To further validate our findings and to give us a path forward we performed both compression testing and a degradation study.
Mechanical Compression Testing
To build edible pneumatic actuators, we melted gummy bears at hot plate setting of 160℃ and 260℃ (the actual temperature of the liquid was not measured) and cast the material into 3D printed molds. After 24 hours we removed the melted gummy actuators from the mold. We noticed that they were rather sticky. This increase in stickiness confirmed our suspicion that melting the gummy bears may cause some sort of chemical and mechanical change in the gummy material. Gummy bears are primarily made out of gelatin, corn syrup, and sugar. We believe that melting gummy bears breaks down the gelatin triple helix and when the melted gummies solidify it is expected that the triple helix structures reform. We performed compression tests on recast samples as well as native gummy bear samples using our Mechanical Testing System over a number of days to attempt to find out after how many days does the melted gummy bear start to have the properties that it originally had. We found that after around 3 days, recast gummy actuators start to have the stiffness of native gummy bears.
| Figure 44: MTS Testing |
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| The above figure displays the MTS testing of a gummy bear. Our Mechanical Testing System has a 100N load cell which compress materials at a constant velocity as data is recorded every 0.1 seconds. |
| Figure 45: Graph of Gummy Bear Stiffness |
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The above Figure displays the graph of the stiffness of the gummy bears with respect to time. The error bars indicate the standard deviation across 4 samples tested at each condition.
We took the average Strain vs. Stress for all of our samples and plotted the average values with associated standard deviations. As seen in Figure 45, after three days, the the data from the recast gummies, irrespective of temperature, overlap with the data from the native bears. While the recast gummy bears at day 7 were more elastic, they lacked visual and tactile appeal.
Upon completion of this experiment using MTS, it became clear that stress-strain relationship while a necessary condition, was not sufficient to define the right properties for a gummy bear like candy. The recast gummy bears (after 3 days of recasting) were sticky to feel and did not have the same texture as that of the native gummies at all. These observations prompted us to continue the search for a better formulation since the touch and feel of a candy is an important attribute.
In an effort to design a new material that achieves the mechanical stiffness of gummy bears in a shorter period of time (note, in the above experiment, we needed to wait 3 days) along with the other qualitative attributes of a gummy bear, we performed mechanical testing to determine the optimal ratios of all of the materials which are used to make gummy bears to make our own formulation.
| Figure 46a: Graph of Material Stiffness |
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Note: Fordmula: n=1, Control and Day 3 160C: n=4. 160C Day 3 refers to the actuators made from gummy bears. We made them by heating gummy bears to 160C and after casting them we waited 3 days. Adding the silica gel desiccant the wait time changes from 3 days to 1 day.
The Fordmula after one day had higher stiffness than the native gummy bears and the recast gummy bears which we used to make our gummy bear actuators. This shows that after less than a day, the Fordmula had a higher elastic modulus than the native gummy bears and the recast gummy bears we used in our gummy bear actuators. This means that after an inflation the Fordmula would want to return to its original form more quickly. We also noticed that the Fordmula was not sticky at all which is a problem we encountered with our gummy bear actuators. This, both quantitative and qualitative, data allowed us to call our patent pending Fordmula a success.
Upon the creation of the Fordmula we performed compression testing on all of the different material actuators we created. We did a comparison between actuators made out of silicone, the Fordmula, and recast gummy bears. We added the control group of native gummy bears because it serves as a good reference point.
| Figure 46b: Graph Comparison of all Materials |
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As seen in the graph the Fordmula is exactly what we had in mind when creating it. We wanted to create an actuator that has the ability to inflate numerous times and was easy to eat. The Fordmula has this exact ability as it is not as stiff and elastic as the silicone material but is still stiffer and more elastic than the gummy bear materials. This is good because if it was as elastic and stiff as the silicone it would be extremely difficult to bite and chew. The fact that it is stiffer and more elastic than the gummy bear materials show that it is stronger and can be actuated more than the gummy bear actuators. This also provided more proof as to the success of the Fordmula.
Degradation Study
Tasked with the challenge of improving the field of soft robotics, the team postulated the primary material of soft robotics should be biodegradable and biocompatible. Traditionally, silicone has been used for the creation of actuators, but this material is neither biodegradable nor biocompatible. Recently, researchers at Harvard University and Boston Children’s Hospital have created a thin silicone heart sleeve. The sleeve serves to help the heart beat. We thought it would be cool to have a biodegradable heart sleeve, but, of course, didn’t attempt because we do not yet have the resources to test such a device at our high school. Instead, we conducted a study as a proof of concept with our gelatin-based candy actuators - for the purpose of showing our material is biodegradable. We used a degradation study to exhibit and simulate how our product’s material would degrade inside the body over time. This study was split into two experiments. In both experiments, the degradation was studied in a 8.0 g/L NaCl solution. We based the solution off of Tyrode's solution.
The first of the two was conducted with an actuator made out of orange melted gummy bears and a small square piece of the material made out of gelatin, water, and corn syrup, referred to as “3.2” or the Fordmula (Refer to The Fordmula section for information on the material).
| Figure 47: Degradation Study |
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| The figure above shows the degradation of two actuators over time. The one on the left made from gummy bears. The one on the right made out of the Fordmula. |
The second experiment was conducted with an actuator made with the Fordmula.
| Figure 48: Graph of Actuation Mass vs. Time in Degradation Study |
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| The above graph shows the actuator mass over time of the Degradation Study. |
| Figure 49: Graph of Actuation Volume vs. Time in Degradation Study |
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| The above graph shows the actuator volume over time of the Degradation Study. |
The results of this study exhibit that our product is indeed biodegradable. Additional products may be added or ratios can be altered with the material to make it degrade at a faster or slower rate in the body. This is shown by the juxtaposition of the degradation of gummy bears and the Fordmula. Combined with the natural biocompatibility, our product’s biodegradable nature makes it a very strong candidate for use in the form of a soft robot in the body.
Success
Below is a video of our gummy bear actuator making multiple actuations which show that we had accomplished our goal and that the actuators work. We performed 8 actuations in the video (below) which is more than double our stated goal of 3 actuations. We also made actuators which tasted good. We gave our actuators to a small group of young boys who all enjoyed eating the actuators proving that they tasted good.
| Videos by Intel Chen |