Developing Biodegradable Acoustic Triboelectric Sensors (BATS)
Marissa Moore - Parallel I
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09/26/2024
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Student’s name: Marissa Moore
Home Institution: University of Missouri - Columbia
NNCI Site: SENIC @ Georgia Tech
REU Principal Investigator: Dr. Blair Brettmann – School of Chemical and Biomolecular Engineering and School of Materials Science and Engineering Georgia Tech
REU Mentor: Harsh Kumar Verma- School of Materials Science and Engineering, Georgia Tech
Abstract: The triboelectric effect occurs when two surfaces come into contact and generate a charge at their interface through rubbing or sliding, commonly known as static electricity. This natural phenomenon can be harnessed for applications, including wearable electronics, self-powered sensors, and energy generation using triboelectric nanogenerators (TENGs). TENGs leverage the combined effect of contact electrification and electrostatic induction to convert mechanical energy into electrical energy without the need for an external power source. However, fluorine-containing polymers, which do not readily break down in the environment, are the most popular choice for the dielectric layers in such devices. Hence, this project focuses on the development of biodegradable acoustic triboelectric sensors (BATS), which are thin film-based biodegradable triboelectric nanogenerators designed for harvesting acoustic energy. This research focuses on creating fully biobased TENGs using silk fibroin, PLLA, and paper to enable battery-free operation. Physical vapor deposition was used for the metallization of polymer films. The incorporation of a small amount of non-biodegradable polyvinylidene difluoride (PVDF) was explored to evaluate the effects of halogenated content over time, providing guidelines for the feasibility of replacing fluoropolymers with biodegradable polymers in TENGs, contributing to the advancement of eco-friendly energy harvesting devices. Device performance was assessed by measuring the voltage output using drop tests.
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- [00:00:00.720]Hey, hello my name is Larissa Moore and I'm a senior studying chemical engineering at the
- [00:00:04.960]University of Missouri in Columbia. This summer I worked with Ben Tworkash Tamar Merriman and Dr.
- [00:00:10.320]Blair Brehman at the Georgia Institute of Technology to develop biodegradable acoustic
- [00:00:14.960]triboelectric sensors or FATs. So first, what exactly is triboelectricity? Basically just
- [00:00:22.000]static electricity or the generation of a charge at the interface between two surfaces in contact.
- [00:00:28.640]This can happen through rubbing, sliding, or vibrations. So for example, when you rub two
- [00:00:33.360]balloons together and then touch your hair, you see your hair come up and that's because of static
- [00:00:38.320]electricity and triboelectricity. And it happens through contact electrification, which is where
- [00:00:44.720]the physical contact and friction between two opposing materials is going to polarize each of
- [00:00:50.560]them. And we can utilize this effect for something like actual electricity in the form of triboelectric
- [00:00:58.160]nanotemperatures. However, there are a few different factors that affect triboelectricity.
- [00:01:04.080]So, for example, we have processing conditions, morphology, and then of course chemical structures
- [00:01:10.000]and especially functional groups. So there are lots of different triboelectric series because
- [00:01:14.640]of all of these factors. There's often discrepancies. However, something that's
- [00:01:19.200]clear for most series is that metal oxides will typically behave more tribo-positively,
- [00:01:24.080]and then halogens and aromatic groups typically behave more tribo-negative.
- [00:01:27.680]So, going back to the example of rubbing two balloons together, you're going to see a small
- [00:01:33.360]charge of that, but when you take two materials that are on opposing ends of a triboelectric
- [00:01:38.000]series, you're going to see a larger charge, which is more usable, and we can maximize this
- [00:01:42.560]for our benefit. We do so in the form of triboelectric nanogenerators or tanks. They
- [00:01:48.160]typically come in two forms, either sliding or vibrational. We're going to focus on vibrational
- [00:01:53.680]because that's something that can use those acoustic and audio effects.
- [00:01:57.840]So there are two main forms of vibrational triboelectric nanogenerators, and that's
- [00:02:04.000]single electrode node and contact separation node. In single electrode node, you have a single
- [00:02:08.960]dielectric material, either tribopositive or tribonegative, directly contacting an electrode
- [00:02:15.040]like a bell. And then in contact separation node, you have two dielectric materials, once again,
- [00:02:20.240]typically from opposite ends, and they each have an electrode coating on the outside. So they'll contact
- [00:02:26.720]each other. And so in a constant cycle of pushing and pulling, you're going to experience this
- [00:02:33.400]polarization, and then a current will flow through the system. So currently, the state of the arctang
- [00:02:41.840]is SATR, or a self-powered audio triboelectric ultrathin rollable nanogenerator. However, this uses
- [00:02:49.920]FEP, which is inviolable. Some of the applications of something like SATR are wearable electronics
- [00:02:56.240]and other electronic devices, and because of that, we don't need something that's going to last forever.
- [00:03:01.120]Typically, SATRs are set up in single-electric mode, and from a top-down view, you can see that
- [00:03:08.240]you have your dielectric material as well as your polymer, and then a couple of wires coming
- [00:03:14.640]out that allow you to collect data. So what exactly do we use SATR and other things for?
- [00:03:21.120]There's things like self-powered sensors, implantable medical devices,
- [00:03:25.760]wearable electronics, and of course, clean energy generation. But once again, these are all uses
- [00:03:32.000]that we don't need the devices to last forever, and especially not in our environment. This
- [00:03:37.520]necessitates something that's fully bio-based and biodegradable, so we can control how long
- [00:03:43.360]we want the device to last. Which brings us to my motivation, which is to develop a fully
- [00:03:49.760]bio-based tribal electrical generator for both clean energy harvesting and reducing
- [00:03:55.280]electronic waste as that's been ever rising. My main research objective this summer was
- [00:04:01.520]to understand how volatile content and material additives are going to affect device performance.
- [00:04:06.800]So, the way that we characterize device performance is by doing drop tests and comparing
- [00:04:13.920]the blended output using an oscilloscope, which will, with each drop, it'll input a peak like this.
- [00:04:19.760]So, if every drop is standardized, and once again, whenever those two layers come
- [00:04:24.800]in contact, that's going to generate electricity, feed it to the oscilloscope, and we get this output
- [00:04:31.680]of voltage. For my project, I specifically focused on the dielectric layers, and I used polymers to
- [00:04:38.320]find what would work best. So, polymers were chosen because not only are they insulating,
- [00:04:44.880]which allow that charge not only to be generated, but also stored up, but there's so many different
- [00:04:50.160]types of polymers, and they're extremely diverse, meaning that we can modify them
- [00:04:54.320]for our desired results. Specifically, for the tribo-negative films, I focused on polylactic
- [00:05:00.320]acid, which is a synthetic bio-blower, and it's easily biodegradable. And then,
- [00:05:07.040]for the tribo-positive film, we looked at sulfiron, which is obtained from silkworms,
- [00:05:11.440]and it's enzymatically degradable, which is really nice because if we have a device that we want to
- [00:05:17.440]last longer, we can decide how long and exactly what we want for that. So, let's talk about PLLA.
- [00:05:23.840]Previously, my mentor did some work where he found that PLLA
- [00:05:28.720]demonstrates decreased output over time. So, what we did with this, rather than the drop test,
- [00:05:34.480]was a frequency sweep test, and on day one, they performed very nicely with about five volts
- [00:05:41.200]of output, but then as time goes on, it steadily decreases, and we were wondering why that was.
- [00:05:47.600]Our theory is that it's because PLLA is special and they use chloroform as a solvent.
- [00:05:53.360]And this is causing the produced output in PLLA devices. So the theory behind this is that
- [00:06:00.240]chloroform is highly halogenated. Once again, halogenation leads to a high tribonegativity,
- [00:06:06.560]and that's going to lead to increased device performance. However, chloroform is also
- [00:06:11.440]extremely volatile, so it's evaporating from the device over time. But to test this hypothesis,
- [00:06:17.360]we needed to look at something that was less volatile, but still halogen bigger.
- [00:06:21.810]So we chose PVDF, which has two borings, which is going to give us more halogen groups.
- [00:06:26.890]And we use this as a co-collaborator with our PLL8, blending them together in various ratios to test this.
- [00:06:32.750]So first, starting with our single electron mode devices, I tested three different blends,
- [00:06:38.690]a 1 to 1, 2 to 1, and 3 to 1 ratio of PLL8 to PVDF.
- [00:06:43.610]And so just as a reminder, single electron mode looks like that on the far left side.
- [00:06:48.650]And then from a top-down view, we used paper coated in silver as our electrode material.
- [00:06:54.290]So after a couple of drop tests over a series of eight days, we still saw a decrease in output,
- [00:07:00.930]which means that PVDF isn't the end-all, the-all solution that we were looking for.
- [00:07:05.270]But something that's really interesting is that on day six, we see this low output,
- [00:07:10.070]and then on day eight, it increases.
- [00:07:12.730]So maybe there's something to say that the chloroform is blocking the PVDF from making these triboelectric connections.
- [00:07:19.770]And then once the chloroform evaporates out of the material, we'll see more behavior from our PVDF devices.
- [00:07:26.490]Something else that's really interesting is that although our theory was that adding more halogens in the form of PVDF would help our devices perform better,
- [00:07:35.930]the groups that had 1 to 1 and 2 to 1 actually performed worse than the group that had the most PLLA in our 3 to 1 devices.
- [00:07:48.170]Now, let's move on to talk about our tribopositive layer with silk fibrin.
- [00:07:54.970]Previously, my mentor did drop tests with this and compared it to that state of Oroteng or Saturn.
- [00:08:00.690]And what we saw was almost nothing.
- [00:08:03.270]So we wanted to be able to explain.
- [00:08:05.910]So we wanted to expand the silk fibrin and see if it was actually useful.
- [00:08:09.370]The reason why we believe that silk fibrin doesn't work as well is because it's extremely hygroscopic and brittle.
- [00:08:15.970]And so because of that, it's causing a reduction compared to the standard devices.
- [00:08:19.890]So just as a reminder, this is what silk fibrin looks like.
- [00:08:23.410]And hygroscopicity is the tendency to attract and absorb moisture from the atmosphere.
- [00:08:28.250]So that means that all of those alcohol and mean groups are pulling in water in the form of H-bond.
- [00:08:36.030]And that's blocking all of this tribopositive indicator.
- [00:08:39.390]So we needed to find a solution that was going to allow us to stop these H-bonds from being front.
- [00:08:45.910]Which is why we're using glycerol.
- [00:08:47.850]So glycerol is a plasticizer, which means that it's going to increase the flexibility.
- [00:08:52.730]So these films, once again, won't be so brittle.
- [00:08:55.410]And then also it has all of these really nice OH groups, which are going to H-bond with silk fibrin, blocking water from doing so.
- [00:09:03.630]So we can have more controlled results.
- [00:09:05.570]So I tested this in the form of a contact separation mode device with both the PLLA and PVF films and the varying ratios.
- [00:09:16.370]And the trend here was really interesting because at a glance, there is no trend.
- [00:09:21.730]Everything just looks very scattered.
- [00:09:23.690]However, something that's really interesting is that despite our three-in-one device performing the best on its own, it actually performed the worst here.
- [00:09:32.430]And so we saw something weird.
- [00:09:34.650]And then additionally, something else that we saw was that two of our groups, our two-in-one and our one-in-one devices, behave the same way where they start at one point, go down, go up, and then go back down.
- [00:09:47.750]Whereas the three-in-one device behaved the exact opposite way.
- [00:09:51.570]And we were wondering why that was.
- [00:09:53.470]So going back to this idea of hybrid soliciting, we were like, maybe it's the environmental humidity.
- [00:10:00.110]So taking a graph of environmental humidity in Atlanta.
- [00:10:04.530]And inverting it actually gave us almost the exact same trend as the two-in-one in one-in-one devices.
- [00:10:09.490]But what about three-in-one device?
- [00:10:12.070]It behaves in the exact opposite way.
- [00:10:15.710]And that might be because PLLA is also hydroscopic.
- [00:10:19.390]It's not nearly as much so as silica hybrids.
- [00:10:21.810]But that slight hydroscopicity could be causing this inverse trend where PLLA is pulling the water and it's actually making the device report that.
- [00:10:30.550]So overall, what we learned.
- [00:10:34.410]Is that although previous literature shows promise for PVDF, they still perform not as well as the state of the art tank.
- [00:10:42.330]So maybe there's more work that needs to be done in this area.
- [00:10:45.110]Whenever I was doing these devices and making up my films, I always noticed spots.
- [00:10:49.970]And we believe that's the PVDF not dissolving in the chloroform and not being fully miscible with PLLA.
- [00:10:56.450]And then additionally, it was clear that despite the use of glycerol in our silica hybrids films, humidity is still impacting them a ton.
- [00:11:04.290]So we'll have to standardize with that.
- [00:11:06.590]Future work should explore a mixture of different biobarable and trigonegative biopolymers that can work together.
- [00:11:13.930]And potentially looking into other solvents other than chloroform so we can analyze the solutility hypothesis.
- [00:11:20.290]And then additionally, we should also closely track environmental humidity so that we can normalize our data better.
- [00:11:27.390]And lastly, we should continue exploring different ratios of silk fibrin to glycerin to see.
- [00:11:34.170]If we can ever eliminate the effect of environmental humidity.
- [00:11:39.430]So I would just like to thank the Bretman Research Group and then all of these different sponsors that worked with Georgia Tech this year.
- [00:11:48.670]And thank you for listening. Does anyone have questions?
- [00:11:58.790]Maybe one question. So you mentioned the PLI.
- [00:12:04.050]Possibly pulling water in during that last trial to explain the opposite trend.
- [00:12:08.310]Can you expand on why that may be? Yeah, so PLLA, if we go pretty far back, I can show you the structure.
- [00:12:15.430]PLLA is also hygroscopic. All right, pretty far back.
- [00:12:21.870]OK, so PLLA, any time something is hygroscopic, it's making H bonds.
- [00:12:26.250]So it has F, O or N. And PLLA has those two oxygens that are sticking right out, that's probably pulling in the water.
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