Method of making self-cleaning skin-like prosthetic polymer surfaces

An external covering and method of making an external covering for hiding the internal endoskeleton of a mechanical (e.g., prosthetic) device that exhibits skin-like qualities is provided. The external covering generally comprises an internal bulk layer in contact with the endoskeleton of the prosthetic device and an external skin layer disposed about the internal bulk layer. The external skin layer is comprised of a polymer composite with carbon nanotubes embedded therein. The outer surface of the skin layer has multiple cone-shaped projections that provide the external skin layer with superhydrophobicity. The carbon nanotubes are preferably vertically aligned between the inner surface and outer surface of the external skin layer in order to provide the skin layer with the ability to transmit heat. Superhydrophobic powders may optionally be used as part of the polymer composite or applied as a coating to the surface of the skin layer to enhance superhydrophobicity.

FIELD

This invention relates generally to the field of prosthetics. More specifically, this invention pertains to the use of nanotechnology to create tough, self-cleaning polymer surfaces that simulate skin-like properties for use as a covering for a prosthetic device.

BACKGROUND

Artificial arms and legs, as well as other prostheses attempt to restore normal function to amputees. Part of this normal function is the physical or aesthetic appearance of the prosthetic limb. One major problem with existing prosthetic devices is that their exterior surface does not exhibit many of the characteristics of human skin, such as toughness, flexibility, heat and pressure sensation, water repellency, and smoothness. The lack of toughness and flexibility in existing prostheses causes the exterior surface of these prostheses to wrinkle or become distorted when they are stretched or compressed. In addition, the polymers currently used in making the prosthetic covering, generally behave as a thermal insulator, thereby preventing the quick detection and transmittal of thermal readings to embedded heat sensors. This time lapse often creates a problem by allowing the polymer to begin melting before the embedded thermal sensor can detect a change in temperature.

Accordingly, while significant advances have been made in the prosthetic industry over the past decade, there exists a continual desire to provide prostheses with enhanced performance and aesthetic appeal. In particular, a prosthetic device having an exterior surface that exhibits the characteristics of real human skin is highly desirable.

SUMMARY

The present disclosure provides an external covering for hiding the internal endoskeleton of a mechanical device (e.g., prosthetic device) that provides skin-like qualities. One embodiment of an external covering constructed in accordance with the teachings of the present disclosure, generally comprises an internal bulk layer in contact with the endoskeleton of the mechanical device and an external skin layer disposed about the internal bulk layer. The external skin layer is comprised of a polymer composite with carbon nanotubes embedded therein. The outer surface of the skin layer has multiple cone-shaped projections that provide the external skin layer with superhydrophobicity. The carbon nanotubes are preferably vertically aligned between the inner surface and outer surface of the external skin layer in order to provide the skin layer with enhanced thermal conductivity, i.e., the ability to transmit heat. The mechanical properties exhibited by the external skin layer may be enhanced by selecting the orientation or alignment angle for the carbon nanotubes in the skin layer. The mechanical properties may further be enhanced by exposing the carbon nanotubes to a high temperature annealing procedure. According to another aspect of the present disclosure, a superhydrophobic powder may optionally be used as part of the skin layer or applied as a coating to the surface of the skin layer to enhance superhydrophobicity of the skin layer.

Another embodiment of the present disclosure provides a method of making the external covering described above for use with a mechanical device whose surface exhibits skin-like qualities. This method generally comprises providing a mold having a cavity with a predetermined shape and at least one surface having a nano-funnel or micro-funnel patterned array. A film comprised of a polymer composite with embedded carbon nanotubes is inserted into the cavity of the mold and allowed to contact the patterned surface of the mold's cavity. Then heat, vacuum, pressure, or a combination thereof is applied to the cavity and/or the film to cause the film to conform to the predetermined shape of the mold and to induce the formation of superhydrophobic cone-shaped projections on the surface of the film. After the shaped film is cooled and removed from the mold, it is adhered to a pre-shaped internal bulk layer to form the external covering.

According to another aspect of the present disclosure the step in which the shaped film is adhered to the pre-shaped internal bulk layer may be replaced with the step of back-molding the internal back layer to be in contact with the film. According to yet another aspect of the present disclosure, the method may further comprise the step of opening a gap in the mold and injecting an in-mold coating into the gap to coat the surface of the film.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is in no way intended to limit the present disclosure or its application or uses. It should be understood that throughout the description and drawings, corresponding reference numerals indicate like or corresponding parts and features.

The present disclosure generally provides an external covering for a mechanical device, such as a prosthetic device, used as a replacement for a human limb that requires the outer layer of the prosthesis to have a skin-like exterior surface. As shown inFIGS. 1(A-B) and2(A-B), examples of such prostheses5include, but are not limited to, those used as transtibial, transfemoral, transradial, or transhumeral replacements for human limbs. By definition, transtibial and transfemoral prostheses replace legs that are missing from below the knee and from above the knee, respectively. Likewise, transradial and transhumeral prostheses replace arms missing from below the elbow and from above the elbow, respectively. InFIGS. 1(A-B) and2(A-B) the prostheses shown are transfemoral and transhumeral prostheses, respectively. One skilled-in-the-art will understand that the external covering20for a prosthesis as described in this disclosure may be incorporated with any type of prosthesis5used as a replacement for a human limb that requires a skin-like exterior surface. One skilled-in-the-art will further understand that the external covering of the present disclosure may be used with the endoskeleton or internal support structure associated with other types of mechanical devices or equipment without departing from scope of the invention.

The prosthetic device5is comprised of an internal endoskeleton25or pylon structure that may include joints30, as well as a means (e.g., belts, cuffs, etc.) to attach the prosthesis to the human body. The internal endoskeleton25is then covered with an external covering20to provide physical protection for the endoskeleton25and to aesthetically provide a skin-like appearance. The prosthesis may be integrated with the functioning of the human body through the use of various biosensors, signal controllers, and actuators. The endoskeleton25may be made from any light weight material, such as plastics, metals, or alloys. Examples of such light weight materials include polypropylene, titanium, aluminum, and carbon fiber composites.

The external covering20is designed and adapted to resemble the appearance and performance of human skin. As shown inFIG. 3A, real human skin35is generally comprised of an inner bulk layer called the dermis layer40and an outer thin layer called the epidermis layer45. The dermis layer40is in contact with subcutaneous tissue41of the human body. The epidermis layer45contains a melanin pigment that helps to counteract the absorption of the light rays from the sun by darkening the color of the skin35.

Similar to human skin35, the external covering20of the current disclosure is comprised of multiple layers as shown inFIG. 3B. Primarily, the external covering20comprises an internal bulk layer50and an external skin layer55. The internal bulk layer50has a first side60and second side65with the first side60being in contact with the internal endoskeleton25of the prosthetic device5. The external skin layer55is disposed about the second side65of the internal bulk layer50. The external skin layer55has an inner surface70and an outer surface75. The inner surface70being in contact with the second side65of the bulk layer50and the outer surface75representing the external surface of the prosthetic device5.

The outer surface75of the external skin layer55exhibits a nano-cone or micro-cone pattern80or structure. The external skin layer55is further comprised of a polymer composite85and an embedded network of carbon nanotubes90. The carbon nanotubes90are preferably vertically aligned or in other words, positioned to be about perpendicular to the outer surface75as shown inFIG. 4A. The carbon nanotubes90may occupy from about 1% to about 40% of the volume associated with the external skin layer55.

The nano-cone or micro-cone80pattern represents nanometer or micrometer size surface irregularities predetermined to be located on the outer surface75of the prosthetic device5. These nano-cone or micro-cone80irregularities may be protrusions arising out of the outer surface75of the external skin layer55. These irregularities, which are formed as an integral part of the external skin layer55, may vary the height, spacing, shape, and characteristics of this layer's outer surface75.

Embedding carbon nanotubes90into the external skin layer55of the prosthesis5provides for enhanced features, such as toughness, strength, and durability. In fact, the external skin layer55is designed to be tougher and stronger than the internal bulk layer50. The carbon nanotubes90exhibit high tensile strength and elastic modulus in the axial direction, while being relatively soft in the radial direction. Thus the external skin layer55may exhibit a tough outer surface75, while maintaining a relatively high degree of flexibility.

The external skin layer55may be constructed to exhibit a gradient in mechanical properties by controlling the orientation or alignment of the carbon nanotubes90, as well as the spacing between the nanotubes90embedded in the polymer composite85of the skin layer55. Referring toFIG. 4B, the external skin layer55may comprise multiple layers of embedded nanotubes90and layers of polymer composite85that are absent of any nanotubes90. When the nanotubes90are vertically aligned with the outer surface75of the external skin layer55they represent a tough, hard segment or layer91. In comparison, the polymer composite85absent any nanotubes90represents a relatively soft segment or layer86. In other words, the mechanical properties exhibited by a layer of the polymer composite85with embedded nanotubes90is enhanced over the pure polymer properties of the composite85layer. Embedding carbon nanotubes90into the polymer composite85provides for anisotropic enhancement of the mechanical properties exhibited by the external skin layer55. The extent of this enhancement depends upon the directional alignment of the nanotubes90, the structure or type of nanotubes90, the amount or loading of nanotubes90in the external skin layer55, and the composition of the polymer composite85, among other factors. In fact, the extent of this enhancement may be larger when a relatively “soft” polymer composite85is used and smaller when a relatively “hard” polymer composite85is used.

When the carbon nanotubes90are vertically aligned with the outer surface75of the external skin layer55, they are orientated in a direction that is parallel to the applied load93, thereby, providing for a large enhancement in mechanical properties. As shown inFIG. 4C, a layer89in which the embedded carbon nanotubes90are perpendicular (90°) to the mechanical load93will not be as “hard” as a layer91where the nanotubes90are parallel (0°) to the mechanical load93and not as “soft” as a layer86comprised of the pure polymer composite85. Carbon nanotubes90that are perpendicular to the mechanical load93are aligned to be parallel with the outer surface75of the external skin layer55. An intermediate enhancement in mechanical properties may result when the layer92has nanotubes90oriented in a direction that is between the vertical (0°) alignment of layer91and the perpendicular (90°) alignment of layer89. InFIG. 4C, the intermediate layer92is shown with nanotubes90aligned at a 45° angle to the mechanical load93. One skilled-in-the-art will understand that the carbon nanotubes90may be aligned or oriented with respect to the mechanical load93at any angle between 0° and 90°.

For example, the mechanical properties exhibited by an external skin layer55comprising about 10 wt. % multi-walled carbon nanotubes90in an epoxy polymer composite85were measured at about 25° C. using a nanoindentor equipped with a Berkovich tip. A summary of the enhancement in mechanical properties observed for the external skin layer55upon incorporation of carbon nanotubes90into the polymer composite85is plotted inFIG. 5Aas a function of nanotube90alignment with respect to the direction of the applied load93. The origin point105of the plot (e.g., 0% enhancement) represents the mechanical properties exhibited by the pure polymer composite85without the incorporation of any carbon nanotubes90. Upon the incorporation of nanotubes90into the polymer composite85an enhancement in mechanical properties, such as Young's modulus110, hardness111, polymer creep112, and scratch resistance120, is observed. An enhancement in polymer creep may be defined as a reduction in the permanent deformation of the polymer that occurs when the polymer composite85is exposed to a constant applied stress.

Referring toFIG. 5A, the maximum enhancement in Young's modulus110of 40% occurs when the carbon nanotubes90are aligned parallel with the applied load93. The hardness of the external skin layer55can be increased between about 17% to about 25% compared to the pure polymer composite85upon the incorporation of nanotubes90into the polymer composite85with the largest enhancement occurring when the nanotubes90are aligned perpendicular to the load93. Similarly, the largest enhancement with respect to reducing polymer deformation or creep112is provided when the embedded carbon nanotubes90are aligned perpendicular to the applied load93.

An optional high temperature annealing procedure that exposes the carbon nanotubes90to a temperature up to about 2,400° C. can be used to further enhance the mechanical properties exhibited by the external skin layer55. It is possible that exposure of the nanotubes90to this annealing procedure reduces the existence of surface/wall defects in the nanotubes90, which in turn increases the crystallinity and overall stiffness of the nanotubes90. Thus this annealing procedure can be used to enhance the mechanical properties exhibited by the external skin layer55with having to alter the orientation of the nanotubes90in the layer55.

Referring now toFIG. 5B, after annealing the carbon nanotubes an enhancement of about 71% in Young's modulus110is observed when about 10 wt. % of the carbon nanotubes90are embedded in the polymer composite85and are aligned parallel to the applied load93. The hardness of the external skin layer55with embedded carbon nanotubes can be increased up to about 34% over the pure polymer composite85, and polymer deformation or creep112can be reduced by about 29% when the carbon nanotubes90are annealed and then aligned perpendicular to the applied load93.

The overall range of enhancement for Young's modulus110, hardness111, and polymer creep112attributable to annealing the carbon nanotubes90, aligning the nanotubes90in the polymer composite85, or a combination of both is between about 22 to 80%, about 13 to 34%, and about 13 to 29%, respectively, when about 10 wt. % nanotubes90are incorporated into the polymer composite85of the external skin layer55. Further enhancement of the mechanical properties can occur upon the incorporation of a greater weight percentage of carbon nanotubes90into the polymer composite85. The incorporation of about 40 wt. % carbon nanotubes90into the polymer composite85may increase Young's modulus110, increase hardness111, and reduce polymer creep112by as much as about 320%, 134%, and 116%, respectively.

Referring now toFIG. 5C, the effect of the alignment or orientation of carbon nanotubes in the external skin layer55on improving the scratch resistance120exhibited by the skin layer is shown. Carbon nanotubes90oriented perpendicular125to the scratch direction in an epoxy polymer composite85provide about a 90% reduction in surface penetration when compared to the pure polymer composite85. When the carbon nanotubes90are oriented parallel130to the scratch direction, the reduction of surface penetration is about 58%. One skilled-in-the-art will understand that although the scratch conditions utilized were 0.1 mN load over 100 mm distance at 0.5 mm/s, similar results would be obtained using a different set of standard conditions. The scratch resistance can be further tuned by controlling the type of nanotube (e.g., single nanotube, few wall nanotubes, or double wall nanotubes), as well as controlling the weight loading, the incorporation of other additives (i.e., metal oxide structures) on the surface or between nanotubes.

The external skin layer55with embedded carbon nanotubes90can also conduct heat and provide for pressure sensation. The presence of the carbon nanotubes90enhances the ability of the outer surface75to conduct electrical current and heat. Thus a variety of different sensors95may be optionally located in the internal bulk layer50that can detect temperature or pressure changes occurring in the external skin layer55, thereby, imitating the performance of real skin nerve cells.

The nano-cone or micro-cone80patterns provide the outer surface75of the external skin layer55with superhydrophobicity, thereby, keeping the external surface of the prosthesis clean and dry. The superhydrophobicity of the outer surface75is preferably described as the surface having a contact angle greater than about 150 degrees when a drop of water is applied to the surface.

Referring toFIG. 3B, superhydrophobic powders100may optionally be incorporated into or onto the outer surface75of the external skin layer55, thereby providing an additional mechanism through which water may be repelled. Preferably, the superhydrophobic powders100comprise at least one hydrophobic material selected as one from the group of perfluorinated organics, fluorinated organics, and self-assembled monolayers. Other examples of superhydrophobic powders include those described in U.S. Publication No. 2009/0042469, herein incorporated by reference.

Carbon nanotubes90exhibit a range of different colors depending on their size, shape, and length. The carbon nanotubes90may be sorted and selected by size to provide a predetermined color to the external skin layer55. Thus the external skin layer55may be designed to match the color associated with the rest of a person's skin35, thereby, making the external surface of the prosthesis appear much more skin-like and natural.

The carbon nanotubes90as embedded in or applied to the external skin layer55may be single-wall (SWNTs) or multiwall (MWNTs) structures exhibiting either metallic conducting or semiconducting behavior. Carbon nanotubes are allotropes of carbon having a nanostructure with a large length to diameter ratio. Preferably, the diameter of the nanotubes is on the order of a few nanometers with a length up to several millimeters. Preferably, the carbon nanotubes are vertically aligned or, in other words, perpendicular to the outer surface75of the external skin layer55in order to enhance the desired affects. The nanotubes are embedded in a polymer composite85or in an applied surface coating in a manner that stabilizes the architecture of the nanotubes90in the predetermined vertical alignment, direction, or pattern.

The carbon nanotubes90can be grown from or deposited by plasma enhanced chemical vapor deposition (PECVD) or any other method known to one skilled-in-the-art, including but not limited to chemical vapor deposition, arc discharge, laser ablation, high pressure carbon monoxide (HiPCO), spray coating, dip coating, and flow coating.

The polymer composite85of the exterior skin layer55or optionally a coating applied to the outer surface75of the exterior skin layer55may be made of any known thermoplastic or elastomeric material, such as polyimides, fluoropolymers, polyamides, polyesters, silicones, polyurethanes, epoxies, or polyacrylates, among others. Preferably, the polymer composite or coating is comprised of a polyimide material.

The interior bulk layer50may be comprised of any fabric, foam, silicone, gelatin, latex, collagen, sponge, wool, cotton, or a mixture or combination thereof. One skilled-in-the-art will understand that the interior bulk layer50may be comprised of any other material that will provide for the physical protection of the underlying endoskeleton25without departing from the scope of this disclosure.

The exterior skin layer55may be fastened to the underlying interior bulk layer50and the interior bulk layer50may be fastened to the underlying endoskeleton25by any means known to one skilled-in-the-art. Such means includes, but is not limited to, the use of adhesives, coupling agents, mechanical fasteners, and melt bonding.

It is another objective of the present disclosure to provide a method199of making or manufacturing an exterior covering20for use with an endoskeleton25to form a mechanical device, such as a prosthetic device5. One aspect of the present disclosure is to provide a method of forming a prosthesis5that includes the previously described exterior covering20. Referring toFIG. 6A, a method of forming an external covering20for use with a mechanical (e.g., prosthetic) device generally comprises the steps of providing200a mold having a cavity with a predetermined shape. The mold includes at least one surface having a nano-funnel or micro-funnel array. A more detailed description of the nano-funnel or micro-funnel array surface of the mold as used herein is provided in U.S. patent application Ser. No. 11/945,865 filed Nov. 27, 2007, which is hereby incorporated by reference.

A film, which will form the external skin layer55, is then inserted210into the cavity of the mold. In such, this film is comprised of the polymer composite85embedded with carbon nanotubes90. If desirable, the carbon nanotubes may be optionally subjected to an annealing205procedure prior to being incorporated into the film. Optionally, superhydrophobic powder100may be applied215to the surface of the film. Heat, vacuum, and/or pressure are then applied220to induce the formation of an external skin layer55exhibiting a superhydrophobic outer surface75. The application of heat may be accomplished by heating the mold or heating the film. The surface of the film is caused to melt or deform and to take on the shape of the funnel array surface of the mold. The translation of the mold nano-funnel or micro-funnel surface onto the film results in the formation of nano-cones or micro-cones80on the outer surface75of the external skin layer55. The external skin layer55is then removed230from the mold. One skilled-in-the-art will understand that the preceding steps200,210,215, and220associated with forming the external skin layer55may be varied according to any known steps used in a thermoforming process without departing from the scope of this disclosure.

The external skin layer55is then fastened or adhered240to a pre-shaped internal bulk layer50to form the external covering20for use in a prosthetic device5. Optionally, the formation of a prosthetic device5may be completed by fastening250the internal bulk layer50of the external covering20to an endoskeleton25. One skilled-in-the art will understand that the external covering20may be applied to an endoskeleton25that represents the internal structure or support for other types of mechanical devices and is not limited to use only with prosthetic devices5.

According to another aspect of the present disclosure, a method299resembling a film-insert molding or in-mold decorating process may be utilized. Referring now toFIG. 6B, in this method299a mold is provided300having a cavity with a predetermined shape. This mold includes at least one surface having a nano-funnel or micro-funnel array as previously described.

A film, which will form the external skin layer55, is then inserted310into the cavity of the mold. In such, this film is comprised of the polymer composite85embedded with carbon nanotubes90. If desirable, the carbon nanotubes may be optionally subjected to an annealing305procedure prior to being incorporated into the film. Optionally, superhydrophobic powder100may be included on the surface of this film. Heat, vacuum, and/or pressure may then be applied320to induce the formation of an external skin layer55exhibiting a superhydrophobic outer surface75. The heat may be applied by heating the mold or heating the film. The surface of the film is caused to melt or deform and to take on the shape of the funnel array surface of the mold. The translation of the mold nano-funnel or micro-funnel surface onto the film results in the formation of nano-cones or micro-cones80on the outer surface75of the external skin layer55.

Finally, the internal bulk layer50may be back-molded330onto the external skin layer55to form the external covering20. The internal bulk layer50may adhere to the external skin layer55by melt bonding or by adhesion when the film includes an adhesive on the surface that is forced into contact with the internal bulk layer50during the back-molding step. The external covering20is then removed340from the mold to complete the process299of forming an external covering20for a prosthetic device5. Optionally, the formation of the prosthetic device5may then be completed by fastening350the internal bulk layer50of the external covering20to the endoskeleton25. One skilled-in-the-art will understand that the preceding steps300,310,320, and330associated with forming the external skin layer55may be varied according to any known steps used in a film-insert molding process without departing from the scope of this disclosure.

According to yet another aspect of the present disclosure, a method399that integrates the film-insert molding method299with an in-mold coating delivery system may be utilized. In order to effectively apply a coating during the molding operation, a rotating stack mold with at least one needle gate to inject the coating formulation is preferably used along with a coating metering unit or delivery cart.

Referring now toFIG. 6C, in this method399a mold is provided400that has a cavity with a predetermined shape and at least one surface having a nano-funnel or micro-funnel array as previously described. The film, which will form the external skin layer55, is then inserted410into the cavity of the mold. If desirable, the carbon nanotubes may be optionally subjected to an annealing405procedure prior to being incorporated into the film. Heat, vacuum, and/or pressure may then be applied420to induce the formation of an external skin layer55exhibiting a superhydrophobic outer surface75. The internal bulk layer50is then back molded430onto the external skin layer55to form the external covering20. The internal bulk layer50may adhere to the external skin layer55by melt bonding or by any other known means.

Finally, a very small mold gap (depending on the desired coating thickness) is then opened440and a measured amount of an in-mold coating (IMC) is injected into the gap. The IMC coating can be injected between the outer surface75of the external skin layer55and the surface of molding containing the nano-funnel or micro-funnel surface. The mold is then closed, clamped, coin/compressed, and the coating cured. Depending on the specific formulation of the IMC coating and the required cure conditions, additional heating or cooling of the mold may be necessary to fully cure the coating. In this way the coating will upon cure exhibit the contour of a nano-cone or micro-cone surface rather than cause the planarization of the outer surface75. Optionally, this coating may include superhydrophobic powder100as a filler material. One skilled-in-the-art will understand that the preceding step of applying440an in-mold coating to the outer surface75of the external skin layer55may be varied according to any known steps used in an in-mold coating process without departing from the scope of this disclosure. In addition, multiple additional steps related to coining and clamping may be required. It is further understood that the in-mold coating step described above may also be incorporated into the thermoforming method described inFIG. 4A.

Finally, the external covering55is cooled, the mold opened, and the covering55removed450from the mold. Optionally, the formation of the prosthetic device5may then be completed by fastening460the internal bulk layer50of the external covering20to an endoskeleton25.

One skilled-in-the-art will understand that the foregoing description is not intended to limit the use of the external covering to the application of prosthetic devices. Rather the external covering of the present disclosure is equally applicable for use in other applications, such as a covering for an endoskeleton, which may include, but not be limited to, mechanical equipment, mechanical devices, or other mechanical structures, that are used in or by other industries, such as automotive or aerospace. For example, the external covering may be applicable for use as a seat covering or an overlay for a dashboard or instrument panel to name a few.