Mainly in the field of biochemistry, substrates with micropores or the like regularly formed and arranged on their surfaces have recently been incessantly proposed and made public as scaffolding materials in cell engineering and culture engineering in scientific literature and articles.
Applications of each of the substrates proposed so far in the art to various art fields, to say nothing of medical fields, are now under study. For instance, their applications to a diversity of fields including semiconductors, low-dielectric-constant materials, scatter layers for electronic displays, magnetic recording material, photonic crystals and cell culture substrates are now under study. Thus, such substrates attract attention as one of promising materials.
However, attempts to prepare structures with such micropoores regularly arranged thereon by means of ordinary micromachining processes run across such problems as mentioned below, and it is very difficult to achieve them; such micromachining processes are still far away from practically proper means. For instance, the micro-machining processes include lithography and laser machining. With these processes, however, there are some limits to the materials to be machined. In addition, an almost unlimitedly extraordinary number of micropores must be formed with regularity in mind, and so by very troublesome operations. It is not hard to imagine that micromachining requires a lot of steps and much time even though it is carried out by those pretty skilled in the art. As a matter of course, micromachining costs much.
Apart from this, there is known a so-called phase separation process by which micropatterns are formed. However, the resulting surface state involves a reproducibility problem; only an inhomogeneous pattern is obtainable. Thus, that phase separation process is still less than satisfactory for the formation of micropatterns having specific regularity.
Anyway, all the prior surface processing methods relying upon micropatterning technology require ever higher levels of techniques, and so have a lot of problems such as the inability to achieve mass production, and unavoidably increased costs.
By the way, some recent literature has reported that regular micropatterns are relatively easily formed by casting a dilute polymer solution on a solid substrate. One typical process has been proposed by a researcher group including the present inventors (see non-patent publication 1). According to this process, a dilute polymer solution is cast, and the solvent is evaporated thereby forming a dot (pillar, protuberance, or projection) pattern of microstructure in the polymer. Even that process is unsatisfactory, because it is still difficult to obtain any microdot pattern with such regularity as to control a dot array.
It has also been proposed to form a porous film having a micro-honeycomb pattern as a microstructure (non-patent publications 2 and 3). In this process wherein a special polymer having a moiety of strong self-aggregation force and a flexibility-developing moiety in combination is used, that polymer is dissolved in a hydrophobic organic solution, and the solution is then cast thereby forming said pattern.
The group including the present inventors has made an intensive study of this process as well, and succeeded in preparing a microstructure having a unique honeycomb structure by choice of a specific polymer. The results have been reported in articles (non-patent publications 4 and 5).
That is, the inventors have succeeded in the formation of a porous thin film having a honeycomb pattern structure by using as the constituent of said polymer an amphiphilic polymer comprising a hydrophilic acrylamide polymer as a main chain and having a dodecyl group as a hydrophobic side chain and a lactose or carboxyl group as a hydrophilic side chain or an ionic complex of an anionic polysaccharide such as heparin or dextran sulfate with a quaternary long-chain alkyl-ammonium salt.
The inventors have also found that porous honeycomb structure films prepared from various biodegradable polymers provide an especially promising material for cell culture substrates, and filed a patent application for them (patent publication 1).
The preparation process proposed by the inventors in that patent application involves an extremely simplified operation wherein a porous honeycomb structure film is obtainable by blowing a high-humidity air onto a cast film of a hydrophobic organic solution having a controlled concentration or the cast film is placed in a high-humidity atmosphere, and so is favorable in terms of preparation cost.
In that case, the pore diameter of the porous film can be controlled in the range of 0.1 to 100 μm by changing the diameter of water droplets acting as pore casts. Thus, the proposal by the inventors is of great originality and excellence.
Non-Patent Publication 1    Chemistry Letters, 821, 1996
Non-Patent Publication 2    Science 283, 373, 1999
Non-Patent Publication 3    Nature 369, 387, 1994
Non-Patent Publication 4    Thin Solid Films 327, 829, 1998
Non-Patent Publication 5    Molecular Cryst. Liq. Cryst. 322, 305, 1998
Patent Publication 1    JP(A) 200-1157574
The studies and proposals mentioned above underlie the present invention. One object of the invention is to achieve a further extension of them so that a structure that has not just a honeycomb texture but also with micro-pillars formed on its surface can be obtained by means of an extremely simplified process.
Another object of the invention is to provide, through a very simplified process, a micro-pillar structure with anisotropy imparted thereto, a micro-pillar structure enriched in water repellency, and a micro-pillar structure enriched in hydrophilicity.