Digital 3-D Printing of Soft Robotic Devices

ABSTRACT

This paper explores the applications and mechanisms behind the process of applying soft robotic technology to 3-D printing, specifically through the method of high precision micro-stereolithography. The ratio of PEGDA 700 to PEG 200 in the printing material of each structure determines the swelling attributes of each print, and we aim to find the optimal swelling ratio and its corresponding solution ratio while documenting the printing process for each structure. It was predicted that more PEG 200 in the solution would increase the swelling capabilities of the structure and deem it more suitable for soft robotics. The data taken via measured swelling of solidified resin shows how an increasing presence of PEG 200 in the solution created a structure more capable of swelling, although a certain limit was reached where too much PEG 200 destabilized the structure, leaving the most usable swelling structure at a solution ratio of one to one.

1. INTRODUCTION 3-D printing is the process of repetitively

laying down thin layers to ultimately for a thicker object. It is an additive process, meaning that products are assembled via compounding of multiple individual layers of printing material.¹ The main limitation of traditional manufacturing is that assembly halts when a specific piece or part malfunctions. With the rise of 3-D printing, this limitation is now nearly eliminated. Creation of products is now only limited to the imagination of the designer. A nearly infinite array of products is now available on an equally large scale ranging from meters to nanometers, a feat nearly impossible to achieve with conventional mass production tactics, especially concerning those of high-quality products. With this newfound ease of precision, previously impossible shapes and designs of higher levels of complexity are more readily available to the public. Despite such advancements, 3-D printing today is still limited by the adaptability of materials with additive printing. In addition, 3-D printing offers the benefit of on-demand, individualized production, rather than mass production of the same product.

Olivia Yao

olivia.yao.98@gmail.com

Livingston High School

Billy Zhang

zbill15@yahoo.com

Fair Lawn High School

Nicolas Ng

nng547@gmail.com

Montgomery High

School

Chris Ko

ckko817@gmail.com

Paramus Catholic High

School

Kavi Munjal

kaviraj@ccomcast.net

Lenape High School

Figure 1: 3-D printed model of the Taj Mahal,

approximately one centimeter in height.

Figure 2: 3-D printed model of an upright piano,

approximately one centimeter in height.

Similarly to many revolutionary inventions, 3-D printers began as large, expensive products that were only available to research institutions and corporations. Recently, 3-D printers have become more cost-efficient, easier to assemble, and more readily available to the public. The populous will eventually be able to create their own innovative products at home, rather than having to rely on larger manufacturers to make prototypes, parts, and solutions. Consumers can now become a direct part of the manufacturing and development process. Consumers can especially take an active role in the 3-D printing industry because of the creation of the micro-stereolithography method. Micro-stereolithography is a process that uses lasers to cure the photopolymer resins at a microscopic level.¹

1.1 History of 3-D Printing: Rapid Prototyping was first

introduced on a patent by Dr. Hideo Kodama in Japan in 1980, but the first patent issued for 3-D printing was issued to Charles Hull in 1986.¹ A year later, the stereolithography apparatus-1 (SLA-1) was developed and Carl Deckard applied for a selective laser sintering (SLS) patent. In 1990, the now renowned 3-D printing company EOS created their first product. A patent for fused deposit modelling was made in 1992 and in 2000, MCP Technologies introduced Selective Laser Melting (SLM).¹ However, widespread use of rapid prototyping was still unrealistic due to extremely high costs. In 2007, SD Systems developed a printer under $10,000, and in 2009, the first commercially available, open-source, self-replicating 3-D printer was introduced.¹ In 2012, many new 3-D printing methods were introduced to the consumer market. Processes like B9Creator and the Form 1 achieved great success.¹ This year could be described as the advent of popularity of 3-D printing in media and the modern public. 1.2 Different printing methods:

The first printing method to be

developed was stereolithography. The process starts with a vat full of resin. When a laser beam is directed across the surface of the resin, the resin hardens where the laser strikes the surface. Once the layer has been finished, a platform in the vat moves down to enable more layers to be made. DLP, otherwise known as Digital Light Processing, is similar to stereolithography. DLP uses a beam of light to solidify resin in a single pass, which makes it significantly faster than stereolithograpy.¹ Laser sintering involves the usage of a bed of compacted material. A laser sweeps over the surface, melding the particles together to make a layer before the layer is smoothed and set aside for further layers. Extrusion is the most common 3-D printing

method available. First, plastic filaments are melted a layer at a time onto a platform according to the given data. Each layer hardens and more layers are added on, which bond to each other. Binder jetting is another method which works like laser sintering, except a binding solution is used instead of a laser. Material jetting is a process where building materials (usually in liquid state) are jetted onto a heater before being cured by a UV light.¹ The selective deposit lamination 3-D printing process builds parts layer-by-layer using standard copier paper. Each new layer is fixed to the previous layer using an adhesive. 1.2 Different printing materials:

Various materials are commonly used

for printing. For example ABS, a type of strong plastic in filament form for extrusion, or PLA, a biodegradable plastic used in filament or resin form commonly used with extrusion or DLP, respectively. Metals like stainless steel and titanium in powdered form are printed using the sintering/melting/EBM processes. There has also been research into printing with ceramics, paper, bio-materials, and even food.

2. BACKGROUND

While most robots nowadays are made of hard metals, soft robotics has the advantage of producing more flexible devices. This reduces risk in many applications, especially medical fields such as surgery. Soft robotics works off of a response system to liquids (acetone in this experiment). The applications of soft robotics are widespread. Claw machines, tracks, and others can be replaced in favor of the automatic soft robotics. This replacement can save energy since the soft robotic devices automatically react and move in a certain way simply by being in contact with a liquid. Soft robotics is crucial in dealing with ever-

changing environments. Terrain travel and grasping unknown objects are hard to deal with standard robotics, while soft robotics would be able to conform themselves to these surfaces by way of their flexibility. 2.1 Photopolymerization:

The process of micro-stereolithography depends upon the photopolymerization of the resin. Photopolymerization utilizes ultraviolet light to induce solidification of the liquid resin into a solid, more stable structure. A projector directs the light in the desired shape and because the resin only reacts in the areas where the UV laser is directed, this type of printing produces extremely accurate results. To further enhance precision, photo-absorber (Sudan I) is used to prevent unwanted curing of resin from stray ultraviolet light.1

The photo-initiators are composed of molecules that react to a specific type of light and split into two or more parts. The initiator used in this experiment reacts only to ultraviolet light, the standard form of light in micro-stereolithography. When exposed to light, the photo-initiator ejects a radical that binds to the chemicals in the resin and starts the reaction. Once a layer is cured by the light, the platform in the vat immerses just enough to cover the layers that have been completed so that the UV laser can solidify a fresh layer of resin. As the reaction continues, the molecules develop a cross-linking pattern that constitutes the final properties of the finished solid product.1 2.2 Channel Size: Swelling is induced in the printed model by touching acetone to a micro-fluidic channel located on one side of the device.2 The channel is a trapezoid shaped cut in a rectangular prism. The depth of the channel should be less than half of the depth of the prism, and the ratio of the sides of the

trapezoid should be 1:2, with the opening of the channel being about half the length of the bottom trapezoid. In the cross section of the channel printed for experimentation of swelling, the ratios of the channels were: 2.4: 2.25: 4.5: 7.5: 6 (depth of cut: opening of cut: bottom of cut: width of entire structure: depth of the entire structure). In addition, the length of the entire structure should be at least ten times the width of the entire structure. 2.3 Hydrogels:

Hydrogels are networks of hydrophilic polymer chains that form a three dimensional cross-linked structure allowing the hydrogels to suspend solutions3. The hydrogel will swell on the side where the channel is cut into it. As the acetone travels up the channel, the structure absorbs the acetone, causing it to swell. In order for this to occur, the printing material must be a hydrogel. Hydrogels are able to absorb hydrophilic solutions (in this case acetone) and swell due to their hydrophilic functional groups.3 Since the channel is only on one side of the structure, there is a difference in swelling, causing the model to bend. 2.4 Capillary Action:

The acetone travels up the channel through capillary action, in which the intermolecular forces between the acetone and the hydrogel structure create movement2. There are London dispersion forces between the non-polar hydrogel and polar acetone and dipole-dipole forces between the polar functional groups of the hydrogel and acetone. Acetone is also volatile, meaning it evaporates quickly. As the acetone evaporates, swelling will decrease and the model will return to normal shape.

3. PRODUCTION OF 3-D MODELS

Digital 3-D printing through usage of projection micro-stereolithography produces high accuracy results in exchange for a relatively slow production rate. Once set up, however, the process is fairly linear and intuitive. 3.1 Construction of the Micro-Stereolithography Printer:

The printer consists of five basic parts: the light source, lens, mirror, stage, and resin bath. The light source of choice is a basic projector that is capable of projecting simple black, white, and red images4. The projector emits rays of lights through the lens, positioned so that the rays converge on a point rather than diverge and weaken in strength.

Figure 3: The above image displays the difference between converging rays (left) and diverging rays (right). Converging rays are applied in micro-stereolithography.

This pathway for light is then redirected from a lateral to a vertical direction so that the PowerPoint image such as that shown in Figure 4 can be projected onto the stage, made flat with the use of a level.

Figure 4: The above image depicts a sample slide of PowerPoint to be used in a print of a piano. The small notes in red indicating that two of these layers should be printed are in red to prevent polymerization of the “x2.”

The stage raises and lowers with high precision to allow for the structure’s multiple layers to form one after another. The resin bath simply holds the liquid resin at a steady height so that as the stage lowers between each photo-polymerization, new layers of resin are able to flow over the top of previously created layers and be solidified by the next projected image. Each layer is dropped by 160 micrometers to allow resin to fill in at a consistent amount.

Figure 5: The above diagram depicts a basic micro-stereolithography setup.2

Figure 6: The printer setup used to create soft robotic devices.

Figure 7: Side view of Figure 5’s setup.

3.2 Synthesis of Liquid Polymer Solution:

In the process of micro-stereolithography, a light source is focused on a layer of resin to harden it. In order to compensate for ambient light, a photo-absorber (Sudan I in this case) is used to prevent unwanted curing of resin outside of where it is intended on the resin’s surface. Curing is the solidification of the resin. By using photo-absorbers, the light is able to more accurately recreate the desired result.5

The photo-initiator used in this project is phenylbis (2,4,6-trimethylbenzoyl

phosphine oxide).4,6 The photo-initiator absorbs UV light from the projector and converts it into free radicals and chemical energy in the form of initiator compounds. This catalyzes a reaction between the initiator compounds and PEGDA in which PEGDA rapidly gels. The resin consequently solidifies on the surface in the area where the light has penetrated.7 Eleven different solutions were created, each containing a different ratio of PEGDA to PEG, (10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, 0:10) respectively. Each bottle contained a combined total of 50 grams of PEGDA and PEG, measured precisely via the usage of a digital balance. Following the addition of the two polymer liquids as the experimental variables, the photo-initiator (PI), phenylbis, was then added at a constant at 2% of the PEGDA/PEG solution by mass, as the ratio of total polymer to PI was kept constant as a control in the experiment. The PI, a powder, was measured using measuring paper and the digital balance before being added to each bottle. After the photo-initiator was added to each bottle, the bottle was quickly closed to prevent the polymer from reacting to outside light, after which bottles were shaken for two minutes. In an ordinary procedure, a magnetic stirrer would be used to combine the chemicals but in this case, the volume of the solution was too small to use the stirrer. Note that in this case, no photo absorber (Sudan I) was added to these solutions, as the entire solution was polymerized at once, not in layers.

Figure 8: From left to right: PEGDA 700, PEG 200,

Sudan I, Phenylbis.

Figure 9: Measuring the correct weight content of PEGDA 700 and PEG 200 in the creation of the

solution.

3.3 Production of 3-D Printed Models:

The printing method used in this project was Projection Micro-Stereolithography (PµSL). In this method, black and white images are projected by a projector through a lens and onto a flat stage. Since projectors typically enlarge images, a concave lens is required to focus the light onto a small platform. The platform is partially submerged in the resin (a liquid that will solidify into a hard lacquer or enamel). The light from the projector will solidify the printing material. Following each photo-polymerization of resin, the stage is lowered by 160 micrometers to allow fresh resin to flow over the surface of the previous layer in preparation for the next layer of photo-polymerization.

The object was first modeled using CAD software such as SolidWorks. The model was then saved as a .stl file so that XYZ coordinates may be saved. It was then uploaded into Creation Workshop to be resized and sliced into several slides of a Microsoft PowerPoint presentation.4,8 Each layer was 160 µm thick, with each print

holding around 100-120 layers, as the stage must be moved manually between each layer. Creation Workshop automatically slices the 3-D model into cross sections, which can then be transferred into Microsoft PowerPoint slides to be projected onto the resin. However, if the structure is simple and the cross section is the same throughout the model, SolidWorks and Creation Workshop need not be used, as a single image can be created in a program such as Photoshop and pasted into a PowerPoint slide. In addition, the PowerPoint can be much simpler. After the slides were focused and the projector and stage assembly were assembled, only two slides needed to be used: the slide with the black and white image of the cross section and a plain black slide. Since the cross section is the same for every layer of the print, the same slide can be alternated with the plain black slide every time.

The backgrounds for all of the slides of the PowerPoint are black so that the resin does not solidify in unwanted places. The first slide of the PowerPoint must be a black slide with red thin stripes or X’s.4 The purpose of this slide is to focus the projector, as the red light does not cause the resin to solidify. The next slide has the largest cross section of the model, but in red instead of black. After projecting this slide onto the resin, the printing assembly can be adjusted to ensure that the print will fit on the stage.

Prior to printing, a CAD model of the intended print was converted to PowerPoint via Creation Workshop software.8 Once focus was achieved using the initial slide of lines or X’s, resin was poured into the surrounding vat so that it barely covered the surface of the stage. The first layer of the entire process was exposed to light from the projector for an extended period of time to ensure complete polymerization and light binding to the stage. Following that, every layer was separated by a 160 micron decrease in stage height during which a blank, black slide was projected to allow for resin to flow across the previous layer so that additive

manufacturing could take place.9 The purpose of the black slide was to prevent any unwanted photo-polymerization while fresh resin flows over the previous layer. The 160 micrometer distance corresponded to a 90˚ turn of the knob on the stage holder used for this experiment.4 The time period during which layers were photo-polymerized was determined based on the ratio of PEG in the solution; more PEG results in a resin that requires a longer curing time, which can last up to forty seconds per slide.

Figure 10: The above images display the finishing of

a sample 3-D printed structure.2

Following the polymerization of all

slides, the resin vat was lowered to remove excess solution, and at this point the projector was shut off to prevent any further polymerization. If necessary, a razor blade was used to remove the printed object from the stage, after which it was placed in a dark container of acetone to continue the curing process.4

3.4 Detection of Swelling Capabilities:

The primary method used to test the swelling capabilities of various resins involved the measurements of discs of solidified polymers prior to and following submersion in acetone. All eleven solutions to be tested were created ranging from ratios of 0:10 to 10:0 of PEGDA 700 to PEG 200. The solutions then needed to be cured with UV light. To ensure that the samples would be cured completely, a thickness of 1 millimeter was maintained by placing two small thin slide pieces over a base slide, and then placing another larger slide on top.

Figure 11: Shown above is the UL-1000 Ultraviolet Crosslinker used to fully polymerize the 1mm thick

plates of resin during the swelling ratio test.

To prevent mess, these slides were placed on top of pieces of aluminum foil labeled with the ratios for easier recognition and transportation of the slides. Each solution was pipetted into the gap between the slides carefully to ensure that no solution flowed over the top slide. To prevent cross-contamination, a different pipette was used for each slide. Then, the slides were placed into a UV producing machine (Ultraviolet

Crosslinker) to harden. After three minutes, the slides were flipped upside down and left to cure in the machine for another three minutes. After the curing process was complete, the slides were easily peeled off and only the hardened resin remained. Each sample was placed into a clear container (labeled with the correct ratio), filled with acetone. Using a disk shaped cutter, small disks were produced on the size range of approximately three millimeters across and precisely measured for diameter using a caliper as soon as they were dry from their period of additional curing in acetone.

Figure 12: Pictured are three disks absorbing

acetone.

Following ten additional minutes of submersion in acetone, the disks were again measured for diameter, when changes in this measurement were expected and easily observed. Following this basic method of swelling detection, the solutions were chosen to be printed into structures that would bend under the swelling caused due to contact and absorption of acetone. This structure utilized capillary action through its one-sided channel that draws acetone up throughout the entirety

of a single side of the channel. A higher rate of absorption was detected via the visible bending of the entire structure as one side of the tube expanded at a much higher rate than the other. To observe this, we used a mechanical clamp to fix the tube in place, with the open channel side of it facing downwards at an angle towards a petri dish of acetone. As the acetone levels were raised to the height of the tube, the channel drew up acetone and induced swelling that would lift up the entirety of the tube on a visible level.

Figure 13: The above is a short modeled section of a channel used to transport acetone along one side of a

resin structure.

Figure 14: Pictured above is a microscopic view of

an acetone channel.

Figure 15: Frontal view of a channel taking up and

absorbing acetone.

4. RESULTS AND DISCUSSION The experimental portion of this research project consisted of two major portions. The first of these two concerned the swelling ratio of disk-shaped prints of resin. The second concerned the flexing action of printed tubes with channels along one side.

Figure 16: Printing of a channel under low light

conditions.

Figure 18: Shown above is a 1mm thick layer of resin

left to cure in acetone.

Figure 19: Shown here are three uniform disks cut out from a dried layer of 1mm thick resin.

Figure 17: The above chart displays the data taken on the average diameters

of disks of photopolymerized resin, before and after soakage in acetone.

Figure 20: The above graph displays the data taken on the average diameters

of disks of photopolymerized resin, before and after soakage in acetone.

Figure 21: The above graph displays the data taken on the ratios between dried

and soaked disks of resin with varying concentrations of PEG 200 in the solution.

Average Diameter of Polymer Disks with

Respect to PEG Concentration

Average Ratio of Wet to Dry Polymer Disks

with Respect to PEG Concentration

Figure 21: The above graph displays the data taken on the ratios between dried

and soaked disks of resin with varying concentrations of PEG 200 in the solution.

4.1 Summary of Results:

The main observation recorded during in this experiment was that more PEG 200 present in the solution resulted in a higher swelling ratio. The soft robotic material is composed of PEGDA 700 and PEG 200. When there was more PEG 200 than PEGDA 700, the material swelled much more once it came into contact with acetone as used in this experiment. This swelling is important because it is a driving force of soft robotics, enabling materials to perform actions like gripping and more. By only needing to be wetted, the material can perform actions applicable to soft robotics.

After eleven different varieties of PEGDA 700 to PEG 200 were tried out, it was discovered that a portion of the ratios did not work. Too much PEGDA in the solution resulted in the cured solution being too brittle and hard for usage, making it unusable. Too much PEG, on the other hand, resulted in the cured solution being too soft. The cured solution turned out too soft to touch without breaking it, so it was unusable as well. The only viable ratios were between 8:2 and 2:8 for PEGDA 700 and PEG 200. 4.2 Discussion:

From this experiment with soft

robotics, it can be determined that more PEG leads to more swelling in the cured solution.5 The experiment’s hypothesis was confirmed to be true. By doing so, one of the main goals was accomplished. It turns out that the original idea of having more PEG leading to having a more flexible structure turned out to be correct. This fact is extremely important because swelling is the driving force behind soft robotics. Swelling compels the robots to contract and by doing so, allows movement to become automatic, needing no programming or energy to do. However, the experiment also concluded that despite how beneficial more PEG is to the solution, too

much will not work. PEG makes the cured solution softer, while PEGDA makes the cured solution harder. Because of the softness PEG gives to the solution, the material is able to more easily bend and swell.5 However, PEGDA is still necessary as a counterbalance to PEG. Too much PEGDA will result in a brittle hard material, completely incapable of swelling without breaking. Too much PEG 200 will, on the other hand, result in a soft material that cannot be moved around without being broken. The swelling would theoretically be superb, if only the material could actually be hard enough to not dissolve upon touch.

Figure 22: Offset view of a printed channel in the

process of taking up acetone.

4.3 Error Analysis:

For the first chemical experiments, the group performed the first part of a test on the swelling ratio of five separate solutes with different ratios of PEGDA and PEG. The formula that was given included PEGDA and PEG, which composed the majority of the resin’s mass, Sudan I, a photo-initiator, and phenylbis, a photo-absorber. Unfortunately, adding Sudan I powder interfered with the outcome of the final structure, which was to be a one millimeter thick plate cured in a UV microwave.

Figure 23: Shown above is the 1mm thick sheet of

resin with an unintended inclusion of Sudan I, distinguishable by its orange color.

Typically, the Sudan I powder is applied to resin that is only cured layer by layer, which consists only of a number of micrometers. Since these layers are more susceptible to stray light radiation resulting in unwanted photo-polymerization, Sudan I is used to limit this negative effect. However, when used at the macroscopic level, Sudan I blocks polymerization of the resin past the very surface of a structure, and thus our original block of millimeter thick resin was not able to be cured all the way through. This situation was remedied by repeating this part of the experiment, this time without the Sudan I powder to prevent unwanted blocking of UV radiation into the block of resin.

Figure 24: Shown above is the corrected solution, without Sudan I, being inserted into the 1mm thick

plate holder.

Another error that occurred was the addition of the incorrect amount of photo-initiator into the mixture. Due to an unfortunate error with the instructions involving inconsistent units of measurement,

an incorrect amount of photo-initiator was put into the mixture. The solution contained 20% photo-initiator instead of the correct 2%. As a result, more samples had to be created with the correct ratio of materials.

Some more additional errors began appearing once outside-lab work was started. The first issue encountered was the lack of a level to ensure the stage was parallel to the floor, which was a problem that had to be solved manually. However, the lack of lighting control in the area that the work took place was far more consequential for the printing. Additional lighting in the dorms as compared to the laboratory resulted in unintended photo-polymerization of the resin; the product of the initial 7:3 ratio print was akin to a mass of undifferentiated resin instead of the intended basic acetone channel that was previously designed. To fix this, all available lights were turned off and all window were covered with sheets to reduce stray lighting. Additionally, the micro-stereolithography printer was covered with an umbrella to create more of a shadowed area. An inconvenience also appeared in the later stages of the initial 3-D printing attempts. The stage dial became increasingly hard to turn until eventually a small pop was heard and the stage dial became extremely loose and ceased to have an effect on the stage. Upon close examination, it was found that the mechanism controlling the stage dial was jamming into the base of the structure. Once this was discovered, the issue was rectified by adjusting the mechanism to its highest point before beginning each print. Another error involved geometrics involved with the actual printing of a 7:3 ratio sample Due to a failure of accounting for the lowering mount of the stage in relation to the mirror, around the fortieth layer of resin, the mirror came into contact with the mount and threw off the focus of the projector onto the stage for the rest of the print. This was remedied by reseating the mirror at a more elevated and retracted position.

A common problem involved in the process of digital 3-D printing via micro-stereolithography is a failure for the first layer to sufficiently adhere to the stage. This is most commonly a result of an inadequate period of exposure for the first layer of photo-polymerization. The consequence for this mistake is typically a loose first layer that is free to move about in the solution, negating any attempt to accurately place following layers on top of the first. Needless to say, this fails the entire print and is most commonly restarted following this failure. To remedy this, the first layer is often given a much longer exposure time to white light, up to two or three times the typical amount.

An experimental inconvenience involved the testing of the sample solution disks. For the PEGDA and PEG solutions of ratios 10:0, 9:1, 1:9, and 0:10, swelling data could not be taken. The 0:10 solution could not be solidified at all using UV light. The 1:9 ratio solution was solidified after several extra minutes of UV light exposure, but after soaking for 24 hours in the acetone bath, the solidified solution was too soft and flimsy to be retrieved from the bath and easily ripped. On the other extreme, the 9:1 and 10:0 solutions successfully soaked for 24 hours and dried out for 10 minutes. Once dried, however, the samples were too brittle to cut into the sample circles and easily chipped apart.

The samples were cut into circles using a circular sample cutter. A potentially large issue arose when the samples were found to easily get stuck in the cutter. However the problem was solved by inserting a pipette into the opposite end of the sample cutter and pushing it out. Tweezers were then used to pull the sample off of the end of the cutter.

Many trends appeared as the samples dried. More PEGDA in the sample correlated with a harder, clearer, more brittle substance. These samples also curled more and some even began to crack. They also were tested after the samples containing a higher ratio of

PEG, so those samples spent slightly more time drying and more acetone was subsequently able to evaporate. The samples with more PEG remained soft, shrunk from their size in the bath, and turned a whitish color.

5. CONCLUSION

Our experiment concluded that a strong correlation exists between the PEGDA 700 to PEG 200 ratio and the swelling capabilities of 3-D printed structures; consequentially, there exists a strong relation between this ratio and the application of 3-D printing in soft robotics. As the PEG 200 concentration grew in strength, the increasingly porous molecular structure of each print allowed for more and more acetone to flow into and expand the entirety of the material. The hydrogel nature of both PEG 200 and PEGDA 700 aided in this activity, but it was undoubtedly PEG 200 that had the greater influence on swelling properties as supported by our disk swelling experiment data. Initially, this finding implied that to induce usable swelling in soft robotic structures, more PEG 200 would be the only relevant factor. However, the increasingly soft texture of PEG 200 concentrated solutions had other implications for actual printed structures using the printer instead of simply a UV oven. Past the 3:7 ratio mark of PEGDA to PEG, the structures became so unstable that any sort of application became unrealistic. Swelling was still observable in ratios of around 1:1, and those prints still held a fairly stable structure. In real-life application, it would be that 1:1 ratio that could still be used whereas the others would either exhibit weak swelling or structural properties. Although there were solutions of resin that exhibited noticeably higher ratios of swelling, such as the 3:7 ratio of PEGDA to PEG solution with a ratio of 1.507 (wet diameter to dry diameter), it was the 5:5 solution that yielded the most effective

balance of stability and swelling capability. For future work to be done, studies must be made into further additions in the solutions that could improve the functional properties of these structures so that swelling capability and stability need not be traded for each other. The porous structure of PEG as a solid polymer acts as both a benefit and a drawback, and alternative options that avoid this situation would greatly improve the viability of 3-D printed soft robotics in various fields such as the medical or surgical practices.10

6. ACKNOWLEDGMENTS

The authors would like to acknowledge their project mentor, Dr. Howon Lee, for his guidance and mentorship during this research, as well as Daehoon Han, Lucas Lu and WonYoung Choi for their assistance in the project. The group would like to thank Residential Teaching Associates Eamon Collins and Edmund Han for enabling the completion of this project and for their help in editing this paper. They would like to acknowledge Dean Jean Patrick Antoine for his guidance and for securing resources and facilities for this project. The authors would like to thank Dr. Ilene Rosen, Director of New Jersey Governor’s School of Engineering and Technology, for making all of this possible. Finally they would like to acknowledge the sponsors: Rutgers, the State University of New Jersey; Rutgers School of Engineering; The State of New Jersey; Morgan Stanley; Silver Line Windows and Doors; Lockheed Martin; South Jersey Industries; Novo Nordisk Pharmaceuticals, Inc.; NJ Resources.

7. REFERENCES

¹ 3D Printing Industry, “3D Basics: The Free Beginner's Guide,” http://3dprintingindustry.com/3d-printing-basics-free-beginners-guide/ (18 July 2015).

² Howon Lee, Biomimetic Microactuator Powered by Polymer Swelling, (ASME, 2008).

3 Enas M. Ahmed, “Hydrogel: Preparation, Characterization, and Applications: A Review,” March 2015, http://www.sciencedirect.com/science/article/pii/S2090123213000969 (18 July 2015).

4Center for Nanoscale Chemical-Electrical-Mechanical Manufacturing Systems, 3-D Printing, (National Science Foundation).

5 Kairu K. et al., “Parameter Optimerization For Photo Polymerization Of Microstereolithography,” 2013, http://www.academia.edu/7710357/Parameter_Optimerization_For_Photo_Polymerization_Of_Microstereolithography, (18 July 2015).

6“Phenylbis (2,4,6- trimethylbenzoyl) phosphine oxide,” http://www.sigmaaldrich.com/catalog/product/aldrich/511447?lang=en&region=US, (18 July 2015).

7 “Applications: Free Radical Initiators,” https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Aldrich/General_Information/photoinitiators.pdf, (18 July 2015).

⁸Michael Weinberg, It will be Awesome if they don’t Screw it up: 3D Printing, Intellectual Property, and the Fight over the Next Great Disruptive Technology, (Public Knowledge).

⁹Mathew Napoli, The Use of Additive Manufacturing Technologies for the Design and Development of Cubesat, (San Jose State University, 2013).

10TE Edwards, “Scientists Use Microstereolithography 3D Printing to Repair Damaged Nerve Connections,” 23 February 2015, http://3dprint.com/46500/ngcs-repair-damaged-nerves/, (18 July 2015