A Robotic Platform to Study the Foreflipper of the California Sea Lion

The overall goal of this procedure is to create a robotic platform to study Sea Lion swimming. This matter can help answer key questions in biofluid dynamics, such as how Sea Lions move the water around them when they swim. The main advantage of this technique is that we can explore Sea Lion locomotion in detail while still being in a controlled laboratory setting.

Though this method can provide insight into Sea Lion swimming, it can also be applied to other animal systems, such as dolphins, tuna, whales, and other fish or aquatic mammals. To begin, digitize a specimen of a Sea Lion foreflipper, or obtain it from an alternate source. Print the resulting point cloud at 68 percent of the full size, and use it to create a three-dimensional mold.

Design three different pieces to mimic the bone structure of the Sea Lion foreflipper, starting with the base. In the CAD software, click on Sketch, and design the base piece so that it is proportional to the distance between the shoulder joint and the wrist of the specimen Sea Lion flipper. Add knuckles to the base by clicking on Sketch and drawing two circles at both ends of the part, then click on Boss-Extrude to extrude the desire length from the plane of the base piece.

Click on the sketch of the smaller circle at the knuckles and click Cut-Extrude to make room for the shaft. To strengthen this joint, click on Fillet to smoothen the sharp joints. Next, design the middle section.

Click on Sketch and draw a trapezoid with a height of 2.25 inches and bases of 1.625 and 0.85 inches. Then, click on Extrude and input the extruded length as 0.165 inches to get the basic three-dimensional shape of the middle piece. As with the base, add knuckles on both ends so that they will fit inside the volume of the foreflipper.

To accomplish this, make the diameter of the extruded cut 0.125 inches. Then, click on Sketch and create a tower as a rectangle on the base of the model. Extrude the sketch by selecting it, clicking on Boss-Extrude, and setting the thickness of the tower to 0.165 inches.

Finally, click on Fillet, and select the model and one edge of the extruded tower to smooth the sharp joint and strengthen where the tower and the base of the middle piece are connected. Make the length of the tip piece proportional to the distance between the knuckle joint and the tip of the longest finger bone of a Sea Lion. Do this by clicking on Sketch and sketching the desired shape on the plane.

Once the geometry is designed, click on Extrude to get the basic three-dimensional shape of the tip piece. Next, form the knuckles on both ends by making an extruded cut that is equal to the diameter of the axle, which is shown here as 0.125 inches. Finally, add a tower to the base end of the piece on both sides.

Once all of the pieces have been created, 3D print the base, middle, and tip pieces of the flipper. Then, reinforce the knuckles of the middle and tip piece by applying epoxy to the printed bone structures, and laying two layers of three-quarter inch long carbon threads onto the epoxy. Next, drill 05 inch diameter holes through the bottom of each tower, which will be use to thread the Kevlar string.

Place all of the bone pieces on a flat table. Align the knuckles of the base and middle pieces, and insert the axle. Then, use the same technique to connect the middle and the tip piece together.

Use an adhesive on each end of each axle to ensure that the axle does not move laterally. Next, cut four plastic tubes the length of the base bone piece, and two tubes the length of the middle piece. Also, cut four piece of Kevlar string, each three feet in length.

Slide one string through an eight centimeter tube and then, a six centimeter tube. Slide a second string through an eight centimeter tube. Repeat the process with the remaining tubes and strings.

Place the tubes on top of the bone structures and use tape to hold them in position temporarily. Using an adhesive, permanently secure the tubes onto the bone structure and then remove the tape. Next, thread the Kevlar string from the tubes through the holes drilled onto the tip and middle pieces.

Make a small, secure knot once the string is through the hole. First, measure 200 milliliters of silicone and silicone medium in two different containers. Pour both these liquids into a steel bowl and then add up to ten percent paint thinner.

Use a stand mixer to mix the components thoroughly for three to four minutes. If desired, add color at this step to achieve the desired visual effects. Insert a rod into the knuckles of the base part and align it with the knuckles of the flipper mold.

When the pegs fit into the cavities of the mold, the bone structure should be perfectly aligned. While holding down on the two parts of the mold, secure the parts by using a clamp for added compression. Then, carefully pour the silicone mixture into the mold, until it reaches the topmost knuckles of the bone structure.

Look for oozing of liquid from the bottom hole in the mold as a sign that the mixture is uniformly distributed. Once the oozing liquid is noticed, plug the hole to avoid further outflow of the liquid. Leave the liquid to cure for four hours before removing the flipper robot from the mold.

Next, connect the flipper to a motor through a three-pulley system, and mount the flipper to the test system as described in the accompanying text protocol. Set the motion of the flipper by selecting the jogging function on the motor driver. Pressing the up button rotates the flipper clockwise and the down button rotates the flipper anticlockwise.

Shown here on the left is a foreflipper from a female California Sea Lion, which was scanned and recreated into this robotic flipper. The digital scanned image shows a wire-frame view of the digital flipper. From this image, one can see that the flipper has an airfoil-like shape that is thicker on the leading edge and is cambered, with its upper surface more convex and its inner surface concave.

Once mounted to a recirculating flume, like the one shown here, dye can be injected upstream of the flapping flipper, and the flow pattern can be imaged. Shown here are three points along the flipper cycle. In the right panel, a vortex can be seen around the tip of the flapping robotic flipper.

Once mastered, this technique can be done in four and a half hours if it is performed properly. After watching this video, you should have a good understanding of how to create a robotic platform to investigate Sea Lion swimming.