Rapid Manufacturing of Thin Soft Pneumatic Actuators and Robots

This rapid method uses a laser cutter and thermoplastic polyurethane to fabricate thin soft actuators with arbitrary geometry and sizes. The main advantage of the technique is that it allows quick design and fabrication of multiple thin soft actuators and robot prototypes in parallel. To calibrate a force sensor to be used in the heat press, first place a glass slide on the force sensor and put a weight on it.

Write down the force and resistance of the sensor and use a digital caliper to measure the area of the sensor. Then calibrate the sensor by dividing the force values by the measured area to obtain the pressure data and use a spreadsheet to fit a linear line to the pressure versus resistance data. Next, sandwich the force sensor between two layers of 50 by 50 by 3 millimeter silicone, place the layers inside the heat press, and turn the pressure knob until a pressure of approximately 200 kilopascals is read from the sensor.

To heat press the TPU films, put on gloves and cut four 30 by 30 millimeter layers of the material. Position the four sheets so that all four of the edges are aligned and place the sheets inside the heat press. Set the temperature of the heat press to about 93 degrees Celsius and close the heat press fully.

Keep the films inside the press for 10 minutes before opening the press and removing the laminated TPU films. To determine the optimal laser parameters, use a computer-aided design program to design a square with 20 millimeter sides and a four by eight millimeter rectangle that will act as the inlet of the square balloon. To laser cut and weld the square pattern out of the TPU layers, in the laser cutter software, set the speed and power to 10%and the pulses per inch to 500 for each value of power.

Cut the end of the inlet of the square balloon with scissors and insert a needle inside the square balloon inlet. Apply glue around the inlet and wrap the inlet and needle with PTFE tape. After allowing the glue to dry for five minutes, inflate the balloon with a precise fluid dispenser until it bursts to identify the average burst pressure of the square balloon.

To fabricate the actuator, design the desired actuator pattern using computer-assisted software and highlight all of the segments of the design. In the taskbar under the properties section, change the line weight of the selected design to zero millimeters in the software and select print changing the printer name to VLS2.30 in the menu. In the printer settings, select the paper size as user-defined landscape.

And in the plot scale section, deselect the fit to paper option and scale the image size to one millimeter for one unit of length. In the plot offset origin set to printable area, check the center of the plot option and press the power button to turn on the air filter. Turn on the laser cutter and in the laser cutter software, set the power to 80%the speed to 60%and the pulses per inch to 500.

Then use the focus view tool to move the laser pointer to the left top corner and bottom right corner of the pattern to make sure the whole pattern fits inside the prepared laminated TPU films. To focus the laser machine, move the lens carriage to the middle of the table and place the focus tool on the table. Move the table up until the top of the focus tool touches the front of the lens carriage and move the table up slowly until the lens carriage hits the notch of the focus tool and bumps the tool forward.

Without changing the position of the TPU sheet, decrease the laser cutter speed to 55%increase the power to 85%and keep pulses per inch to 500 and run the laser again. Then set the speed to 50%increase the power to 90%and keep pulses per inch at 500 for a third run of the laser to ensure that there are no leaks in the actuator. To bond a stainless steel dispensing needle with a Luer lock connection, cut the end of the balloon actuator with inlet scissors and insert a needle inside the balloon actuator inlet.

Apply glue around the needle and actuator and wrap PTFE tape around the connection. When the glue has dried, mount a camera over the actuator at a sufficient distance so that the actuator is in full view of the camera in both its pressurized and unpressurized states and hold the actuator in an orientation such that its deflection upon pressurization is orthogonal to the camera. Increase the pressure of the actuator with a precise fluid dispenser until it deflects into its full range without bursting.

Then increase the actuator pressure until it reaches about 20%of its full range and note the pressure. Finally, take a picture of the actuator and use an image processing software program to measure the X and Y coordinates of the tip of the actuator in the image. After repeating the pressurization and coordinate measurement until a full range of actuator deflection is reached, plot an XY graph of the actuator's deflection versus the inflation pressure.

Wrinkles in the laminated sheets can result in issues with bonding during the laser cutting step. Therefore, ensuring a perfectly smooth surface is critical for reproducible results. Here, a 2D design of the pneumatic actuator as drawn using a computer-aided design program is shown.

Using laser cutting, the laminated four-layer stack of TPU can be cut and welded to fit the 2D pneumatic actuator design as demonstrated. To couple the actuator to an air supply unit, a stainless steel needle can be inserted into the actuator and the interface of the actuator and stainless steel needle can be wrapped tightly with PTFE tape to prevent leakage. Finally, using a digital fluid dispenser, the pneumatic actuator can be inflated to a pressure of five pounds per square inch to allow observation of a deflection within the region in which the array of lines was designed.

Given these by which its design can be iterated, this technique has the potential to expand the use of thin soft actuators for many fields of studies.