabricated, including lines, planes, laminated structures, and tubes, and now, experiments to construct various hydrogel structures are being carried out in our laboratory.

The next step beyond conventional scaffold-based tissue engineering is cell-based direct biofabrication techniques. In industrial processes, various three-dimensional (3D) prototype models have been fabricated using several different rapid prototyping methods, such as stereo-lithography, 3D printing and laser sintering, as well as others, in which a variety of chemical materials are utilized. However, with direct cell-based biofabrication, only biocompatible materials can be used, and the manufacturing process must be performed under biocompatible and physiological conditions. We have developed a direct 3D cell printing system using inkjet and gelation techniques with inkjet droplets, and found that it had good potential to construct 3D structures with multiple types of cells. With this system, we have used alginate and fibrin hydrogel materials, each of which has advantages and disadvantages. Herein, we discuss the roles of hydrogel for biofabrication and show that further developments in biofabrication technology with biomatrices will play a major part, as will developments in manufacturing technology. It is important to explore suitable biomatrices as the next key step in biofabrication techniques.

The utility of hypromellose acetate succinate (HPMCAS), a cellulosic enteric coating agent, as a carrier in a solid dispersion of nifedipine (NP) was evaluated in comparison with other polymers, including hypromellose (HPMC), hypromellose phthalate (HPMCP), methacrylic acid ethyl acrylate copolymer (MAEA), and povidone (PVP). An X-ray diffraction study showed that the minimum amount of HPMCAS required to make the drug completely amorphous was the same as that of other cellulosic polymers, and less than that in dispersions using non-cellulosic polymers. Hypromellose acetate succinate showed the highest drug dissolution level from its solid dispersion in a dissolution study using a buffer of pH 6.8. This characteristic was unchanged after a storage test at high temperature and high humidity. The inhibitory effect of HPMCAS on recrystallization of NP from a supersaturated solution was the greatest among all the polymers examined. Further, the drug release pattern could be modulated by altering the ratio of succinoyl and acetyl moieties in the polymer chain. Our results indicate that HPMCAS is an attractive candidate for use as a carrier in solid dispersions.

Tissue engineering has been developed with the ultimate aim of manufacturing human organs, but success has been limited to only thin tissues and tissues with no significant structures. In order to construct more complicated tissues, we have developed a three-dimensional (3D) fabrication technology in which 3D structures are directly built up by layer-by-layer printing with living cells and several tissue components. We developed a custom-made inkjet printer specially designed for this purpose. Recently, this printer was improved, and the on-demand printing mode was developed and installed to fabricate further complicated structures. As a result of this version, 3D layer-by-layer printing based on complicated image data has become possible, and several 2D and 3D structures with more complexity than before were successfully fabricated. The effectiveness of the on-demand printing mode in the fabrication of complicated 3D tissue structures was confirmed. As complicated 3D structures are essential for biofunctional tissues, inkjet 3D biofabrication has great potential for engineering complicated bio-functional tissues.

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