pdms molds were made of polydimethylsiloxane ( pdms , dow corning , usa ) at 1 : 10 mixing ratio . the mixed pdms was degassed in a vacuum chamber for 10 minutes to remove trapped air and poured at 500 μm thickness onto a micropatterned cyclic olefin copolymer ( coc ) mask consisting of parallel grooves with 2 μm period , 1 μm groove width and 0 . 6 μm groove depth . subsequently , the pdms was briefly degassed for a second time and cured for 4 hours at 60 ° c . the cured pdms molds were separated from the mold with tweezers and cut into squares of 1 cm 2 with a scalpel . blank molds were similarly created by pouring pdms onto flat coc substrates for comparison purposes . subsequently , all patches were left in ethanol overnight to dissolve any uncrosslinked material . the molds were then treated with oxygen plasma to increase the hydrophilicity of the surface . a process time of 120 seconds at 10 w was chosen after testing a range of intervals from 30 to 150 seconds as the one yielding the lowest contact angle ( 20 . 2 ± 0 . 5 °). fig8 shows the testing so the water static contact angle measured on the active pdms patch surface upon different plasma treatments , the contact angle of untreated pdms patches is compared with the contact angle of patches treated with low power ( 10 w ) plasma for 30 , 60 , 90 , 120 , and 150 seconds and with the contact angle of gelatin coated pdms . the stiffness of the resulting patches was measured by uniaxial testing and their young &# 39 ; s modulus was calculated to be 1 . 53 ± 0 . 057 mpa . the mold 2 has grooves 6 with a width f and ridges 5 with a width e . this shall be illustrated in somewhat more detail in the context of fig7 , specifically fig7 a , in which a cut essentially perpendicular to the running direction of the pattern on the mold 2 is shown . in this case the pattern is a regular rectangular pattern , where both widths e and f are equal , and where the pattern angle α is 90 °. the length 1 of the actual pattern should have a minimum length , namely the pattern period p should be smaller than 10 pm [ micrometer ] and the pattern length 1 should be larger than 1 mm . normally this length 1 is equal to the full with d of the mold 2 as illustrated in fig1 . the ridges have a height h ( or the grooves have a depth ), which can be within the boundaries as outlined above . the shape of the pattern does not need to be a regular rectangular shape as illustrated in fig7 a . the ridges can also be of at least partly trapezoidal shape as illustrated in fig7 b , they can be of triangular shape as illustrated in fig7 c ( it is also possible that the triangles meet at the bottom of the ridges leading to a zigzag shape ), and they can also be rectangular with rounded edges as illustrated in fig7 d ( the rounded edges can be at the top corners of the ridges as illustrated in fig7 d , they may however also be or alternatively be at the bottom edges of the grooves ). within fig1 and 7 only situations are shown where the pattern essentially extends along a single linear direction . it is however also possible to have a bent structure along the direction 9 , if growth of the cellulose producing cells is to be induced along such a bend . the length 1 with the limits as outlined above is in this situation to be understood as the length along such a bent shape . possible geometrical shapes of topographical arrays on healing / dressing patches are illustrated in fig9 . while the basic geometry is gratings , as longitudinal arrays of alternating ridges and groves , this may only be useful and appropriate to longitudinal wounds or burns , provided preferably that the anisotropic topography is aligned perpendicular to the wound when the patch is applied . however , more complex geometries can be realized to interact with circular or complex - shaped wounds or burns . these geometries refer to the macroscopic pattern while at the micro - scale or submicron scale the topographical features may retain the same or similar feature size and periodicity . visual examples are given below for the case of a longitudinal wound a ), of a circular wound b ) or of a complex shaped wound c ) in fig9 . 1 a . xylinum belongs to the family of bacteria that ferment carbohydrates to vinegar and is commonly found in soil and decaying fruit . it is peculiar for its cellulose production . also other bacteria can be used having similar properties . the resulting solution is autoclaved for 30 minutes at 121 ° c . after cooling down to room temperature , 50 ml of a filtered glucose solution ( 50 % in distilled water ) is added . take 1 ml of homogenized cellulose ( containing bacteria ) and mix with 20 ml of medium within a sterile falcon tube ( 50 ml ) pour the mixed cellulose and medium into the final dish place the pdms mold on top taking care that no air bubbles are trapped between the liquid medium and the pdms mold . bubbles may result in inhomogeneities in the final cellulose patch , so should be avoided . the pdms patch will be floating on the medium , therefore it is generally more convenient to use a container that is slightly bigger than the mold . the bioreactor is carefully placed in the incubator and maintained at 29 ° c ., in a humidified environment ( 70 % humidity ) let bacteria grow for a sufficient time . the incubation time is proportional to the desired total thickness of the resulting cellulose patch . the following table provides some experimental indication of this correlation : these values can be influenced by increasing the oxygen ( partial ) pressure across the mold . at the end of the fermentation period the pdms mold and the cellulose patch are removed from the bioreactor . they will stick together . the pdms mold and the cellulose patch are immersed into a naoh ( 1 m in distilled water ) solution at room temperature the cellulose patch is carefully peeled off the pdms mold within the solution . the pdms mold is removed from the solution the cellulose patch is left in the naoh solution . the naoh solution with the cellulose patch is then kept at 80 ° c . in an oven for 80 minutes ( i . e . the whole patch is immersed in the naoh solution ; prior to that , bacteria and medium are trapped within and at the surrounding of the cellulose ; this step removes / washes ( or “ anneals ”) the bacteria ; the next steps are to replace naoh with water ). the naoh solution is then removed and distilled water is added . the cellulose patch is washed with fresh distilled water 4 times for 1 h each to remove media residues the cellulose patch ( which has the character of a hydrogel ) is then washed with a 95 % ethanol solution and then stored in ethanol until use . this procedure yields a semi - transparent cellulose patch ( in the visible spectrum ). to improve transparency ( up to 90 % of incident light ) longer incubation in ethanol ( up to 1 week ) can be used . fig1 illustrates that the resultant structures are stable upon dehydration / rehydration . the surface topography of the initially produced hydrated patch was measured with the resulting characterization illustrated on the right side . then the patch was dried out and subsequently the patch was rehydrated again and the surface structure of the same region of the patch was measured , the results being given on the right side . the same depth and periodicity is established after rehydration as before the drying . no deterioration or change in terms of periodicity and / or depth and / or shape of the structures initially established could be observed . fig1 and 12 schematically illustrate possible topographies as implemented in the corresponding mould , so the pillars to be generated in the cellulose element are corresponding indentations in the mold surface . in fig1 a hexagonal structure is shown , where a regular array of regular ( equal side length , fully symmetric ) hexagonal cross - section indentations , finally giving rise to corresponding hexagonal pillars in the cellulose patch , are arranged in a 2d array , in lines along three skewed axes tilted by 60 °/ 120 ° relative to each other . the maximum inclination angle of the vertical surfaces is preferably , in the mould , smaller than 1 °. fig1 illustrates a respective arrangement where the cross section of the indentations is circular , leading to cellulose patches with circular cross - section pillars . as pointed out above , pillar structures are preferred , but also the corresponding negative structure is possible , i . e . a structure where there is a regular two - dimensional array of indentations , and in which the indentations are all essentially identical . in fig1 a mold structure is shown for making a cellulose patch having a regular array of hexagonal indentations as a surface topography . animal tests were performed on male c57bl / 6j mice with a body weight of 25 to 35 g ( n = 18 ) in order to assess : 1 . wound healing performance of cellulose patches with topographically engineered surface . to study the revascularization process the modified dorsal skin fold chamber ( mdsc ) was used . briefly , for chamber implantation , two symmetrical titanium frames were mounted on a dorsal skin fold of the animal . one skin layer was then completely removed in a circular area of 15 mm in diameter , and the remaining layers ( consisting of striated skin muscle , subcutaneous tissue and skin ) were covered with a glass cover slip incorporated into one of the titanium frames . before skin grafting , a recovery period of 3 days was allowed . then , skin and most parts of the hypodermal fat layer were carefully removed in a circular area of 7 mm in diameter from the back of the chamber in order to create an artificial full - thickness wound . the defect on the back of the chamber was then covered with bacterial cellulose substrates and covered with a glass cover slip incorporated into the other titanium frame . surface - structured bacterial cellulose substrates were placed with gratings directed towards the wound bed . additionally , to investigate the biocompatibility of bacterial cellulose substrates , all animals received a replica of the same bacterial cellulose substrate in a skin pocket in the groin . the overall results have shown minimal inflammation in the wound bed in the presence of bacterial cellulose patch . this was as low as with a full skin graft of the same animal ( used as control ). therefore bacterial cellulose is confirmed as an ideal material for wound treatment and implantation . additionally , insights from quantitative histological collagen formation analysis and distribution revealed that the group treated with topographically engineered cellulose had ( after 21 days ) already re - established a dense , homogenously distributed layer of collagen fibers . therefore the performance of topographically engineered cellulose patches was superior to the one of identical non - structured patches . 3d cellulose structures are obtained by introducing 3d silicone molds with the bacterial culture . the silicone molds feature surface topography , an example is illustrated in fig1 . the surface topography is imprinted during the molding process itself ( with e . g . pdms ) or applied as additional layer ( e . g . by gluing with the pdms itself an already structured layer , as previously described ). the silicone mold has a thickness between 0 . 5 - 2 mm and an inner cavity of the shape , sizes and dimensions of the object to be covered with cellulose . the silicone mold can feature an additional layer at its top for facilitating its placement into the bacterial culture . the silicone mold with surface topography is placed in the bacterial culture so to allow for : the placement of the silicone mold can be helped with a bioreactor consisting of two chambers , for air and bacteria in medium , respectively . oxygen circulation within the air chamber can be facilitated by leaving the chamber open or by controlling the oxygen flow in it , by using e . g . a pump or a gas bottle with a system of valves . a schematic representation of a bioreactor configuration is illustrated in fig1 . after the culturing time , a cellulose layer is formed at the mold interface . the cellulose pouch / cover / pocket features surface topography on its internal surface and can easily be removed , washed , processed and sterilized as previously described for the flat cellulose patches . the cellulose pocket is eventually flipped inside - out in order to feature surface topography on its external surface . the target object can eventually be inserted within the cellulose pocket . the enclosing of the object can be optimized by suturing the open side of the cellulose pocket . fig1 illustrates as an example a 3d cellulose pouch / cover / pocket on a pacemaker .
a method for the self - assembled production of a topographically surface structured cellulose element . first , a mold is provided having on one side a first surface which is in a complementary manner topographically structured and which is permeable to oxygen . next , a liquid growth medium containing cellulose producing bacteria is provided . then , the mold is placed to form a interface such that the side of the mold with the first surface is in direct contact with the liquid growth medium , and an opposite side is facing air or a specifically provided oxygen containing gas surrounding . this allows bacteria to be produced and deposit cellulose on the first surface and developing on the interface a surface structured surface complementary thereto , until a cellulose layer with a thickness of the element of at least 0 . 3 mm is formed . finally ; the element is removed from the mold .