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METHODS FOR MODIFYING THE SURFACES OF A SOLID AND MICROSTRUCTURED SURFACES WITH ENCREASED ADHERENCE PRODUCED WITH SAID METHODS
 The invention relates to methods according to the preamble of claim 1 or 2, in particular methods for microstructuring surfaces of solids; methods for producing a bond from objects with modified surfaces; objects with adhesionenhancing modified surfaces; as well as connections made with such objects.
 Adhesive connections between the same type or different types of materials, which adhesive connections reliably create specified adhesive forces while at the same time being reversibly detachable, are of great importance in the field of technology. Applications comprise for example the detachable connection of components by means of hook and loop connections; the manipulation of objects by means of tools; magnetic holding devices for objects; connection elements on textile products; temporary fixing of objects (e.g. note pads) in offices, and the like. Numerous connection technologies have been developed which are specially adapted to the respective task. For example, it is know in the field of automation technology to reversibly attach objects with suction connections to manipulators and to move said objects with said suction connections. However, such suction connections are associated with considerable technical expenditure. Furthermore, reversible adhesive connections can also be made using adhesive materials. However, this is associated with a disadvantage relating to possibly limited chemical compatibility between the materials connected and the adhesive material used, and also relating to the forming of residues.
 Adhesive connections which are free of residues are known; such connections are particularly based on chemical and/or mechanical modification of the surfaces of the materials to be connected. For example, U.S. Pat. No. 5,755,913 describes various options of chemical modification of polymer layers to enhance their adhesive power. For example, the surface components of a polymer layer are modified by chemical treatment or radiation such that free bonding positions, electrostatic interactions, ionic interactions or other bonding appearances occur. However, chemical modification is associated with an important disadvantage in that it is limited to certain polymer materials.
 From U.S. Pat. No. 6,099,939, U.S. Pat. No. 6,107, 185 and U.S. Pat. No. 4,615,763, mechanical surface modifications are known which are essentially based on the roughening of surfaces. Roughing takes place for example by selective etching. By way of changed surface morphology, the effectively adhering surface of the material is enlarged. Again, this technology is associated with a disadvantage in that it is limited to certain materials, e.g. polymers. Furthermore, these materials also have to be sufficiently soft, because otherwise contact formation is impeded even by the highest elevations on the roughened surface. Lastly, the techniques for roughening surfaces are difficult to control so that quantitatively defined adhesive forces can be set only to a limited extent.
 From WO 99/32005 it is known to modify the surfaces of objects by an adhesive element which consists of a layer-shaped carrier and rod-shaped or bar-shaped projections. The carrier is glued onto the object to be modified so
that the projections protrude into space, thus causing anchorage to a respectively modified surface of a further object. The attachment element thus does not form an adhesive connection but instead a mechanical anchorage. The rodshaped or bar-shaped projections necessarily have to be produced so as to provide good stability and matching geometric shapes. Thus, the projections have characteristic cross-sectional dimensions and mutual spacing in the mm range. Additionally, the establishment of a reliable adhesive connection requires an adhesive material.
 Modification of the surface morphology of solids is also known in other technical contexts. For example, WO 96/04123 and WO 00/50232 describe a self-cleaning effect of surfaces with a structure comprising elevations and depressions. It has been shown that structured surfaces comprising hydrophobic polymers with typical structuring dimensions above 5 fim have an adhesion-reducing effect.
 Research by the inventors shows that in the process of evolution, nature also has developed adhesive systems. They make it possible, for example, for insects to walk on surfaces irrespective of the orientation of said surfaces, or to fix body parts to each other under certain conditions. These adhesive systems are based on the presence of extremely fine hairs, for example on the legs of insects, as has been described by M. Scherge and S. N. Gorb in "Biological Micro- and Nanotribology" (Springer).
 Adhesive systems based on adhesive microstructures have also been developed after investigating the structures present on the legs of geckoes (see WO 01/49776). However, these attempts to use the adhesive systems of the gecko in technical applications were limited to transferring the gecko structures to technical objects or to copying said structures in a synthetic geometric sense. Pure copying of gecko structures is however associated with a disadvantage in that the application of respective adhesive systems is limited to ideal smooth surfaces and has substantial limits in respect of the holding forces and their application involving real surfaces, in particular rough or fractal surfaces. It is thus the object of the present invention to provide
 improved methods for surface modification, with which methods the adhesive power of the modified surfaces is increased, and with which the disadvantages associated with the conventional technologies can be avoided. The method according to the invention is in particular intended to make it possible to form detachable adhesive connections for an enlarged range of materials and surfaces; to provide improved adhesive power on real surfaces, if applicable on uneven surfaces; and to set specified adhesive forces or adhesive characteristics. It is a further object of the invention to provide a method for producing an interconnection involving several objects whose surfaces have been modified so as to enhance their adhesive power. Further objects of the invention relate to the provision of improved surface modifications which make it possible to achieve increased adhesive power.
 The above-mentioned objects are met by methods and structured surfaces with the characteristics according to claims 1, 2, 29 or 30. Advantageous embodiments and applications of the invention are defined in the dependent claims.
 An important characteristic of the methods for surface modification of an object, which methods are
described in this document, provides for the surface of a structure to be subjected to the formation of a multitude of projections each comprising a front surface, wherein the projections are dimensioned such that all the surfaces project to the same vertical height above the surface, and thus provide an adherent contact surface which is only interrupted by mutual spacing or gaps between the front surfaces. By means of this measure, the originally closed (unmodified) surface changes to a contact area in which the front surfaces form a multitude of individual contacts (so-called microcontacts). Advantageously, with this splitting-up into microcontacts enhanced adhesion is achieved, as it has been possible to demonstrate by experiments and the theoretical considerations mentioned below. This result is surprising because the contact surface as a sum of the front surfaces at first seems to offer a smaller adhesive surface than the originally unmodified surface. Nonetheless, the adhesive power is increased, and this can even be adapted in a specified manner to meet the requirements of a given application. Arranging the front surfaces at a constant vertical height above the surface provides an additional advantage in that the adhesive connection is not weakened by projecting microcontacts.
 According to the invention the projections are formed so as to be inclined in relation to the surface. As a result of the inclination of the projections, when an adhesive connection is made between two objects, a shear component is applied when contact is established, which shear component enhances the adhesive power. Advantageously the inclination, elasticity, dimensioning of the projections and/ or their elasticity parameters explained below can be optimised depending on the application.
 According to a first important aspect, the abovementioned object of the invention is met in that the projections are made inclined in relation to the surface, from an elastic material, with a gradient of the elasticity module. If the flexural strength of the projections towards their free end and/or radially across their longitudinal extension is reduced, this results in the following advantages. The inventors have found that an adhesive connection with positive attraction between the structured surface and an adjacent body is formed if the energy gain which results from the adhesion of the contact surface to the body exceeds the elastic energy which is required for bending the projections. By generating the elasticity gradient or flexural strength gradient, advantageously the elastic energy required for bending the projections is reduced, so that the positive adhesive effect is achieved more easily. This advantage is particularly pronounced in the application of the invention to real surfaces, in particular to rough irregularly structured surfaces. Such surfaces have unevennesses which lead to bending of the projections. By the introduction, according to the invention, of the elasticity gradient, the energy loss as a result of bending of the projections is reduced and adhesion is thus improved.
 If according to a preferred embodiment of the invention the gradient of the elasticity module is formed between an upper and a lower elasticity value, wherein the upper elasticity value is between 10 MPa and 10 GPa, and the lower elasticity value is between 20 kPa and 10 MPa, advantages can result in relation to a particularly soft bending of the projections, and thus particularly good adhesion improvement can be achieved.
 According to a further important aspect of the invention, the above-mentioned object is met in that the projections are formed with an effective elastic module whose size depends on the elasticity module of the material of the projection, its density per unit area, and its geometric dimensions relating to length and diameter, wherein the effective elastic module is set at a value ranging from 20 kPa to 10 MPa. With this measure, the energy loss during bending of the projections on real surfaces is again reduced, and consequently the adhesive effect is improved.
 The projections of the surface structure formed according to the invention preferably form an angle of 89° to 45°, in particular 80° to 40 degrees, in relation to the local alignment of the structural surface. Surprisingly it has been shown that even a minimal inclination of the projections at the time contact is established with an adjoining body leads to bending the projections into the desired direction. When these angles of inclination are set, advantages for improved adaptation of the contact surface to a real surface can result.
 A contact surface is formed by the totality of the front surfaces. When establishing an adhesive connection, the contact surface touches the surface of the respective other object without establishing an anchorage and without any interlocking of the projections. The adhesive connection is achieved by van der Waals forces. Additional contributions can be made by electrostatic forces or capillary forces. The spacing between the projections is smaller than the cross-sectional dimensions of the front surfaces.
 According to a preferred embodiment of the invention, the projections are arranged such that the front surfaces form a regular pattern (or grid). In contrast to conventional roughening of surfaces, periodic arrangement of the projections provides an advantage that weak points in the adhesive connection are avoided and homogenisation of the contacts is achieved.
 According to further advantageous embodiments of the invention, the contact surface is formed such that the mutual spacing between adjacent front surfaces is less than 10 fim, in particular less than 5 fim (e.g. 4 fim or less). Furthermore, the characteristic cross-sectional dimensions of the front surfaces are preferably less than 5 fim. These dimensions, which are achieved mutually or independently and if applicable relate only to one reference direction, provide the advantage of particular adhesion improvement. Selecting a dimension depends on the adhesive force to be achieved in a particular application. With a given overall area of microcontacts, the adhesive force increases with the square root from the number of microcontacts. With surfaces modified according to the invention it is even possible to reliably interconnect macroscopic objects, e.g. tools, without the weight of one of the objects being able to interrupt the adhesive connection.
 According to an advantageous embodiment of the invention, the front surfaces of the projections, which together form the contact surface, have a curved front surface shape which is preferably implemented as a hemispheric shape, a cylindrical shape, a torus shape and/or a bowl shape. The front surface shape can vary within the multitude of projections. The curved front surface shape, which has resulted from calculations first carried out by the inventors, is associated with an advantage in that the adhesive force of the individual contacts is improved as a result