A low-cost wearable device for portable sequential compression therapy

In 2020, cardiovascular diseases resulted in 25% of unnatural deaths in the United States. Treatment with long-term administration of medication can adversely affect other organs, and surgeries such as coronary artery grafts are risky. Meanwhile, sequential compression therapy (SCT) offers a low-risk alternative, but is currently expensive and unwieldy, and often requires the patient to be immobilized during administration. Here, we present a low-cost wearable device to administer SCT, constructed using a stacked lamination fabrication approach. Expanding on concepts from the field of soft robotics, textile sheets are thermally bonded to form pneumatic actuators, which are controlled by an inconspicuous and tetherless electronic onboard supply of pressurized air. Our open-source, low-profile, and lightweight (140 g) device costs $62, less than one-third the cost the least expensive alternative and one-half the weight of lightest alternative approved by the US Food and Drug Administration (FDA), presenting the opportunity to more effectively provide SCT to socioeconomically disadvantaged individuals. Furthermore, our textile-stacking method, inspired by conventional fabrication methods from the apparel industry, along with the lightweight fabrics used, allows the device to be worn more comfortably than other SCT devices. By reducing physical and financial encumbrances, the device presented in this work may better enable patients to treat cardiovascular diseases and aid in recovery from cardiac surgeries.


Supplementary Text
Experimental Procedures: To measure the relationship between the lateral tensile force exerted by the pouch motors, Fstrap, and the pneumatic input pressure, Ppouch, we used a universal testing machine (Instron, Model 68SC-2), an electronic pressure gauge (SSI Technologies, Model MGA-300-A-9V-R), and an in-house laboratory source of pressurized air. We secured the ends of the strap into the universal testing machine's grippers and set the testing method to prevent displacement (i.e., measuring blocked force), as shown in Figure S1. We connected the pneumatic inlet of the strap to the air supply, which was set to a specific pressure on a manual regulator, and we measured the lateral tensile force exerted by the strap via a 2-kN load cell integrated in the universal testing machine. We used the digital pressure gauge to measure the value of the supplied pneumatic pressure fed into the strap.
To experimentally determine the inward radial force exerted by the strap, Fleg (which is ultimately correlated directly to the Laplace pressure, PL, applied to the leg), we used the following devices: a digital pressure gauge (SSI Technologies, Model MGA-300-A-9V-R), a data acquisition device (DAQ) (National Instruments, Model USB-6218), a force-sensing resistor (Interlink Electronics, FSR 402) with surface area of 1.77 cm 2 , several solid cylinders of various diameters, a layer of silicone elastomer (Smooth-On, Dragon Skin 10 Very Fast), and a laboratory air supply. We adhered the force-sensing resistor to the outer face of one of the solid cylinders and then encompassed the cylinder by wrapping it with the sheet of elastomer to form a pseudo-appendage. Then, we wrapped one of the straps containing a pouch motor around the elastomer-wrapped cylinder and secured the strap using hook-and-loop fasteners. We pressurized the strap to a specific pressure by reading the digital pressure gauge. A surface area correction factor of α (in this work, valued at 2.4) was empirically determined when calculating the Laplace pressure from the input pressure to correct for distribution of force through the elastomer sheet onto the sensor. The DAQ, connected to both the sensor and a laptop, read the change in voltage of the sensing circuit and sent the corresponding data to the laptop. Using the manufacturer's provided technical documentation for the sensors, we calculated the exerted radial force based on the change in voltage in the circuit.
After finding the inward radial force on the solid cylinders, we ran a similar test on a mannequin leg to simulate and observe the device's operation on a geometry with better anatomical representation (main text Fig. 4A). The devices used were a DAQ (National Instruments, Model USB-6218), three force-sensing resistors (Interlink Electronics, FSR 402), a layer of silicone elastomer (Smooth-On, Dragon Skin 10 Very Fast), the electrical control system of the presented SCT device (main text Fig.  2B, Fig. S2), and a laptop running MATLAB. Similar to the test for the solid cylinders discussed above, we embedded the three force-sensing resistors underneath the elastomeric layer that was wrapped around the mannequin leg. After connecting the device to the electronic and pneumatic equipment, the device was used to apply sequential compression through the three straps. The DAQ sent the laptop the real-time data for the voltage for each of the force-sensing resistors and was then able to calculate the force exerted on the mannequin leg over time. We performed the same test with the device mounted on a human leg. Lastly, to understand the relationship between the exerted force and the duty cycle of the micropump (Skoocom, SC3101PM), we ran an experiment similar to the one described above investigating radial force on the mannequin leg in which we recorded the force S-2 Supplementary Material for one strap. Different duty cycles (DC) were applied to the micropump, and the exerted force on the leg showed a linear relationship between exerted force and DC percentage (main text Fig. 3C, D), providing an empirically derived curve to relate these two quantities (main text Eq. 6 and Fig. 3D).

Details on the Fabrication Process:
To fabricate the enclosure for the electrical components of the device, the provided files (Electrical_Box.STL, Electrical_Box_Top.STL) can be used to 3D print the box and its cover, respectively, out of polylactic acid (PLA) or other reasonably durable materials. After printing the box, a strip of fabric with hoop-and-loop fasteners can be threaded through the slit above the base of the box to act as a strap which mounts the box to the leg. Depending on the spatial management of the circuit components that are intended to fit inside the box, the casing may be altered before printing with free-to-download computer-aided design (CAD) programs.
To fabricate the emulated leg, we used a polyethylene mannequin leg (Econoco, 00841134106370) and a 1-cm-thick layer of silicone elastomer (Smooth-On, Dragon Skin 10 Very Fast). Following the provided instructions from Smooth-On, we mixed the elastomer components together and poured the resulting liquid into a rectangular plastic box, where it cured in its final 1-cm thickness. We calibrated three force-sensing resistors (Interlink Electronics, FSR 402) and embedded them between the elastomer and the mannequin's leg. We used the provided MATLAB code (FSR_Record.m) to record the force data to a laptop connected to a DAQ (National Instruments, Model USB-6218). We ran the attached Arduino code (Leg_Force_Experiments.ino, SCT_Run.ino) onboard the Arduino Nano IOT 33 to supply the necessary pneumatic pressure, Ppouch, to the straps for our experiments.
Detailed model numbers for all the parts used are listed in Table S2 at the end of this document.
S-3 Figure S1. The setup for measuring the lateral (in-plane) force exerted by the pouch motor at a given pneumatic pressure input on a universal testing machine. Figure 4A(ii) contains a battery for electrical power, a pump for pneumatic power, solenoid valves for directing pressure to the straps, adjustable knobs to change the duty cycle of the pump (and the resulting applied pressure to the straps), and an Arduino Nano microcontroller to control all the inputs and outputs. (Right) The circuit diagram for the electronic wiring is shown.   Figure S3) measuring input pressure; (A-ii) a similar setup is attached to human leg with a thigh-mounted electropneumatic controller containing the micropump, solenoid valves, and microcontroller. (B) Open-loop controlled input pressure for the results seen in Figure 4C for the 90 mm Hg of PL. (C) Open-loop controlled input pressure for the results seen in Figure 4D for the 50 mm Hg of PL.

Figure S2. (Left) The thigh-mounted electropneumatic controller in
S-7 Table S2. The cost ($) of sequential compression therapy for different time frames. For the cost comparison provided in the manuscript, we selected 1 month as the time frame because it is a commonly suggested timespan for rehabilitation after surgery. However, some patients need longterm SCT, which would incur additional incremental costs for some of the choices such as hospitalization or equipment rentals. For those products that are widely available for rental, we used their rental prices, and therefore their costs are incremental in this chart. For other products, we used their one-time purchase price. Note that Hospitalization column depicts the rental price of Kendall 700 plus medical service, as it is the dominant product used in the hospitals. The Kendall 700 column suggest the one-time purchase price of the Kendall 700. With longer time-frames, the low-cost benefit of the our SCT device becomes increasingly more advantageous.