US-12618660-B2 - Implantable, stretchable sensor for continuous biomechanical monitoring of the heart

US12618660B2US 12618660 B2US12618660 B2US 12618660B2US-12618660-B2

Abstract

A modular approach for strain sensors for large deformations is provided. The strain sensor separates the extension and signal transduction mechanisms using a soft, elastomeric transmission and a high-sensitivity microelectromechanical system (MEMS) transducer. By separating the transmission and transduction, they can be optimized independently for application-specific mechanical and electrical performance. The durability of the strain sensor was evaluated by conducting cyclic loading tests over one million cycles, and the results show negligible drift. Applications of the strain sensor are suitable for human health monitoring as for example an implantable cardiac strain sensor for measuring global longitudinal strain (GLS).

Inventors

Ali Kight
Mark R. Cutkosky
Doff McElhinney
Ileana Pirozzi
Xinyi Liang
Seraina Dual
Kyung Won Han

Assignees

The Board of Trustees of the Leland Stanford Junior University Office of the General Counsel
SEOUL NATIONAL UNIVERSITY

Dates

Publication Date: 20260505
Application Date: 20240205

Claims (11)

1 . A strain sensor, comprising: (a) a transmission element, wherein the transmission element is an elongated homogeneous elastomer defining a width and a height of a cross-sectional area, a length and a Young's modulus of the elongated homogeneous elastomer; and (b) a transduction element comprising a MEMS transducer with integrated signal processing and digital communication, wherein the MEMS transducer is a barometric pressure sensor with a capacitive diaphragm, wherein the capacitive diaphragm is attached to the cross-sectional area of an end of the transmission element, such that the transmission element couples a mechanical signal of interest applied to an aspect of the transmission element to variations in stress onto the capacitive diaphragm, which is then converted into an electrical signal, wherein the transmission element acts as a spring with a stiffness, and the transduction element acts as a deformation normal force sensor that is mechanically attached to the cross-sectional area of end of the transmission element, and wherein when the spring stretches, the normal force at the cross-sectional area of the end of the transmission element increases in proportion to the spring stiffness and that normal force is then transduced into the electrical signal.
2 . The strain sensor as set forth in claim 1 , wherein the transmission element is adapted in shape to conform to a shape of a heart.
3 . The strain sensor as set forth in claim 1 , further comprising multiple transduction elements organized in a three-dimensional pattern or a cube, and further comprising multiple transmission elements, wherein each of the capacitive diaphragms of the multiple transduction elements is attached to the cross-sectional area of the end of the respective transmission element of the multiple transmission elements.
4 . The strain sensor as set forth in claim 3 , wherein the multiple transmission elements are adapted in shape to conform to shapes of a heart.
5 . The strain sensor as set forth in claim 3 , further comprising an arm cast to the three-dimensional pattern or the cube, wherein the arm is cast in a more or less perpendicular fashion to the three-dimensional pattern or the cube.
6 . The strain sensor as set forth in claim 1 , wherein the transmission element is cast with a material that is at least four times less stiff or substantially less stiff than a material of the transmission element.
7 . The strain sensor as set forth in claim 6 , wherein the material of the cast is a silicone gel at least four times less stiff than the material of the transmission element.
8 . The strain sensor as set forth in claim 6 , wherein the material of the cast is a silicone, a polyurethane, or a material with a Shore 000 hardness.
9 . The strain sensor as set forth in claim 1 , wherein the length is in a range of 10 mm to 100 mm.
10 . The strain sensor as set forth in claim 1 , wherein the Young's modulus is in a range of 40 kPa to 500 kPa.
11 . The strain sensor as set forth in claim 1 , wherein the elongated homogeneous elastomer is made from silicone, polyurethane, sterene-isoprene- rubber, or natural rubber.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority from U.S. Provisional Patent Application 63/443,421 filed Feb. 5, 2023, which is incorporated herein by reference. FIELD OF THE INVENTION This invention relates to strain sensors. BACKGROUND OF THE INVENTION The ability to accurately sense large deformations of soft structures is becoming increasingly valuable in various fields, such as soft robotics, wearables, textiles, and implantable medical devices. Despite this interest, there have been few demonstrations of highly stretchable sensors that provide consistent and repeatable results over many cycles. Conventional strain sensors, such as semiconductor and piezoresistive gauges and microelectromechanical systems (MEMS), can be manufactured with high precision for robustness and are often used in industrial applications that require a high cycle life. Nevertheless, they are limited to measuring small strains and are typically manufactured out of rigid components, inhibiting their use in applications with large deformations and soft structures. To overcome these limitations, recent research has focused on the development of highly-stretchable, large deformation strain sensors using novel materials, including, but not limited to, ionic hydrogels, conductive polymer composites, and liquid-metal-in-rubber. These methods can produce sensors with impressive signal-to-noise (SNR) ratios, but they often suffer from baseline conductive drift and unreliable interconnects between soft and rigid conductive components that limit their cyclic durability, with stable performance up to 60,000 cycles at best. Higher cycle life has been demonstrated but with evidence of sensor drift and changes in signal amplitude over time. Others have developed large deformation strain sensors by patterning inextensible conductors with rationally designed geometrical structures that impart stretchability (e.g. with a serpentine pattern), but this approach only permits stretch in a particular direction. Moreover, there is a large mismatch in bulk material stiffness between the conducting material and the soft substrate that can lead to undesirable local stress concentrations, particularly when stretch is perpendicular to the preferred direction of electrode patterning. This can inhibit the sensing of soft structures that stretch multidimensionally, which is often the case for human health and biomechanical monitoring applications. It is also worth noting that most of these sensors are intrinsically sensitive to pressure and bending, leading to difficulties in discerning between mechanical phenomena. Finally, others have attempted stretchable strain sensors that leverage the patterning or structural forming (e.g. wrinkling) of soft conductive materials, bypassing the limitation of mismatched mechanical properties but still leveraging the advantage of geometry-imparted stretchability for signal generation. However, the actual integration of such sensors remains challenging, and the development of robust, miniaturized signal amplification and readout circuitry and shielding mechanisms for practical applications of these sensors has yet to be demonstrated. In these traditional strain sensing approaches, the transduction mechanism relies on the extension of the sensing unit, which results in a fundamental tradeoff between mechanical properties and electrical integrity under repeated large deformations. SUMMARY OF THE INVENTION Clinical research has established that imaging-derived measures of global longitudinal strain (GLS) of both the right and left ventricle are informative and predictive measures of cardiac health. Such functional monitoring can be particularly valuable in post-surgical settings, such as a post-mitral valve replacement, left ventricular assist device (LVAD) implantation, or heart transplantation. Nevertheless, echocardiographic imaging methods traditionally used to obtain GLS measurements require an expert clinician and measurements are person-dependent, inhibiting their use in remote settings; moreover, these measurements are intermittent rather than continuous and may miss critical changes in cardiac function. This challenge is relevant for doctors monitoring patients as well as drug and medical device companies trying to assess the effect of drugs and therapies in pre-clinical animal models. To-date, no commercially available cardiac strain sensor exists. Direct cardiac mechanical monitoring has challenged the durability of conventional mechanical sensors due to the number of cycles of the beating heart (40M cycles/year) at high strains (15-20%). Additionally, this particular application bears requirements for mechanical and material biocompatibility, given the soft nature of live heart muscle. Although many soft sensing technologies have been developed in a research setting, they lack the ability to operate over many cycles and suffer from integration challenges. Embodiments of this invention use a