Hydraulically actuated artificial muscle for ventricular assist

A ventricular assist device comprises a sheet of hydraulically actuated material that can be affixed to prescribed locations on the surface of the heart to assist areas of the heart that do not contract normally. The material is comprised of a network of contractible unit cells that individually contract when fluid is pumped into them. These unit cells are connected together in a network that causes the sheet to contract radially inward. This contraction causes the sheet to transmit forces to the heart to assist in its natural contraction. A sensing function coordinates the contraction of the sheet with the contraction of the heart. The change in shape of the device is accomplished by distributing pressurized fluid throughout the spaces of the device by way of a network of channels. When pressure is removed from the fluid system, it assumes a deenergized “rest” position in which it does not transmit any forces to the surface of the heart. This property of the device prevents the device from inhibiting the heart's natural contractions in the event of a failure of the device or a loss of hydraulic power.

TECHNICAL FIELD

The invention relates to implantable cardiac assist systems and more particularly to a hydraulic implantable system that can apply a controlled spatial and temporal distribution of forces and pressures to portions of the heart surface to aid it in its natural function.

BACKGROUND OF THE INVENTION

Many of the pathologies of the heart lead to a deterioration in its ability to pump blood. In this situation, the heart itself and the organs of the body are at great risk for sustaining irreversible damage. Ventricular assist devices can be implanted to ensure that the organs of the body are supplied with an adequate flow of blood by taking over the pumping action of the heart. Ventricular assist devices can help heart transplant patients survive until a suitable donor heart is found. They can also help the heart return to normal function after heart surgery. Or they can be used as permanent devices in cases of severe heart failure where heart transplantation is not a viable option for the patient.

Most commonly used ventricular assist devices are pumping devices, which shunt blood from the left ventricle of the heart to the aorta. Use of a ventricular assist pump was shown to help promote improved heart function in some patients with heart failure. The most significant problem that patients with ventricular assist pumps face stems from the fact that the patient's blood is constantly circulated through the man-made surfaces of the device. This frequent contact with the surface of the device increases the likelihood that components of the patient's blood will react to the presence of a foreign object and result in blood clots, infections, and immune system reactions.

Another type of ventricular assist devices consists of an assembly that serves to apply pressure to the surface of the heart in order to augment or replace its pumping action. This type of device has the advantage of minimal contact with the patient's blood, and so the risk of blood clots, infections and immune system reactions is significantly reduced. The drawback of this type of device is that repeated cycling of pressure to the exterior of the heart can, lead to mechanical trauma to the surface of the heart. In addition, these types of devices do not localize the pressure to the damaged area of the heart but also apply pressure to healthy areas of the heart.

Thus there is a need to supply mechanical energy in a localized fashion to the non-functional portions of the heart muscle, thereby minimizing the potential to traumatize otherwise healthy portions of the heart, as well as minimizing the total energy required to restore the patient's heart function.

“Smart skin” or artificial muscle could in theory be used to cover the nonfunctional area of the heart and restore functionality. However, a challenge in implementing this approach is the ability to distribute sufficient energy and power in the artificial muscle, especially on the relatively small scales of interest, since in many cases the nonfunctional areas of the heart are on the order of only tens of millimeters in diameter.

Any assist device must have sufficient energy and power density in order for it to perform the work necessary to aid in circulating blood. A critical issue is whether the energy or power can be distributed in an appropriate fashion throughout the volume of the device, especially as the total physical size of the device scales down. Although electrical energy can be easily dispersed throughout the volume of a ventricular assist device, conversion of this electrical energy to mechanical energy by means of a multiplicity of small-scale-localized actuators is difficult, due to the unfavorable scaling of many of these actuators. Magnetic and electrostatic actuators cannot supply sufficient energy density for this type of device on the small scale. Piezoelectric actuators can supply the required energy density but must utilize very high electric potentials, a condition that is not ideal for biocompatibility. Shape memory alloys can also supply the required energy density but require heating and cooling cycling which would not be good for the heart, and in addition may not be able to withstand the cycling of the heart without undergoing fatigue-based failure. Thus none of these types of actuating mechanisms are well suited to the demands of a ventricular assist device.

Still another consideration in the design of a ventricular assist device is the fact that, if the device were to fail, the presence of the failed ventricular assist device should not increase the demands on a partially dysfunctional heart. If the device fails and the heart has to pump against the failed device, heart failure is rendered more likely. Thus a desirable characteristic of a ventricular assist device is that, if it were to fail, it should place a neutral demand on the unassisted heart and so not contribute to adverse clinical consequences.

SUMMARY OF THE INVENTION

Hydraulic energy is a reliable mechanical energy distribution approach with the required energy density that is a good candidate for a ventricular assist system. Therefore, an attractive approach is to utilize larger scale (e.g., cm-scale) electrical-to-mechanical energy conversion where the required forces are available in, e.g., magnetic actuators, and then distribute this mechanical power by means of hydraulics to the artificial muscle. Such microhydraulics can then form the basis of the actuation of the artificial-muscle-based ventricular assist device.

Microhydraulic technology has several advantages for use in a ventricular assist system. First, it does not require detailed computer control technology. Additionally, the device can cycle between two different conformations: one being a pressurized or “energized” form, and another being the depressurized or “de-energized” form. The shapes of these two forms can be prescribed by the device's internal microhydraulic geometry. Lastly, the device could be specifically designed on a patient-by-patient basis in much the same way that eyeglasses are prescribed according to the measured eyesight of the patient.

This invention thus relates to an implantable cardiac assist system that utilizes hydraulics to actuate a material that can supply forces and pressures to portions of the surface of the heart. The device consists of a sheet of hydraulically actuated material which is comprised of a network of mechanically-linked contractile unit cells that can individually be filled with fluid or emptied of fluid. The physical arrangement and interconnection of the unit cells prescribes the overall motion and force application of the device. Each unit cell is comprised of, e.g., a central actuating cavity inside an expandable membrane. In some cases, natural elastic forces define the contracted shape of the membrane, and expanded, pressurized unit cells reorganize the shape of the membrane to define the expanded form of the membrane. In a preferred embodiment, the pressurized device is in its most contractile position, and the relaxed, unpressurized device is in its most expanded position, so as to not place additional strain on the heart in the event of device failure.

Objects, features, and advantages of the present invention will become apparent upon reading the following specification, when taken in conjunction with the drawings and the appended claims.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENT

Referring now to the drawings, in which like numerals indicate like elements throughout the several views,FIG. 1shows a heart10with a diseased area12on its left ventricle. A ventricular assist device14is attached to the heart10over the diseased area12of the left ventricle. A length of tubing16connects the ventricular assist device14to a source of hydraulic power and control18, as will be more particularly described below.

FIGS. 2 and 3are schematic diagrams illustrating how the expansion of unit cells20can cause an associated mechanical linkage22to contract. The unit cells20are fluid-impervious membranes of a biocompatible elastic material. In this example the mechanical linkage22comprises a first connector24, a second connector36, and a third connector48. The first connector24comprises a first end leg26, a downturned leg28, a horizontal wall30, and an upturned leg32. Coupled with the first connector24is the second connector36. The second connector36includes a first end wall38, an upper horizontal wall40, an intermediate vertical wall42, a lower horizontal wall44, and a second end wall46. Coupled with the opposite end of the second connector36is the third connector48which has a first end leg50, an upturned leg52, a horizontal wall54, and a downturned leg56.

Looking first atFIG. 2, it will be seen that the first unit cell20is located between the upturned leg32of the first connector24and the first end wall38of the second connector36. Similarly, the second unit cell20is located between the downturned leg56of the third connector48and the second end wall46of the second connector36. InFIG. 2the unit cells20are deflated or deenergized, i.e., they have no hydraulic fluid in them. The linkage inFIG. 2has a length L1.

InFIG. 3the unit cells20have been inflated by the infusion of hydraulic fluid. As the unit cells expand, the upturned leg32of the first connector24is displaced away from the first end wall38of the second connector36, and the downturned leg56of the third connector48is displaced away from the second end wall46of the second connector36. The result of this displacement is that the overall length L2of the mechanical linkage has decreased by an amount equal to the combined increase in diameter of the two unit cells20. In other words, L2is substantially shorter than L1.

In summary, to create a mechanical device that contracts in size in response to the expansion of an inflatable member therewithin, the device should have a first non-extensible member having a first end and a cell-contacting portion contacting the cell on a side opposite the first end, and a second non-extensible member having a first end and a cell-contacting portion contacting the cell on the same side as the first end of the first structural member, so that the cell-contacting portions of the first and second non-extensible members are on generally opposite sides of the cell. Thus when the cell is expanded, such as by the infusion of a hydraulic fluid, the cell-contacting portions will be displaced apart from one another, with the consequence that the first ends of the non-extensible members will be drawn toward one another, thereby contracting the device.

It will be understood that such unit cells may be linked together to form chains of longer length, thereby increasing the amount by which the device will contract when actuated, assuming all of the unit cells are energized simultaneously. This action effectively duplicates the shortening movement of a muscle fiber.

Reference is now made toFIGS. 4-11as an exemplary embodiment of a ventricular assist device60will now be described. As is shown inFIG. 4, the embodiment60comprises several chains of these linked unit cells20and associated mechanical linkages22joined to a central point62at one end and radially arranged so that the opposite ends64of the chains66are equally spaced along a circle68of particular radius. This arrangement of unit cells20and mechanical linkages22can be used to form a circular sheet70that can be attached to the surface of a heart10(FIG. 1). If the cells20are empty, as shown inFIG. 4, the radius of the circle68will be at a maximum, and the sheet70will provide no force on the surface of the heart. If the central chambers of the unit cells20are pressurized, as shown inFIG. 6, the sheet70will contract radially and exert an inward force to the portions of the heart to which it is attached. The dashed line72inFIG. 6indicates the expanded, or relaxed, radius of the sheet66.

A system of channels76within the sheet70has the ability to either supply pressurized fluid to each of the unit cells20or allow drainage of fluid from them. The actuation of the hydraulic sheet70can be coordinated with the natural rhythm of the heart. This can be achieved by having a sensor that can supply a signal that indicates the onset of the natural contraction of the heart, as, known in the art. This signal can then be supplied to a control system that directs the pumping mechanism to pump fluid to the unit cells20by way of the network of channels76.

Cutouts78are provided between radially adjacent unit cells20to facilitate radial contraction of the device.

Provision to fasten the device to the heart in order to transfer the hydraulically-communicated mechanical energy from the ventricular assist device to the heart is included. Such features include suture points80in the rigid and other sections of the device60, and provision for windows in the embedding skin to expose the suture points (and also to have the beneficial attribute to alter the mechanical properties of the skin as a function of position).

Dimensions of the device are not constrained by any particular limitation, but in a typical left-ventricular assist device application dimensions might range from 20-100 mm in diameter in its relaxed, deenergized state, and might have an energized contraction to approximately 20-50% of its deenergized state, and might have a thickness of 3-4 mm.

The device has the property that if hydraulic power or control of the valves is lost the cells20will return to their emptied states. In this state, the sheet70will be in a “rest” state, and no active forces will be applied to the heart.

Although a two-dimensional radial arrangement is indicated inFIGS. 4-11as a specific embodiment, it should be noted that no particular restriction to two-dimensionality exists within the contemplated microhydraulic framework. Such multidimensionality may be attractive in more advanced implementations of the ventricular assist device, for example, to more closely mimic the true action of heart muscle. Multidimensional approaches to manufacturing will be discussed below.

A particular sequence of unit cell actuation can be prescribed by designing a fluid system that has channels76with varying cross-sectional areas and lengths. The combination of cross-sectional area combined with the total path length of the fluid to the cavities of the unit cells20will determine a time constant for the pressurization of each unit cell after the onset of pressurization of the device through its main inlet port.

Several approaches exist for the manufacture of the skin. For example, cavities, hydraulic passages, and interconnections could be formed in individual laminate layers and the layers laminated together as known in the art to form a microhydraulic structure of substantial three-dimensional complexity. However, the extreme requirements on reliability of the device (e.g., in excess of 400 million hydraulic cycles in a typical long-term application) may preclude the use of manufacturing technologies where excessive numbers of joints and adhesive bonds are formed. A more desirable structure is one in which the flexible material of the skin with all of its three-dimensional complexity is formed as a single, integral whole. Such approaches are possible using extensions of current micromachining techniques. For example, additive or subtractive micromachining techniques can be used to create a millimeter-scale interconnect network of two dissimilar materials, a first material100and a second material102(FIG. 12). The first material100has the property that it can be removed selectively using chemical, thermal, or other means in the presence of the second material as well as the material which will ultimately form the skin. The second material102has the property that it can form the rigid elements which will ultimately remain embedded within the skin and prescribe its kinematic motion. This network can then be immersed or cured within an elastomeric or other material104which will form the body of the artificial muscle (FIG. 13). Upon curing or otherwise forming the muscle body material104, advantage is taken of the fact that each central actuation cavity must ultimately be in hydraulic communication with a common fluid channel to remove the first material100, using chemical (e.g., wet etching) or thermal means.FIG. 14shows the completed device after selective removal of the sacrificial material100to form the network106of fluidic channels and common fluidic channel. Rigid elements102remain embedded in the body104of the artificial muscle.

In some applications, it may be desirable not to utilize elastomeric materials (which may have difficulty maintaining the extreme reliability required) but instead to utilize non-elastomeric, inflatable bladders or “bags” of highly durable material such as PEEK (poly-ether-ether-ketone) or other ultrareliable materials known in the medical art. The incorporation of these materials into the manufacturing process described above is straightforward, and can be accomplished by surrounding the first material100inFIG. 12with the ultrareliable material prior to the immersion in the embedding skin as inFIG. 13. This would result in the structure ofFIG. 14, with the exception that the fluidic channels would be lined with ultrareliable, pressurizable material which would potentially extend the working life of the device.

Finally, it will be understood that the preferred embodiment has been disclosed by way of example, and that other modifications may occur to those skilled in the art without departing from the scope and spirit of the appended claims.