Compliant Buckled Foam Actuators and Application in Patient-Specific Direct Cardiac Compression

Abstract

We introduce the use of buckled foam for soft pneumatic actuators. A moderate amount of residual compressive strain within elastomer foam increases the applied force ∼1.4 × or stroke ∼2 × compared with actuators without residual strain. The origin of these improved characteristics is explained analytically. These actuators are applied in a direct cardiac compression (DCC) device design, a type of implanted mechanical circulatory support that avoids direct blood contact, mitigating risks of clot formation and stroke. This article describes a first step toward a pneumatically powered, patient-specific DCC design by employing elastomer foam as the mechanism for cardiac compression. To form the device, a mold of a patient's heart was obtained by 3D printing a digitized X-ray computed tomography or magnetic resonance imaging scan into a solid model. From this model, a soft, robotic foam DCC device was molded. The DCC device is compliant and uses compressed air to inflate foam chambers that in turn apply compression to the exterior of a heart. The device is demonstrated on a porcine heart and is capable of assisting heart pumping at physiologically relevant durations (∼200 ms for systole and ∼400 ms for diastole) and stroke volumes (∼70 mL). Although further development is necessary to produce a fully implantable device, the material and processing insights presented here are essential to the implementation of a foam-based, patient-specific DCC design.

Keywords: direct cardiac compression; elastomer foam; patient-specific device; pneumatic actuation.

FIG. 1.

The direct cardiac compression device concept. (A) The device fabrication process involves the following steps: collection of a chest MRI or CT of the patient's heart, reconstructing a digital 3D model of the heart, digitally isolating the heart, 3D printing a mold from the digital heart model, casting elastomer foam within the mold, and cropping and assembling two foam chambers to form the final DCC device. Scale bar, 3 cm. (B) A device schematic showing elastomer and strain limiting layers surrounding the foam actuation chambers. Scale bar, 250 μm. (C) The device before inflation (left) and during inflation (right) with arrows showing volume displacement upon pressurization. Scale bar, 3 cm. CT, computed tomography; DCC, direct cardiac compression; MRI, magnetic resonance imaging. Color images available online at www.liebertpub.com/soro

FIG. 2.

The compressive behavior of polyurethane foams. (A) Mechanical compression of the foams used in this study, (B) a schematic of constrained compression method, and (C) a foam cube before (left) and after constrained triaxial compression to ∼40% original volume (right). Scale bar, 5 mm. (D, E) μCT 3D reconstruction (left), a representative 2D slice (right), and pore size distribution (lower) of uncompressed and triaxially compressed foam. Scale bars, 500 μm. μCT, microcomputed tomography. Color images available online at www.liebertpub.com/soro

FIG. 3.

The effect of foam compression on actuation. Extending actuation of an uncompressed (left) and compressed foam actuator (right). Scale bar, 5 mm. Color images available online at www.liebertpub.com/soro

FIG. 4.

Fabrication of the DCC device. (A) The porcine heart that served as the basis for the digitally designed mold (B) that was used to form an organ-specific foam shell. Scale bar, 2 cm. (C) The compressed foam inflation chambers that were coated in carbon fiber and sealed with elastomer to form the final DCC device (D) that fit well around the original heart (E). Color images available online at www.liebertpub.com/soro

FIG. 5.

DCC device benchtop and ex vivo performance. (A) Photo sequence of chamber inflation and deflation corresponding to maximum and minimum actuation pressures (B) recorded by an in-line analog sensor. The shaded areas indicate the duration of inflation triggered by a simulated ECG reading (C). Scale bar, 2 cm. Video available in Supplementary Data. Color images available online at www.liebertpub.com/soro

FIG. 6.

Ex vivo demonstration on porcine heart. (A) The device setup for the ex vivo demonstration on the porcine heart with (B) the collected air line pressure measurements and corresponding water flow rate during ex vivo demonstration on the porcine heart. Video available in Supplementary Data. Color images available online at www.liebertpub.com/soro