JP7483312B2 - Antibacterial materials - Google Patents

Description

The present invention relates to an antibacterial material that can inhibit the occurrence, growth, and proliferation of bacteria.

Traditionally, antibacterial materials have been used in a variety of fields, including the food industry, the medical industry, and hygiene-related fields. For example, in the food industry, they play a role in preventing food spoilage and the occurrence of food poisoning. They also extend the expiration date and improve the shelf life of food. As a countermeasure to the recent problem of food waste, improved shelf life can reduce food waste and contribute to reducing food costs. In addition, with the rise in health consciousness, antibacterial materials are also used in various items used by individuals and items used by many people in public places.

In most cases, antibacterial materials are made antibacterial by applying a chemical agent to the surface of the base material or by mixing a chemical agent into the surface material. In the case of chemical agents, there are concerns that they may leach into food. There are also concerns about the emergence of resistant bacteria that have acquired resistance to the chemical agents used. Other antibacterial materials that are used include those that use silver or silver ions.

Since around 2012, it has been reported that the nano surface structures present in the wings of dragonflies, cicadas, and other insects have antibacterial properties, and Patent Document 1 describes how antibacterial properties can be achieved by forming a structure that mimics the shape of a cicada's wing. The antibacterial material in Patent Document 1 forms columnar protrusions by etching a silicon substrate, and in particular, the columnar protrusions are formed to satisfy Asp ≧ 1.80exp (-0.00083p). Here, Asp is the aspect ratio of the vertical cross-sectional shape of the columnar protrusions, and p is the center-to-center distance between two adjacent columnar protrusions. It is described that by forming columnar protrusions that satisfy this condition, the survival rate of bacteria can be reduced to 1% or less.

In Patent Document 1, the specific method of forming columnar protrusions is to lay out polystyrene beads, irradiate them with plasma to carbonize them, form a thin gold film on a silicon substrate in the gaps that have shrunk due to the carbonization, and then remove the thin gold film and the silicon oxide film on the surface to complete the process. This type of processing requires large-scale equipment, which is very expensive, and since precious metals such as gold are used in the manufacturing process, the antibacterial material produced is inevitably very expensive. Furthermore, only limited materials suitable for etching, such as silicon substrates, can be used as substrates, which limits the applications.

On the other hand, polishing is a widely used processing method, and is much cheaper than the above-mentioned etching. Although it is a completely different technical field, there is a method described in Patent Document 2 as "F-polishing" as a technique for preventing powder adhesion. Conventionally, polishing has been performed to reduce or eliminate unevenness, but the method described in Patent Document 2 makes it possible to form the desired unevenness while still using polishing.

JP 2019-072475 A Japanese Patent No. 4064438

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide an antibacterial material that is inexpensive and can be used for a wide range of purposes.

The invention described in claim 1 of the present application is an antibacterial material characterized in that a predetermined unevenness is formed by polishing on part or all of the surface of a base material , and the predetermined unevenness is an unevenness having an average wave height to wavelength ratio of 0.0001 or more and 0.01 or less at a wavelength of approximately 1 μm .

The present invention uses polishing, which allows for much cheaper processing than conventional etching processes, and can be used with many different types of substrate materials, providing an antibacterial material that can be used in a wide range of applications.

1 is a schematic diagram showing a first embodiment of an antibacterial material of the present invention. 1 is a graph showing an example of the relationship between the wave height and wavelength ratio with respect to the wavelength when each rank of F polishing and mirror finishing are performed. FIG. 2 is an explanatory diagram of an example of test results of the antibacterial effect using the first embodiment of the antibacterial material of the present invention and a mirror-finished product. FIG. 2 is a schematic diagram showing a second embodiment of the antibacterial material of the present invention. FIG. 2 is a schematic diagram showing a modified example of the second embodiment of the antibacterial material of the present invention.

Figure 1 is a schematic diagram showing a first embodiment of the antibacterial material of the present invention. In the figure, 11 is a metal body and 12 is an irregularity. In the example shown in Figure 1, the base material of the antibacterial material is the metal body 11. In the example shown in Figure 1 (A), the metal body 11 is plate-shaped, and a predetermined irregularity 12 is formed by polishing on part or all of its surface (one side or both sides). In the example shown in Figure 1 (B), the metal body 11 is a sphere, and a predetermined irregularity 12 is formed by polishing on part or all of its surface. The irregularity 12 may be formed on the part where an antibacterial effect is desired, or on the part that comes into contact with an article such as food where an antibacterial effect is desired.

Although stainless steel or the like can be used as the base metal body 11, various metals such as aluminum and titanium alloys can be used, and the metal is not limited to metal as long as the unevenness 12 can be formed by polishing. In addition, the shape of the metal body 11 is not limited to the examples shown in Figures 1(A) and (B), and it goes without saying that various shapes are acceptable, including a thin sheet shape, a rod shape, a cube shape such as a tetrahedron or hexahedron, a stand shape with a base and an upright part, a comb shape, a mesh shape, an inner lid shape of a container, and the like.

The predetermined unevenness 12 should be one whose type is known, and should have roughly uniform frequency characteristics. For example, it is preferable that the unevenness is formed so that the average wave height to wavelength ratio is 0.0001 or more and 0.01 or less at a wavelength of approximately 1 μm. An example of the predetermined unevenness 12 will be described later using measurement results. Furthermore, in the portion where such predetermined unevenness 12 is formed, antibacterial action has been confirmed, as shown in the experimental example described later, and the metal body 11 can be used as an antibacterial material by forming the predetermined unevenness 12.

The predetermined unevenness 12 to be formed may be stripe-like unevenness in any direction, or unevenness in several directions may be formed so as to intersect. For example, in the case of a plate-shaped metal body 11 as shown in FIG. 1(A), the unevenness may extend in the longitudinal direction or in the lateral direction. In addition, in the case of a spherical metal body 11 as shown in FIG. 1(B), the unevenness may intersect approximately in the latitude direction and approximately in the longitude direction. Of course, even if the unevenness is formed randomly, it is sufficient as long as the unevenness satisfies the above-mentioned conditions as a whole. In addition, the formation position and shape of the unevenness 12 are not limited, and it may be formed, for example, in a band shape on a part of the surface of the metal body 11, or, for example, in the case of a plate-shaped metal body 11 as shown in FIG. 1(A), on only one side, or even if on both sides, it may be formed in different positions, or formed like a pattern.

Polishing is used as a processing method for forming the predetermined unevenness 12 on the metal body 11. In general polishing, the unevenness existing on the surface is scraped off to form a smooth surface, but here, polishing is used to form the predetermined unevenness 12 as described above. Polishing that leaves unevenness has been performed in the past, but the state of the remaining unevenness is unknown and there is no reproducibility. In the present invention, the predetermined unevenness 12, the unevenness of which is known, is formed on the surface of the metal body 11. As one method for forming the predetermined unevenness 12, for example, the method described as "F polishing" in the above-mentioned Patent Document 2 can be used. When this F polishing is performed, the unevenness that existed until then is eliminated, and the predetermined unevenness is formed by polishing, so that the predetermined unevenness 12 is formed in a stable state. This F polishing technology is described in Patent Document 2 as a technology to prevent powder from adhering to the metal surface, but its antibacterial effect has not been considered.

The F polishing described in Patent Document 2 is a method of polishing metal surfaces using an abrasive consisting of hard abrasive particles such as diamonds, which do not lose their sharpness even as the polishing process progresses, attached to paper or cloth. The abrasive particles are ranked according to their nominal roughness, and the surface finish is designated in order of roughness as "F-2" (#60), "F-1" (#120), "F0" (#240), "F1" (#320), "F2" (#400), "F3" (#500), "F4" (#600), "F5" (#800), "F6" (#1000), "F7" (#2000), etc.

A wave number analysis using Fourier transform was attempted for the predetermined irregularities 12 formed by the above-mentioned F-polishing. This analysis quantifies the characteristics of the irregularities 12, and specifically, it is possible to numerically express the height difference of the irregularities at what intervals. The irregular shapes of the steel sheet surface by F-polishing at several ranks can be calculated using the following formula (1):
X(k) = Σ _{n = 0} ^N-1 x(n) · exp(-2πknj/N) ... (1)
A discrete Fourier transform was performed according to the formula (1), and the relationship between the wavelength (unevenness pitch) component L of the unevenness and the wave height component H was analyzed. For comparison, a Fourier analysis was also performed on the case where mirror finishing was performed, and a comparison was made with F polishing. In addition, in formula (1), x(n) is the height direction value (representing the unevenness of the surface) measured by the probe sensor at a predetermined sampling interval when the steel sheet surface is scanned in a predetermined length (distance) direction with the probe sensor, that is, the nth digital sampling value out of the total sampling number N. In addition, k is a value corresponding to the number of waves (spatial frequency) f [times/μm] per unit length (k=0, 1, 2, ..., N-1), and k=fD is expressed by assuming that the total measurement distance is D [μm].

That is, X(k) is a vector that represents the signal strength after Fourier transform for the wave number corresponding value k per unit length, and the absolute value (length) of this vector is proportional to the wave unevenness. Therefore, for the transformed signal strength X(k) for the value k, the wave height H of the uneven shape is expressed as 2|X(k)|/N, and the ratio of the wave height H to the wavelength L (called the "wave height to wavelength ratio" or "wave height ratio") H/L is H/L=2|X(k)|/NL ... (2)
It is expressed as:

Figure 2 is a graph showing an example of the relationship between the wave height-to-wavelength ratio and the wavelength for each rank of F polishing and when mirror finishing is performed. When the above-mentioned wave height-to-wavelength ratio is shown for each wavelength, an example result such as that shown in Figure 2 is obtained. Figure 2 shows the cases of F polishing ranks F-2, F-1, F0, F1, F5, and F7, and the case where mirror finishing is performed (marked "mirror"). In actual measurements, measurements were taken at sampling intervals of 0.1 μm or less for measurement distances of 100 μm or more, and such measurements were taken at several locations, and the analysis results are shown as an average.

Referring to the analysis results shown in Figure 2, a clear difference appears in each graph at a wavelength L of approximately 1 μm. When viewed at a wavelength L of 1 μm, when F polishing is performed, the value of the wave height to wavelength ratio is larger than when mirror finishing is performed. More specifically, the value of the wave height to wavelength ratio at a wavelength L of 1 μm is 0.0001 or less when mirror finishing is performed, whereas it is 0.0001 or more in all cases when F polishing is performed, and the ranks are approximately 0.0001 to 0.0003 for F7, 0.0002 to 0.0004 for F5, 0.0006 to 0.001 for F1, 0.001 to 0.002 for F0, 0.005 to 0.007 for F-1, and 0.008 to 0.01 for F-2.

Furthermore, according to the analysis results shown in Figure 2, for example, when wavelength L is 1 μm, the amplitude of the graph is smaller when F polishing is performed than when mirror finishing is performed. This amplitude is about 20 to 30 percent of the average value at each wavelength. Therefore, it can be seen that F polishing stably forms roughly uniform unevenness. However, when mirror finishing is performed, the wave height to wavelength ratio varies by about two orders of magnitude, indicating that the unevenness is not stably formed. Furthermore, on unpolished surfaces and surfaces where the unevenness before polishing remains, the analysis results differ depending on the location of measurement, and there is also a large variation, and it was found that uniform unevenness as shown in Figure 2 does not exist.

Of course, even in the case of F polishing, there is variation in each unevenness, but when viewed as a surface, unevenness with uniform frequency characteristics is formed, and unevenness with such roughly uniform frequency characteristics is stably reproduced. This shows that the specified unevenness 12 is stably reproduced on the surface of the metal body 11.

The unevenness formed by polishing is not thought to be columnar as described in Patent Document 1, so a general comparison with the conditions described in Patent Document 1 cannot be made. If we assume that the wavelength L corresponds to the center-to-center distance of the protrusions and the wave height to wavelength ratio corresponds to the aspect ratio, then in Patent Document 1 the aspect ratio is approximately 0.3 or more, whereas in the present invention the wave height to wavelength ratio is 0.0001 or more and 0.01 or less, a much smaller value. In this way, it can be seen that in the present invention, an antibacterial effect is obtained outside the range of conditions shown in Patent Document 1.

Figure 3 is an explanatory diagram of an example of the test results of the antibacterial effect using the first embodiment of the antibacterial material of the present invention and a mirror-finished product. In order to confirm that the antibacterial material of the present invention actually has antibacterial effect, the following test was conducted. First, a plate-shaped stainless steel metal body 11 as shown in Figure 1 (A) was prepared, with the surface polished to the above-mentioned F polishing ranks of F-2, F0, F3, and F7 to form the specified irregularities 12. These are shown as "F-2 product", "F0 product", "F3 product", and "F7 product". As a comparative example, a metal body 11 with a mirror-finished surface was also prepared. This case is shown as "mirror product".

Approximately ¹⁰⁵ Escherichia coli were seeded on the surfaces of the five types of metal bodies 11 prepared, and left to stand at 37°C for 24 hours. After that, the Escherichia coli liquid was recovered with 10 ml of phosphate buffer, and the number of bacteria in the recovered liquid was counted. The counting results are shown in Figure 3. Figure 3(A) shows the counting results, and Figure 3(B) shows each counting result as a bar graph.

As a result of this test, the number of bacteria in the "mirror-finished product" was 8.4 x 10 ⁴ , while the number was 1.8 x 10 ⁴ in the "F-2 product", 1.2 x 10 ⁴ in the "F0 product", 6.0 x 10 ³ in the "F3 product", and 53 in the "F7 product". Overall, the number of bacteria was reduced in the case where the specified irregularities 12 were formed compared to the "mirror-finished product", demonstrating the existence of an antibacterial effect. In particular, the number of bacteria in the "F7 product" was reduced to below 0.1%, indicating a very high antibacterial effect.

The results of this test show that there is almost no antibacterial effect when the surface is almost completely uneven, such as in a "mirror-finished product," but that antibacterial effect is achieved by forming the unevenness 12 on the surface. Therefore, as shown in Figure 2, it is advisable to provide the unevenness 12 with a wave-height-to-wavelength ratio of 0.0001 or more when the wavelength L is 1 μm. Also, since the wave-height-to-wavelength ratio of the unevenness 12 formed by F polishing is 0.01 or less, it is sufficient for the wave-height-to-wavelength ratio of the unevenness 12 formed on the surface to be 0.0001 or more and 0.01 or less.

Furthermore, from the F polishing rank, it was found that finer irregularities have a stronger antibacterial effect than coarse irregularities. For example, if the number of bacteria is to be reduced to about 1/10, it would be sufficient to make the wave height to wavelength ratio smaller than that of the "F0 product", and form irregularities 12 on the surface with a wave height to wavelength ratio of less than 0.001. Furthermore, if the number of bacteria is to be reduced to about 1/1000, it would be sufficient to form irregularities 12 on the surface with a wave height to wavelength ratio of about that of the "F7 product", that is, about 0.0001 to 0.0003.

These predetermined irregularities 12 are formed by a common processing method called polishing, including F-polishing technology. Therefore, compared to conventional etching processes, processing can be performed with simpler equipment and at relatively low cost. In addition, the substrate is not limited to conventional Si substrates, and various substrates can be used, allowing for use in a wide range of applications. Of course, since the predetermined irregularities 12 are simply formed on the substrate and no chemicals are used, no chemical damage is caused and the process is safe.

For example, in the food industry, it is necessary to prevent the growth of germs. Of course, it would be best if it were possible to completely sterilize and completely prevent the intrusion of germs. However, when considering cost-effectiveness, sterilization may not be realistic. Even if it is possible to eliminate germs to a certain extent and to suppress the growth and proliferation of germs to a certain extent through antibacterial action, it is possible to extend the expiration date of food and reduce food waste, and it can also contribute to reducing food costs, and the effect is immeasurable. Such an effect can be obtained by using a metal body 11 with the specified unevenness 12 formed by F polishing. Furthermore, in the above-mentioned test, the bacteria were almost completely killed in the "F7 product", and the effect is very large.

The Kansai University team believes that the reason why the formation of the specified irregularities 12 can provide an antibacterial effect is that the protrusions of the irregularities break the cell membrane of bacteria. However, the specified irregularities 12 formed on the metal body 11 have an average wave height to wavelength ratio of about 0.0001 to 0.01 at a wavelength of approximately 1 μm. The protrusions of such specified irregularities 12 are small, and the physical damage they cause to bacteria is unlikely to be significant. On the other hand, it is known that charges accumulate in the protrusions on an irregular surface, and the inventors believe that changes in the surface potential of the protrusions affect bacteria, or that bacteria come into contact with the protrusions and are affected by the difference in surface potential, suppressing the growth and proliferation of bacteria or killing them.

Figure 4 is a schematic diagram showing a second embodiment of the antibacterial material of the present invention. In the figure, 21 is a container, 22 is an inner surface, 23 is an inner bottom surface, and 24 is an outer surface. In this second embodiment, an example of a container 21 whose inner surface has antibacterial properties is shown. In the example shown in Figure 4, the cross section has a shape in which the inner surface 22 is approximately perpendicular to the inner bottom surface 23, but this is not limited to such a shape, and the shape of the container 21 is arbitrary, and it may be a wide-mouthed or narrow-mouthed shape, or the cross section in the planar direction may be a shape other than a circle, such as a square shape.

A predetermined unevenness 12 is formed on a part or the whole of the inner side surface 22 or the inner bottom surface 23 of the container 21, or on both. This predetermined unevenness 12 is similar to that of the first embodiment described above, and is formed by polishing. For example, unevenness having an average wave height to wavelength ratio of 0.0001 or more and 0.01 or less at a wavelength of approximately 1 μm may be formed by polishing. In addition, this predetermined unevenness 12 is approximately uniform regardless of the location where it is formed, and it is preferable that unevenness with approximately uniform frequency characteristics is formed when viewed as a surface. By forming such predetermined unevenness 12 in the container 21, it is possible to suppress the generation and proliferation of bacteria in the contents stored in the container 21.

As an example of a polishing method for forming such predetermined irregularities 12, the above-mentioned F-polishing can be used as in the first embodiment. The predetermined irregularities 12 to be formed may be unevenness extending in the vertical direction (the direction connecting the opening of the container 21 and the inner bottom surface 23) on the inner surface 22, unevenness extending in the circumferential direction, or spiral unevenness, and may be stripe-like unevenness in any direction. Also, the inner bottom surface 23 may have various stripe-like irregularities, such as concentric circular unevenness or radial unevenness. In any case, the unevenness is not limited to stripe-like unevenness, and may be randomly formed unevenness.

Of course, as mentioned above, the predetermined unevenness 12 may be applied to the entire inner surface 22 and inner bottom surface 23, or may be applied only partially, such as to the inner surface 22 up to a predetermined height from the inner bottom surface 23 where the contents are likely to come into contact, or to only a portion of the inner bottom surface 23.

Figure 5 is a schematic diagram showing a modified example of the second embodiment of the antibacterial material of the present invention. 31 is a metal part. In this modified example, the container 21 is made of glass or the like, and a different material, here a metal part 31, is arranged on the lower part of the inner surface 22 of the container 21. In the example shown in Figure 5, the inner bottom surface 23 and the lower part of the inner surface 22 of the container 21 are made of metal part 31. In the example shown in Figure 4, an example is shown in which the container 21 is made of metal, but for example, as in this modified example, the area forming the unevenness may be made of metal part 31, and the other parts may be changed to a different material.

The metal part 31 has a predetermined unevenness 12 formed on a part or the whole of the inner bottom surface 23 or the inner side surface 22, or both. As the predetermined unevenness 12, it is preferable that unevenness with an average wave height to wavelength ratio of 0.0001 or more and 0.01 or less is formed by various polishing processes such as F polishing as described above in a wavelength range of 1 μm or less. The periphery of this metal part 31 is joined to the glass part of the container 21. Any joining method may be used, for example, by gluing or welding.

The glass part of the container 21 does not need to be transparent, and may be colored glass, translucent glass, opaque glass, decorated glass, etc. Of course, the material of the container 21 is not limited to glass, and plastic or other materials may be used. In addition, various materials may be used for the metal part 31 as long as they are capable of forming the specified irregularities 12.

As can be seen from the first and second embodiments and their modifications described above, the present invention can provide an antibacterial effect even with a very simple structure of forming the specified irregularities 12 as described above. This makes it possible to provide an antibacterial material that is safe, inexpensive, and can be used for a wide range of purposes without using chemicals, etc.

It goes without saying that the configuration shown in the first embodiment in which the metal body 11 is provided with the predetermined irregularities 12 may be combined with the configuration shown in the second embodiment in which the inner surface of the container 21 is provided with the predetermined irregularities 12, and the metal body 11 on which the predetermined irregularities 12 are formed may be placed in the container 21 on which the predetermined irregularities 12 are provided.

In the first and second embodiments and their modified examples described above, the material used for the metal body 11 and the metal part 31 may be various metals such as aluminum, titanium, copper, and tin, as well as various alloys, in addition to the commonly used stainless steel. Furthermore, in these embodiments and modified examples, an example is shown in which a metal is used as the base material, but this is not limited to this, and materials other than metals may also be used, and members of various materials may be used as long as the material can be used to form the specified irregularities 12 by polishing. Furthermore, for example, the irregularities 12 may be formed in a mold by polishing, and the mold may be used to mold with resin or the like, or post-processing may be performed using the mold, and it is sufficient that the specified irregularities 12 are formed on the surface after molding.

11...metal body, 12...concave and recess, 21...container, 22...inner surface, 23...inner bottom surface, 24...outer surface, 31...metal part.

Claims (1)

An antibacterial material characterized in that a predetermined unevenness is formed by polishing on part or all of the surface of a substrate , and the predetermined unevenness has an average wave height to wavelength ratio of 0.0001 or more and 0.01 or less at a wavelength of approximately 1 μm .

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