Making DIY gecko tape - work in progress

today on Applied Science I'd like to talk about my adventures in making gecko tape if you haven't heard gecko tape is sort of an alternative to the currently available adhesive tapes it actually works by a different mechanism and it's not really a commercial product yet but it's gotten a lot of press in sort of popular science articles and so it has a few attractive qualities that make it better suited than these commercial adhesive tapes so first let's talk about how these actually work each one of these tapes has a layer of adhesive on it there's actually a chemical on here and the reason it's sticky is because it makes a chemical bond with whatever you're sticking it to and so the problem with this is if you get it wet or if it gets dirty the chemical becomes coated with water or the dirt and then that the tape is no longer sticky so if you're constantly taking the tape off and replacing it eventually the adhesive will wear out and the tape is no good anymore the difference with gecko tape is that this works by a mechanical method it's actually using a different sort of molecular bonding and so in this video I'm going to talk about all the things I've tried so far unfortunately it's not working just yet but with your help hopefully it will like the name implies gecko tape is actually modeled after the foot of a gecko and so part of the reason that the geckos foot wouldn't work with a conventional adhesive like this is because it would quickly become loaded with dirt and the gecko wouldn't be able to stick and walk upside down which is critical to its survival so the gecko tape is sort of our answer or at least our you know modeling of this sort of natural phenomenon and the trick is that the geckos foot has a whole bunch of hairs on it and the hairs get into very intimate contact with the foots with the surface that the foot is stepping on and it's these small intermolecular bonding forces that produce an attractive force between the hair ends and the surface that it needs to stick to so most surfaces aren't perfectly flat they're not atomically flat so if you want to get something into very close contact with it it needs to be very conformable and even something soft like this silicone rubber it's really not you know you'd think well if the silicone rubber has a flat surface here and we put it down on something flat at the end back but not really at the microscopic level there's always going to be you know hills and valleys and it's not going to make really great contact so nature's solution to this problem was to have a structure that has very small hairs and when you push that down onto a surface sort of the end of each hair will make really good contact with the surface and then you know if you have millions of these little hairs it's basically almost as good as having a solid flat piece into you know perfect atomic contact so whereas the commercial adhesive tapes use perhaps hydrogen bonding or something like that where you have this chemical adhesive layer that's going between the tape and the surface the gecko tape works by Van der Waals forces which are present between all molecules as far as I know but the trick is that you have to get the two surfaces really really close together like less than you know one nanometer a few nanometers or something it basically has to be in contact and as I mentioned even if you cast a perfectly flat piece of tape and you put it on to a perfectly flat object like this it's not even close to good enough contact you really need this conformable architecture so the gecko has adapted sort of a two layer system where it has pillars coming off of its foot and then there's even hairs that are a finer structure even coming off of that and so between the pillars and the hairs it can really conform and have a huge amount of surface to surface contact the actual material itself of the hairs and the thing that it's walking on is not particularly important it's just the fact that it's such in such close contact is what makes this whole system work so anyway so I'm really interested in this and I searched the internet for instructions on how to make gecko tape or at least you know what the techniques are being used in these research labs and lo and behold I actually found detailed dedicated instructions for how to make your own gecko tape at home and if you had part numbers for all the you know stuff that you'd need and this is sort of the key ingredient here so the idea is that you make the Gecko tape by casting the silicon rubber and it's pretty easy you just mix it up kind of like two-part epoxy but the the ratio is much greater than one to one and you pour this onto a mold and hopefully the mold has you know holes in it just like these these we want to form pillars and so the mold has to have holes in it so we're going to pour the silicone on to that and then D mold it and then you will have like you know this forest of pillars coming off and that's the structure we need to make this whole thing work so it sounds reasonable and the instructions are very clever so they're going to use this interesting membrane here too to make our silicone cast and of course I'll put links to all of this in the description so definitely check that out these are actually pretty cool this deserves a little one minute side note all by itself this is a track edged membrane filter and so the idea as I mentioned we want a whole bunch of little holes in here so that when we cast our silicone rubber it goes down into the holes and we end up with this forest of pillars so the way that you make this track itched membrane is you start with a really thin sheet of polycarbonate standard you know just just like unbreakable safety glass or whatever and you expose it to a radiation source I believe it's probably a beta source but I couldn't quite find out for sure and the trick is the beta particles are going really fast and they'll actually go all the way through the polycarbonate sheet because it's so thin we're talking 10 or 20 micron or something like that and then you put this into an etchant and where the beta particles have gone through this really thin polycarbonate sheet it's actually weakened the chemical structure of the plastic a little bit and the etchant will attack only the weakened areas so as the beta particles go screaming through these sheets then you put this into the etchant bath you'll end up with a hole edge in just the places where the particle has gone through and the longer that you leave it in the etchant bath the wider the holes will become because it sort of edges from the inside of the hole out so it's a really easy way to get a very uniform hole size so let's take a look at one of the what if one of these looks like under the scanning electron microscope this is the millah pore membrane that was mentioned in the instructions for making gecko tape and it has a 5 micron hole size and it's pretty much identical on the top and the bottom I also bought GE Walkman brand filters and these have a three micron hole size but I can tell they're built or they're made with a different process because the one surface is very shiny and the other surface is very rough so clearly the etchants that they used to make these filters was a little different it actually attacked the polycarbonate a bit more so you can kind of tell which sizes which side of the filter was exposed to the etchant anyway so I thought that was pretty cool what you do according to the instructions is you take one of these filters with the tiny holes in it and put it down onto some double-stick tape and then you pour the silicon you know you mix up a batch of silicon this is actually tin cure silicone RTV silicone from tap plastics and amazingly enough the silicone will actually flow down into those five micron holes all the way to the bottom even so through the thickness of this membrane and then when it cures and you peel this off you end up with the structure we wanted to make this you know simulated gecko foot and it all works the way it's supposed to but the structure that it creates doesn't function as well as a gecko foot in fact it functions less well than a solid slab of silicon as I'll show you so just to give you a rough idea of how good the silicone is by itself even though I spent a lot of time saying well you know if you put something in contact like this even though the contact area even though it's flat is not very good because there'll be you know microscopic hills and valleys it's actually pretty good so just to give you sort of an idea of how sticky this thing is I just sort of stuck it down like that with almost no pressure applied I can pull this heavy piece of plastic around if you can see then okay so then this is the silicone that I made by casting the silicone in the membrane just like it was said in the instructions and if we put this on here I know that it's kind of hard to see on the camera you really need to get a tactile sense of this no matter how much I baby this and no matter how careful I am the friction coefficient with the so-called gecko tape is way way worse in fact this is almost sort of an anti friction material like this is really kind of it's worse than then just flat I mean it's basically worse than a lot of other things so it doesn't work and then I started searching around on the Internet a bit more and found someone's thesis or dissertation where they actually mentioned these instructions and how it couldn't possibly work for a lot of reasons that we're going to get into so you know I wanted to see it first so let's take a look with the scanning electron microscope again at what these pillars look like okay you can see that yeah the casting process did work and you can also see that the direction of the tracks that are etched through these plastic is not you know they're not perpendicular to the surface they're actually kind of all different directions and that's because the radiation source doesn't just shoot out you know beta particles in one direction they're kind of going all different directions so that's one problem the other problem is that the pillars have a really high aspect ratio they're kind of like wet noodles just you know falling down over there because they're too long and so I mentioned that the thickness of this membrane is like 10 or 20 micron or something like that this is one of them right here you can see that it's really really thin in fact it's so thin that just the static attraction makes it really kind of difficult to handle it's it's is thin stuff but it's still way too thick so the relative thickness of this thing compared to the 5 micron you know pillar diameter is so high that the pillar just folds over and we don't really want it to fold over like that what we want is the thing to be you know fairly stiff so then we put this down on to the surface the pillars kind of have enough rigidity so that the ends of the pillars touch the surface and make a good contact there if they're all just flopping around then you know the whole gecko thing doesn't work so I did a little bit more searching and found another group that moves instead of doing straight pillars like this they were doing angled kind of wiper blades almost you know again microscopic we're talking you know a couple microns thick or a couple microns long and maybe a pitch of 500 nanometer and I thought well that's pretty cool maybe there's actually a structure that I can find kind of around in everyday life that sort of mirrors this because they were using photolithography to make their parts which I'll talk about a little later and I came up with the idea of casting the silicone on to really really fine metal files so I did a little bit of research and found out that you can get these swiss metal files that are quite a bit finer than the the so called american grades of files so typically a smooth file is the smoothest thing you can get commonly at least you know in this country around here if you go to your local hardware store in the US a smooth file is considered the final the finest thing you can get but if you go online to a specialty store you can get this Swiss file and I've never seen a file this fine this thing is really really fine I think the pitch is around 20 or 30 thousand inch I'll put the exact numbers in the description but anyway I made a whole bunch of different castings with different files and different viscosities of silicone you can kind of thin the silicone and make it into a stiffer or more flexible stuff and you know as you might expect the results aren't that great but they are unidirectional surprisingly so I did find that with a specific file pitch I kind of have to put a little bit of weight on the tape too to make this work but you can kind of see in one direction it works about that well and then in the other direction it's not even close to as good and the reason is that the the teeth are you know asymmetric and when the teeth are getting pulled sort of in their direction they kind of fold down and create more contact with the surface that they're bonding to if they're going the wrong way when you pull them this way they just sort of buckle and flap around and they don't create that great contact so if you are interested in using gecko tape is kind of a robotic foot or grabber or something having a unidirectional adhesive surface is actually very nice because you can make like a wall climber you know gravity is always trying to pull you down so you can make the wall climber you know you can make the foot basically work just the way you want so if you put a lot wait on it you can get Goods diction and then to lift the foot you actually push it upwards and it will instantly detach because it's you know a one a one directional gripper so after this I realized there's no way I could get a metal file that was fine enough to get you know close enough contact again another problem with the metal file is that the teeth are sharp on the top but unfortunately we're making a casting and we actually want the teeth to be really sharp in the bottom because when we do the casting and peel it off it's actually the tips of the casting that are going to be interacting with our surface and that's going to be formed by the bottom of the of the file so you know that basically just the manufacturing this isn't really made for it even though it's a good idea it was it's not really going to work out so then I had another great idea what about using a CD as the former so that's pretty cool so we're talking this thing has a pitch between the pits and lands with you know a few micron or something on that order but I didn't know how deep they were so I was able to make a silicone cast of a CD and it is this one so I came home one day after letting my silicone cast there and tried it out and I was actually really excited a it works really well in fact way way better than the files way better than the you know official gecko tape instructions and then I you know of course tried it compared to the actual just flat cast on glass silicone and it's it's not even quite as good as that so so clearly there's still some more work to be done here the biggest problem is that the aspect ratio of these magic pillars is not quite right it needs to be about four to one so if the if the spacing between the pillars or if the size of the pillars is on the order of 500 nanometer that means the depth has to be 2 micron and in the CDs case if it's like a few micron in diameter then we're going to be like maybe 4 to 8 micron deep or something like that and so far I haven't found any kind of easily accessible mold that has this correct aspect ratio and a high density of the pillars the other problem with the track etched membrane is that these you know wet noodles are just spread out way too far if you want like dense packing to make this thing work so all of the research groups that are doing this are pretty much using photolithography which involves you know casting a photosensitive layer and you can control the thickness of the layer by spinning it in a spin coater and if you'd the faster you spin it the more you know so difficult force we get it makes the layer thinner so that we can control the thickness that way and then what you do is expose your you know micron scale pattern with ultraviolet light and etch it away so we can basically make little holes of almost any width and depth that we want with this system the problem is that I don't have any way to print micron scale stuff so I've done photolithography before but I've always used a laser printers transparency and that's there's no way to get down to microns with that so I need like a lensing system but then the lenses don't pass ultraviolet light and you know there's a lot of problems involved so I need your help to figure out how to pull this off there's a few interesting ideas that well I'm sure will come up in the comments but basically we need to create this you know about 500 nanometer by 2 micron pillar and have it really dense on a surface and something that we can cast like silicon and then I'm pretty sure it's going to work and we'll actually have deco tape okay see you next time bye

Channel: Applied Science

Views: 218,088

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Id: 9XQfYKYO380

Length: 16min 48sec (1008 seconds)

Published: Fri Jan 01 2016