Self-sealing Pressurized Limb Enclosure
Method and system are provided for creating a self-sealing pressurized limb enclosure for the assessment of pressure effects on the limb. Embodiments can be self-sealing in that the seal is created by the positive pressure in the enclosure relative to the external environment and does not necessitate contact pressure at the seal location that exceeds the pressure in the enclosure. The seal accounts for anatomical size differences as well as deformations in the size and shape of the limb due to pressure. Furthermore, the seal maintains function in the presence of skin and tissue movement. In operation, the system can be used by an individual without external assistance.
This application is a continuation of U.S. application Ser. No. 16/074,773 filed 2 Aug. 2018, which application was a 371 application of PCT/US2017/062356 filed 17 Nov. 2017, which application claimed priority to U.S. provisional application 62/423,768, filed 17 Nov. 2016. Each of the foregoing is incorporated by reference herein
TECHNICAL FIELDThe present invention relates to the field of methods and apparatuses for managing the pressure around a limb.
BACKGROUND ARTIn some medical applications it is desirable to study the effects of pressure on a limb. One example is the optical determination of central venous pressure from the dorsal hand veins as described in U.S. provisional patent application 62/423,768, incorporated herein by reference. In such applications, it can be important that the method used to generate pressure around the limb does not create additional contact pressures on the limb that exceed the pressure of interest. The creation of a pressurized enclosure around a limb in a manner that does not utilize contact pressures exceeding the enclosure pressure is a challenging problem. The difficulty is exacerbated by the physical complexity and anatomical variability inherent to human limbs, as well as by the desire that the sealing mechanism be easily used by a single operator.
SUMMARY OF INVENTIONEmbodiments of the present invention enable the creation of a self-sealing pressurized limb enclosure for assessment of pressure effects on the limb by successfully addressing many nuances associated with human physiology and anatomy. Embodiments address criteria associated with the intended use by providing a system where the contact pressure at the seal location does not exceed the enclosure pressure or create significant local pressure gradients along the limb. Due to the physiological properties of the limb, the seal mechanism should function in the presence of skin and tissue deformations as well as movement of the tissue relative to the enclosure boundary.
Embodiments also provide other advantages associated with usability and comfort. Embodiments function in a manner that allows an individual to operate the system without additional assistance. Embodiments facilitate user comfort by not requiring the user to resist the forces acting on their limb due to the positive enclosure pressure.
An example seal mechanism comprises a rigid outer aperture and an inner flexible seal. The rigid outer aperture couples with the rigid enclosure and allows entrance of the hand into the enclosure. The aperture size can be adjusted to accommodate various sizes and shapes of the limbs under examination. The inner flexible seal compresses radially on to the limb due to the positive pressure in the enclosure, and is therefore self-sealing. The flexible seal accommodates deformation of the soft tissues and subtle movements of the limb within the aperture. The system maintains seal integrity in the presence of skin movement relative to the underlying bone structure.
Embodiments provide physical and geometrical properties of the inner seal that are important to creating an effective air seal. The seal is sufficiently compressible in the radial dimension to uniformly and consistently restrict airflow. At the same time, the seal resists forces in the axial dimension; in some embodiments this is achieved via friction with the limb, axial rigidity, or other means of stiffness or resistance to deflection in the axial dimension. The circumference of the inner seal is equally important: the inner seal must also allow entrance of the terminal aspect of the limb (i.e., the hand or foot), which may have a larger diameter than more proximal aspects of the limb, and in general is constructed such that it does not generate any circumferential pressure on the limb that exceeds the enclosure pressure.
The distance or gap between the rigid outer aperture and the surface of the limb is an important parameter. A large gap increases the axial forces acting on the seal and the limb; excessive force will result in user discomfort and potentially eject the inner seal and limb from the enclosure. A smaller gap reduces the axial force such that an air seal can be maintained. Embodiments offset these axial forces via limb support mechanisms so that the user does not have to activate muscles or otherwise resist limb movement. Embodiments' use of an elbow stop or alignment of the limb such that movement is opposed by gravity, are examples of solutions to mitigate the axial force.
Seal Junction describes the area over which there is contact between the flexible sleeve and the tissue of the limb.
Seal Location is the location of the seal junction relative to definable location, such as the plane defined by the rigid aperture.
Radial Pressure is the pressure normal to the limb surface acting towards the center of the limb.
Axial Force is the pressure acting along the axis of the limb. A positive axial force acts to push the limb out of the enclosure.
Pressure Tolerance defines the permissible limit of variation in pressure relative to a set or desired value. The pressure tolerance for typical applications is roughly 1 cm H2O.
Pressure Consistency defines a static condition where the pressure across a surface is consistent to within the pressure tolerance, i.e., local pressure gradients larger that the pressure tolerance are not present.
Non-positive angular progression configuration: as used in this document defines a configuration where progression around the circumference of the seal material results in a condition where the angular relationship between sequential point on the circumference does not result in an increase of the angle define by an line from the center of the object and the intersection with the material forming the seal. As illustrated in
Tube: as used in this document simply defines a cylindrical object for transporting with a proximal and distal opening. The object can vary in circumference along the length of the tube.
Sealing engagement, or sealingly engaged, or seal, refers to an engagement between two entities, such as between a sleeve and a limb, that provides adequate resistance to airflow. Sealing engagement does not require absolute airtightness or zero air flow through the engagement, but only sufficient restriction to air flow that the engagement facilitates the desired pressure differential across the engagement.
Properties and Features of Example EmbodimentsFor the intended use of studying the effects of pressure variations on a limb, the following system capabilities are provided by various example embodiments:
The pressure at the seal junction should not exceed the pressure of the enclosure by more than the pressure tolerance.
The seal junction creates pressure consistency around the circumference of the limb. Spatial variances in the seal quality can create failure points that allow air to escape via high velocity flow. Air leakage creates localized pressure gradients and areas of skin deformation, permitting further air leakage. A seal with pressure inconsistency around the limb is unstable and unreliable, and unsuitable for the intended uses.
The seal system can compensate for large anatomical variations in the size and shapes of limbs. This includes both variances between individuals in a population, as well as the variance in the geometry of the limb within an individual. The typical limb increases in diameter as one moves proximally toward the point of attachment, though the diameter of the terminal limb element (i.e., hand or foot) can often exceed the limb diameter at more proximal locations. The seal mechanism can accommodate varying limb diameter and maintain functionality if the seal location moves along the limb.
The system can allow for some variance in the placement of the limb within the seal mechanism. It is anticipated that individuals will move their limbs slightly within the seal mechanism during any measurement protocol. Embodiments of the present invention will tolerate or adapt to these expected small variances in limb position.
Because the limb is a non-rigid object that deforms under forces, the seal can accommodate for changes in the size and shape of the limb. Limbs are complex, non-uniform objects composed of multiple tissue layers including bone, muscle, fat, vasculature and skin. The different tissue layers vary in their physical properties and some are easily deformable. Specifically, the skin has a moderate degree of elasticity and can be compressed or stretched relative to the bones of the limb. In addition, the volume of vascular tissues is highly affected by surrounding pressures. Embodiments can accommodate for changes in the size and shape of the limb which that occur in response to variations in the enclosure pressure.
Positive enclosure pressure relative to external environment will act to push the limb out of the enclosure, potentially creating an uncomfortable experience for the user.
To facilitate overall usability, embodiments can be operable by a single individual without assistance from another party. Specifically, the user is able to insert a limb into the device such that effective seal is formed without the assistance of a second individual. In some embodiments, the user can simply place their limb through the aperture. Many other user-friendly scenarios exist, but the general goal is to minimize the number of actions that must be performed by the user.
Embodiments of the present invention provide the advantages described above, and are effectively self-sealing because the pressure used to create the seal is generated by the pressure difference between the positive pressure in the closure relative to the external environment.
System Components
Embodiments of the present invention involve the integration of three components working in concert. Components include (1) an outer rigid aperture with variable opening that allows entrance of the limb into the enclosure; (2) an inner radially flexible material that is compressed radially to create an effective air seal; (3) a design element that enables the seal to oppose the axial forces of positive pressure. The properties of each component and their integrative function are described below.
The outer aperture is sufficiently rigid such that it is not deformed by the enclosure pressure. A variable opening size is provided in some embodiments to accommodate limbs of different sizes. The variable aperture can take many forms. For example, the system can use a continuously variable aperture, such as an iris diaphragm. Such an aperture can be opened to easily allow the limb entrance into the enclosure, and then can be closed to reduce the gap between the aperture and the limb. Alternatively, the system can employ a set of interchangeable fixed apertures that are sized to be as small as possible while avoiding contact with the limb and allowing entrance of the limb into the enclosure.
An inner flexible material forms the air seal around the limb. The seal is created using the radial forces generated by the pressure in the enclosure, and in this way, is self-sealing. The radial force places the material used to create the seal under compression. Compression is a term associated with the general forces on an object and used with an awareness that any bend of a material creates both tension and compression. As used to describe the formation of the seal at around the limb, the seal material is compressed around the arm to create a seal. The material properties of the seal are an important element of the invention, and the seal must have sufficient radial flexibility such that it can be compressed to create pressure consistency.
The examples depicted herein generally show the limb extending past the end of a sleeve, for example having a tube encircle an arm while the hand extends past the end of the tube. The invention also contemplates sleeves with closed ends, for example a portion encircling an arm with a glove-like or mitten-like portion that also covers the hand. In example embodiments, an optical measurement is made of a limb while at least a portion of the limb is surrounded by a sleeve. The portion being measured can be outside of the sleeve, or can be covered by, or even completely enclosed in, the sleeve, provided that the portion being measured is accessibly to the measurement system. As an example, an optically transparent glove end to an opaque tube can be suitable in some example embodiments. As used herein, the term sleeve contemplates both structures with ends through which a portion of the limb protrudes, and structures that similarly surround a limb while also enclosing the end of the limb while still providing access as required for the measurement, e.g., an optically transparent portion.
The system also includes design elements that confer axial resistance or rigidity, enabling the seal to oppose the axial force of positive enclosure pressure. Opposing forces can be generated by the material, geometrical, or structural properties of the seal. Examples of opposing forces include, but are not limited to, friction generated between the seal and the limb, stiffness associated with tension of the seal, stiffness associated with compression of the seal, and any combination of the above.
System Operation
The constraint that the pressure on the limb not exceed the pressure in the enclosure is satisfied by using a flexible seal whose primary mechanism for creating pressure on the arm is the result of the pressure difference between the interior of the enclosure and the outside of the enclosure.
As the pressure in the enclosure is increased the seal system must respond in a manner that allows a positive enclosure pressure to be created.
The requirement that the pressure at the seal junction not exceed the pressure in the enclosure by a pressure tolerance necessitates examination of the distal aspect of the sleeve.
To create a functional seal, the forces acting on the sleeve function must sum to create a static condition. Otherwise, the seal would fail.
A concurrent consideration is associated with minimizing the sleeve force. The force on the sleeve is a function of the gap, g, between the aperture and the limb, as shown in
Under preferable conditions, the limb does not contact the rigid aperture since such contact can create pressures that exceed the pressure tolerance. Contact sensors can be used to ensure that no contact with the rigid aperture.
Pressure sensors can also provide valuable information to determine whether the contact pressure is negligible. For example, when testing individuals with less elastic skin, the gravitational pull on the tissue creates a significant sag in the skin, resulting in contact with the rigid aperture. The contact pressure due to sagging skin is often small and beneath the pressure tolerance. Thus, the use of pressure sensors in the aperture can distinguish between cases when contact pressure is negligible and when it can interfere with the measurement and must be addressed, e.g., by increasing the gap size.
Understanding of the system also requires evaluation of the forces acting to push the limb out of the enclosure.
The use of a flexible sleeve creates a system that allows the seal to move in the axial direction as the pressures on the skin create stretch of the skin. As the pressure in the enclosure increases, the sleeve force will increase and stretch the skin in the axial direction.
When using a flexible sleeve as the mechanism to create a seal, the formation of an effective seal around the limb is dependent upon material selection with attention given to the fold radius. The fold radius is the radius or curvature defined by the material under defined pressures. For visualization purposes, consider a very thin pliable piece of plastic folding back on itself. The material effectively folds back, and the resulting fold radius is remarkably small. In contrast, a piece of carpet when folded back on itself has a significant fold radius. The fold radius is defined by the physical and geometric properties of the material.
A primary material property affecting fold radius is the elastic modulus; the geometrical properties of the material, primarily thickness, are also important. A flexible sleeve can be selected such that the thickness and elastic modulus properties enable a small fold radius and create an air seal at the enclosure pressures. Materials that can satisfy these criteria include, but are not limited to, elastic materials such as latex or silicone, moderately inelastic material such as high-density polyethylene or low-density polyethylene, and fabric material such as nylon, Kevlar, and terylene. The above list is not considered an exhaustive list of materials that may satisfy the flexible sleeve criteria but rather a list of example materials.
The fact that the terminal limb diameter is often larger than the more proximal limb diameter in most individuals makes it desirable, but not necessary, to use a sleeve element with elastic properties. In this case, the sleeve stretches over the larger diameter appendage and forms a distal circumference more consistent with the size of the limb. Elastic material properties are also desirable because they allow a sleeve to return its original size and position when the deforming forces are removed. Inelastic or viscoelastic materials may not return to their original size and shape without the application of other forces, or may return slowly, limiting the temporal response of the system.
The example embodiments satisfy all the criteria described. The use of radial compressive forces to create a seal around the limb meets the requirement that the seal pressure does not exceed the enclosure pressure. The concurrent use of the flexible sleeve with sufficient friction with the limb and a minimal gap between the limb and the rigid aperture creates an overall seal system that is effective and easy to use. In use, the user simply places their limb into the enclosure through the flexible seal. As the pressure increases in the enclosure, the flexible sleeve creates a self-sealing closure around the limb, and the axial pressure force on the seal and the limb is opposed by friction and other design elements.
Additional EmbodimentsAxial Rigidity-Based Seal System
The embodiments described above used the example of a seal system where the opposing force to the axial pressure was provided by friction between the seal and the limb. The present invention also provides a seal system based upon axial rigidity of the seal. These example embodiments are not based upon a consideration that the forces due to static friction oppose the air pressure; rather, the seal provides axial compressive strength that opposes the air pressure.
The concepts demonstrated in
The resulting seal system satisfies the design requirements but accomplishes these goals without creating significant axial stress at the skin surface. Depending upon application nuances, the reduction of skin stress might be a desirable attribute. The reduction of skin stress can be important in older individuals that have more fragile skin. Additionally, the degree of skin stress can be influenced by material selection and specifically by use of materials that have a minimal coefficient of friction of the material including the distal sleeve location.
A second embodiment of an axial rigidity-based seal system is shown in
A third embodiment of an axial rigidity-based seal system is shown in
Variable-Sized Apertures
A variable aperture system can be implemented by using a set of fixed apertures that vary in size.
Variable Iris with Flexible Sleeve
A continuously variable aperture system is illustrated in
Demonstration of Applications
We include experimental data to demonstrate the principles outlined above. Data were collected from a single subject using an enclosure around the forearm. Aperture sizes were varied using a set of rigid disks, as described and shown in
The present invention has been described in connection with various example embodiments. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those skilled in the art.
Claims
1. A limb seal apparatus for use with a pressurizable enclosure, comprising a limb engagement system that allows a limb to pass therethrough, wherein the limb engagement system changes its physical configuration to create an air flow restriction responsive to a pressure gradient between the inside and outside of the enclosure, wherein the air flow restriction is sufficient to result in transmural pressure of zero in the veins of a limb passing through the limb engagement system into the enclosure.
2. A limb seal apparatus as in claim 1, wherein the limb engagement system comprises a flexible sleeve.
3. A limb seal apparatus as in claim 2, wherein sleeve mounts with the enclosure with a gap between an opening in the enclosure and the limb, and wherein the force due to friction between the seal and the limb when the enclosure is pressurized plus the sleeve's resistance to axial deformation is at least equal to the force on the seal due to pressure on the gap at pressures above a first predetermined threshold.
4. A limb seal apparatus as in claim 3, wherein the force due to friction between the seal and the limb when the enclosure is pressurized plus the sleeve's resistance to axial deformation is less than the force on the seal due to pressure on the gap at pressures below a second predetermined threshold.
5. A limb seal apparatus as in claim 2, configured to mount with an opening in a pressurizable enclosure, wherein the sleeve is configured to sealingly engage the pressurizable enclosure at the opening, wherein the sleeve has a length sufficient to engage the opening and to extend a distance along a limb within the enclosure, and allow the distal end of the limb to extend into the enclosure without exerting pressure above a predetermined threshold on the limb when the enclosure is not pressurized relative to ambient, wherein the sleeve is flexible enough to sealingly engage the surface of the limb when the enclosure is pressurized above ambient.
6. A limb seal apparatus as in claim 1, wherein the limb engagement system comprises a closure for an opening into the pressurizable enclosure, wherein the closure is configurable to a first size that allows ingress of a limb into the enclosure, and configurable to a second size that matches the outer shape of the limb closely enough to provide the air flow restriction.
7. A limb seal apparatus as in claim 6, comprising a continuously variable aperture providing an adjustable opening by adjustment of the aperture.
8. A limb seal apparatus as in claim 6, comprising a plurality of overlapping leaves, flexibly mounted with the pressurizable enclosure such that the leaves accommodate ingress of a limb into the volume, and overlap to provide a reduced opening that approximates the size of a limb placed within the overlapping leaves.
9. A limb seal apparatus as in claim 1, wherein the limb engagement system comprises a circulate ring of fibers comprising multiple overlapping fibers that allow entrance of the limb into the pressurizable enclosure through the fibers and that together provide air resistance, wherein the fibers have longitudinal stiffness so as to oppose pressure forces from within the pressurizable enclosure, and radial flexibility to allow entrance of the limb into the enclosure and radial flexibility to allow the fibers to form a seal about the arm when the pressure in the enclosure is above ambient.
10. A limb seal apparatus as in claim 2 where the seal changes its physical configuration to obtain a non-positive angular progression configuration at pressures less than 30 cm H20.
11. A limb seal apparatus as in claim 2 where the sleeve does not exert pressure above a predetermined threshold when the enclosure is pressurized above ambient.
12. A limb seal apparatus as in claim 2 wherein the flexible sleeve is attached to the enclosure and is subject to increasing axial tension due to increasing pressure.
13. A limb seal apparatus as in claim 2 wherein the sleeve has a proximal attachment to an aperture allowing access to the interior of the enclosure, and a distal aperture that circumferentially encloses the limb, wherein the sleeve's change in physical configuration are responsive to pressure increases in the enclosure causing the portions of the distal sleeve to compress against the limb while elements of the sleeve in proximity of the aperture experience axial tension.
14. A limb seal apparatus as in claim 2, wherein the sleeve comprises an air resistant material with asymmetric material properties, in a configuration such that the material's resistance to compression is greater aligned with the axis of the limb than orthogonal to the axis of the limb.
15. A limb seal apparatus as in claim 2, wherein the sleeve further comprises a plurality of battens, each batten being stiff in the axial direction, mounted with the sleeve such that the battens resist deformation of the sleeve out of the enclosure.
16. A limb seal apparatus as in claim 2, wherein the sleeve has a rigidity that is greater near the engagement with the opening than distal from the engagement with the opening.
17. A limb seal apparatus as in claim 16, wherein the sleeve has a rigidity that smoothly decreases from the engagement with the opening to a region distal from the engagement with the opening.
18. A limb seal apparatus as in claim 16, wherein the sleeve's thickness, material composition, density, or a combination thereof, changes from a region near the engagement with the opening to a region distal from the engagement with the opening.
19. A limb seal apparatus as in claim 2, wherein the sleeve is configured with accordion folds and has resistance to folding such that pressure above ambient in the volume compresses the accordion folds.
20. A limb seal apparatus as in claim 19, wherein the sleeve material has a low coefficient of friction with the surface of the limb.
21. A limb seal apparatus as in claim 19, wherein the sleeve accordion folds compress at a lower pressure than the sleeve compresses to sealingly engage the limb.
22. A pressure management system, comprising a pressurizable enclosure and a sealing apparatus as in claim 1.
23. A medical instrument configured to measure a limb in a pressure condition above ambient, comprising a pressure management system as in claim 22, and a measurement system mounted with the pressure management system to measure a portion of the limb within the pressure management system.
24. A limb seal apparatus as in claim 1, further comprising one or more pressure sensors, contact sensors, or both, mounted with the limb engagement system.
Type: Application
Filed: Aug 3, 2018
Publication Date: Nov 29, 2018
Inventors: Mark Ries Robinson (Albuquerque, NM), Elena A Allen (Albuquerque, NM), Fahimeh Salehpour (Albuquerque, NM)
Application Number: 16/053,798