Nanofiber adhesives used in medical devices

[0001] The present disclosure relates generally to medical devices and methods, and more particularly to adhesives used with medical devices, such as sensors.

[0002] This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present embodiments. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

[0003] In medicine it is often desirable to attach materials to patient tissues, such as the skin, as part of the treatment process. For example, it may be desirable to use non-invasive techniques to continuously monitor a patient's physiological state. Such techniques may utilize data collected by a sensor, lead, or contact that is typically attached to the skin of the patient. For example, a patient's blood-, heart-, and/or brain-related data are often measured using various sensors placed on the patient for extended periods of time and relayed to some external monitor that can then be assessed by an appropriate healthcare professional. Typically, the sensors are held in place using conventional adhesive backings, such as those made from polymeric viscoelastic materials. After the patient no longer needs to be monitored, the sensors may be peeled from the patient's skin by overcoming the adhesive forces associated with the adhesive backing.

[0004] Despite the ubiquity of adhesive-backed sensors, relatively few advances have been made towards the mitigation of the discomfort associated with the peeling of the adhesive layer from the patient's skin while retaining strong adhesive properties. While this represents a mild annoyance for some, it can be a painful process for others who have sensitive skin, such as children, the elderly, those with allergies to various adhesive compositions, and those who have extended periods of stay in a healthcare facility.

[0013] One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

[0014] The present disclosure relates to the use of nanofiber adhesives in medical devices. Such devices may include a sensor, such as for use with a patient monitoring system, having a layer of nanofiber adhesive. The nanofiber adhesive may be in a variety of forms and may be formed from different materials consistent with any number of desired properties. In one embodiment, the nanofiber adhesive may be formed from smaller subunits of nanotubes, which may adhere to various surfaces including patient tissue (e.g., skin). In other embodiments, the nanofiber adhesive may be formed from an array of nanofibers that are not made from smaller subunits, but are formed from a bulk material. In yet another embodiment the adhesive layer may be formed from a mixture of nanofibers formed from nanotubes and bulk materials. Due to the nature of the adhesive properties of the nanofiber layer, the removal of such a device from a patient may be a relatively painless process. Additionally, a device utilizing nanofiber adhesives may be further designed to allow monitoring of the adhesive state of the adhesive layer, thus enabling a user to determine attachment to a patient surface.

[0015] With the foregoing comments in mind, a nanofiber adhesive as discussed herein may be used in various medical contexts, such as in the placement of different types of medical sensors, leads, or contacts on a patient. As may be appreciated, such sensors may be associated with monitoring systems for monitoring the sensors and processing data obtained via the sensors. By way of illustration, FIG. 1 is a perspective view of an embodiment of a pulse oximetry system 10. The system 10 includes a sensor 12 that includes a nanofiber adhesive and a pulse oximetry monitor 14. The sensor 12 may include an emitter 16 for emitting light at one or more wavelengths into a patient's tissue. A detector 18 may also be provided in the sensor 12 for detecting the light passing through or reflected by the tissue of a patient.

[0016] In one embodiment of the sensor 12, the emitter 16 and detector 18 may be on opposite sides of a digit such as a finger or toe, in which case the light that is emanating from the tissue has passed completely through the digit. However, in another embodiment of the sensor 12, the emitter 16 and detector 18 may be arranged so that light from the emitter 16 penetrates the tissue and is reflected by the tissue into the detector 18, such as a sensor designed to obtain pulse oximetry data from a patient's forehead. In one embodiment, the sensor 12 may be connected to and draw its power from the monitor 14 as shown. For example, the sensor 12 may be communicatively coupled to the monitor 14 via a cable 24 and connector 30. The connector 30 of the sensor 12 may be engaged with a complimentary connector on the monitor 14.

[0017] The monitor 14 may be configured to calculate physiological parameters based on data received from the sensor 12 relating to light emission and detection. Further, the monitor 14 may include a display 20 configured to display the physiological parameters, other information about the system, and/or alarm indications. In the embodiment shown, the monitor 14 also includes a speaker 22 to provide an audible alarm in the event that the patient's physiological parameters are not within an expected range, as defined based on patient characteristics.

[0018] In the illustrated embodiment, the pulse oximetry system 10 also includes a multi-parameter patient monitor 26. The multi-parameter patient monitor 26 may be configured to calculate physiological parameters and to provide a central display 28 for information from the monitor 14 and from other medical monitoring devices or systems. For example, in one embodiment where the monitor 14 is a pulse oximetry monitor, the multiparameter patient monitor 26 may be configured to display a patient's oxygen saturation reading generated by the monitor 14, pulse rate information from the monitor 14, and/or blood pressure from a separate blood pressure monitor on the display 28. Additionally, the multi-parameter patient monitor 26 may emit a visible or audible alarm via the display 28 or a speaker 30, respectively, if the patient's physiological characteristics are found to be outside of the expected range. The monitor 14 may be communicatively coupled to the multi-parameter patient monitor 26 via a cable 32 or 34 coupled to a sensor input port or a digital communications port, respectively. In addition, the monitor 14 and/or the multi-parameter patient monitor 26 may be connected to a network to enable the sharing of information with servers or other workstations. The monitor 14 may be powered by a battery or by a conventional power source such as a wall outlet.

[0019] As noted above, in one embodiment the sensor 12 may include a nanofiber adhesive, which may facilitate attachment of the sensor 12 to a patient. FIGS. 2 and 3 illustrate an embodiment of a sensor 12 having a layer 36 of nanofiber adhesive disposed on a backing layer 40 of the sensor 12. The shape and extent of the nanofiber adhesive layer 36 may vary to suit the type or function of sensor 12. That is, the nanofiber adhesive layer 36 may cover all or most of the patient contacting surface of the sensor 12 or may cover select portions of the sensor 12 (such as patterns, patches, strips, edges, and so forth) that provide sufficient surface area to supply the desired degree of adhesion, as discussed herein.

[0020] In one embodiment, the nanofiber adhesive layer 36 includes a population of nanofibers 38. In one embodiment, the nanofibers 38 of the adhesive layer 36 are composed of carbon. In other embodiments, the nanofiber materials may be composed of one or more of boron, silicon, boron carbide, silicon carbide, boron-silicon, cellulose and its synthetic analogs, collagen and its synthetic analogs, keratin and its synthetic analogs, or any suitable polymer.

[0021] The nanofiber adhesive layer 36 may be disposed on a backing layer 40 of the sensor 12 using a properly selected glue or polymeric bonding agent, such as poly(methylmethacrylate). In some embodiments, the bonding agent may be a combination of acrylate polymers or related compounds. In certain of these embodiments, the nanofiber adhesive layer 36 may be formed separately from the medical device on various surfaces including but not limited to metallic and/or semi-metallic catalyst substrates or beds, polymer matrices (such as a polymer film), and the like. The resulting bed, matrix, or other substrate of the nanofiber adhesive layer 36 may be glued or bonded to a backing layer 40 of the sensor body. Conversely, in one embodiment, the nanofiber adhesive layer may be bonded to the sensor body upon polymerization of a matrix of selected monomer or monomers, such as methyl methacrylate and related compounds. Thus, the polymer resulting from polymerization of the selected monomers bonds the nanofiber adhesive layer or population of nanofibers directly to the sensor body. In one embodiment, the sensor body may be made from any number of synthetic or natural polymeric materials that allow bonding by glues or viscoelastic adhesives.

[0022] Returning to FIG. 2 and, in particular, to the close-up of adhesive layer 36, in one embodiment the nanofibers 38 are aligned in a substantially parallel orientation with respect to one another and in a substantially perpendicular relation to the body of sensor 12. In one embodiment, the nanofibers 38 have a generally cylindrical or tapered cylindrical morphology and may have diameters in the range from about 100 nanometers (nm) to about 500 microns (μm) (e.g., between 100 nm and 1 μm, or between 0.8 μm to 3 μm, or between 50 μm and 500 μm). The diameters of the nanofibers 38 comprising the adhesive layer may be mono- or polydisperse.

[0023] FIG. 3 depicts a side view of a portion of the nanofiber adhesive layer 36 providing a closer view of nanofibers 38. In the embodiment shown, each nanofiber 38 is generally cylindrical, though the nanofibers may also have cross sections that are generally rectangular or rhomboidal. In some embodiments, the nanofibers may have cross sections (such as triangular cross sections) that result in morphologies resembling tapered pillars. In some embodiments, the nanofibers may be formed from a lithographic and/or etching process of an amorphous or patterned bulk material, such as a polymer or other material suitable for lithography or etching techniques. As depicted in FIG. 3, the nanofibers 38 may be formed from a tightly-packed bundle of nanotubes 42. The nanotubes 42 may be made from some or all of the materials previously listed with respect to nanofiber material composition.

[0024] The nanofibers 38 may be generated by a variety of processes including but not limited to: the formation of a superstructure such as a tightly packed bundle of nanotubes, fiber spinning of a solution or suspension of nanotubes, or a lithographic, etching, or similar process from a nanotube carpet or bulk polymer on a semi-metallic and/or metallic substrate or catalyst bed. Generally, the nanofibers 38 have lengths ranging from 100 nm to 500 μm. In one embodiment, the nanotubes 42 that form the nanofibers 38 have an average length of between 30 μm and 500 μm (e.g., between about 200 μm and about 500 μm) and are mono- or polydisperse. In one embodiment, the nanotubes 42 may have a diameter between 1 nm and 12 nm (e.g., between 2 nm and 8 nm or about 8 nm). The diameter of the nanotubes 42 may be mono- or polydisperse, or a combination of both with respect to the population of nanotubes 42. Further, in some embodiments the nanotubes 42 may be single-walled, double-walled, or multi-walled, or may be a combination of these wall configurations with respect to the population of nanotubes 42. In one embodiment, multi-walled nanotubes may contain between 3 and 8 walls, such as between 3 and 5 walls. Nanofibers 38 formed from bulk polymeric materials may have the same or different dimensions than the nanotubes 42 used to construct nanofibers 38. For example, the nanofibers 38 formed from bulk polymer materials may have lengths in the range of about 200 nm to about 500 nm.

[0025] In an embodiment, it may be desirable to use chemically-altered nanofibers, thus, in some embodiments, the nanotubes 42 may also be chemically functionalized or treated. Possible chemical methods for functionalization include, but are not limited to: acid-base chemistry, arylation chemistry, cycloaddition chemistry, diazonium chemistry, organometal chemistry, free-radical chemistry, vapor or gas chemistry, electrochemistry, and the like. In some embodiments, the functionalization of the nanotubes 42 may result in a chemical change of the sidewalls of some or all of the nanotubes 42 within a nanofiber 38. In one embodiment, the nanotubes 42 may be substantially functionalized at the patient contacting surface (i.e., the ends of the nanotubes 42) to alter their adhesive properties. In some embodiments, it may be desirable to chemically treat the formed nanofibers 38 instead of or in addition to the nanotubes 42 to alter their properties (e.g. in the case of nanofibers 38 formed from a bulk polymer). For example, the nanofibers 38 may be coated with a material to increase their adhesive capabilities. The material may include chemical functionalities that have an affinity for patient surfaces. For example, in one embodiment, the nanofibers 38 may be coated with oxidized dextran to increase interfacial adhesion strength.

[0026] In one embodiment, an emitter 16 and/or detector 18 of a sensor 12 are uncovered by the nanofiber adhesive layer 36 (as depicted in FIG. 2). In this embodiment, the nanofiber adhesive layer 36 may or may not be transparent to the wavelengths of light emitted and detected by the emitter 16 and detector 18. Turning to FIG. 4, in another embodiment one or both of the emitter 16 and detector 18 may be covered by the nanofiber adhesive layer 36. In such an embodiment, the composition of the nanofiber adhesive layer 36 may be selected so that the nanofiber adhesive layer 36 is generally transparent to the wavelengths of light that are to be emitted and detected.

[0027] FIG. 5 represents a view of an embodiment of nanofiber adhesion to a patient's tissue 44. In one embodiment, the nanofibers 38 (and nanotubes 42 forming the nanofibers 38) adhere to the patient's tissue 44 by approaching the tissue at a given angle. In some embodiments, the angle of approach may be any angle between 0° and 90° (e.g., between 45° and 90° or about 90°). In one such embodiment, the nanofibers 38 and the nanotubes 42 that form the nanofibers 38 adhere to the patient tissue 44 via an end-on approach. In such an approach, upon contacting the patient surface 44, a downward force applied to “set” the nanofiber adhesive layer 36 deforms the ends 46 of the nanofibers, causing individual nanotubes 42 to splay outward, as depicted in the figure inset. The surface area of contact between the nanofibers 38 and the patient tissue 44 may therefore be greatly increased. In one such embodiment, van der Waals forces, usually responsible for intermolecular interactions, increase in proportion to the increased contact surface area, creating a strong force of attraction between the population of nanofibers 38 (and the nanotubes 42 forming the nanofibers 38) and the patient's tissue 44.

[0028] In further embodiments, the nanofibers 38 of the nanofiber adhesive layer 36 may also be arranged in such a way such that in addition to van der Waals forces, the nanofiber adhesive layer 36 may also have an adhesive component arising from capillary forces. For example, a nanofiber adhesive layer 36 of a properly selected nanofiber 38 material may be arranged in such a way and with such dimensions as to allow water to be taken up by capillary forces, thereby allowing adhesion to wet patient surfaces, such as internal tissues or wet skin. Thus, the nanofiber adhesive layer 36 may be used not only on dry patient skin, but any patient tissue 44 where an increased interfacial contact is attainable.

[0029] In one embodiment, the nanofiber adhesive layer 36 can be removed by peeling the sensor 12 from the tissue 44 at an angle such that the energy of detachment is reduced or minimized. For example, the relationship between the peeling angle and the energy of detachment may be represented by the equation:

[0030] $\begin{array}{cc}G=\frac{F\left(1-\cos \theta \right)}{w}& \left(1\right)\end{array}$
where G is the energy of detachment, F is the peeling force, w is the width of the adhesive layer 36, and θ is the angle that the adhesive layer 36 is peeled back for all angles greater than or equal to 45° (other variables may play a role at lower angles, such that the energy of detachment may also depend on the elastic stiffness of the adhesive layer, the weight of the sensor, and the like). In accordance with equation (1), the energy of detachment of a given sensor from a standard surface (e.g., the patient's tissue 44) is determined by the peeling angle θ, where the force used to remove the adhesive layer 36 has a minimum value at a given angle (such as 45° in one embodiment). Thus, by removing the adhesive layer 36 (and attached medical device, such as sensor 12) at a suitable peeling angle θ, detachment energy may be reduced or minimized, thereby reducing patient discomfort.

[0031] As will be appreciated, a nanofiber adhesive layer 36 may be provided on various types of medical devices in addition to the spectrophotometric sensor 12 discussed above. For example, FIG. 6 illustrates a plan view of an embodiment of a nanofiber adhesive layer 36 disposed on an EEG or ECG lead 48. In one embodiment, a patient-contacting surface of the lead 48 is covered, partially or completely, by a nanofiber adhesive layer 36. As discussed above, the nanofiber adhesive layer 36 may be disposed on a non-conductive backing material of the lead 48 such that, when the adhesive layer 36 is contacted to a patient's tissue, the lead 48 becomes adhered to the patient. While sensors and leads, such as those discussed above, represent certain types of medical devices on which a nanofiber adhesive layer 36 may be employed to adhere all or part of a medical device to a patient, any type of medical device that is adhered or attached to a patent may benefit from the use of a nanofiber adhesive layer 36. For example, bandages, wraps, dressings, portable monitoring devices, and so forth may also benefit from the addition of a nanofiber adhesive layer 36 as discussed herein.

[0032] Turning now to FIG. 7, a peel-away view of an embodiment of a medical device (provided as a sensor 12) with a nanofiber adhesive layer 36 is depicted. In the depicted view, at least two electrical contacts 50 capable of conducting a current modulated by the state of the adhesive layer 36 are disposed on the device body. In one embodiment, the electrical contacts 50 are made from a metal such as copper, gold, silver, aluminum, and the like, though the electrical contacts may be made from any suitable conductive material or composition or any combination of the above. In one embodiment, a current between the electrical contacts 50 may be responsive to or indicative of the adhesive state of the adhesive layer 36. For example, the adhesive state may be represented by a binary function wherein the sensor is attached to the patient's tissue 44 in one state and unattached in the other state. Therefore, a characteristic of the current carried (or not carried) between the electrical contacts 50 may be evaluated or measured to determine whether the sensor is “on” or “off”, i.e., adhered or not adhered.

[0033] For example, in one embodiment, the electrical contacts 50 may be electrically isolated from one another when the nanofiber adhesive layer 36 is not applied to a patient, i.e., no current flows between the electrical contacts when the sensor 12 is not adhered to a patient. Such a non-conductive state may signify that the sensor 12 is not applied, i.e., is “off”. Upon contacting a patient's tissue, a basic electrical circuit may result wherein an electrical current is able to pass between the electrical contacts 50. Such a circuit may result from the current passing through the patient skin or tissue in contact with the nanofibers 38 of the adhesive layer 36 or may pass through the adhesive layer 36 itself in response to a conformational change in the nanofibers 38 associated with adhesion. Such a state of positive conductivity may signify that the sensor 12 is applied, i.e., is “on”.

[0034] In another embodiment, the electrical contacts 50 may not be electrically isolated. Instead, an electrical current may be capable of passing between the electrical contacts 50 through the continuous structure of the nanofiber adhesive layer 36, so that some amount of current flows between the electrical contacts 50 regardless of the application state of the sensor. In such an embodiment, a characteristic (such as the measured current or voltage) of the flow of current between the electrical contacts 50 may vary when the sensor 12 is adhered to a patient as compared to when the sensor 12 is not applied to the patient. In such an embodiment, the value and/or variation of the varying characteristic may be utilized to determine the adhesion state of the sensor 12. While the preceding describes an example in which conductive characteristics of a nanofiber adhesive layer 36 may be used to determine whether a sensor 12 is applied to a patient, it should be appreciated that such conductive characteristics may be used to evaluate the adhesive state of a nanofiber adhesive layer 36 in other contexts, including non-medical contexts.

[0035] While the disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the embodiments provided herein are not intended to be limited to the particular forms disclosed. Indeed, the disclosed embodiments may be applied to measurements of blood oxygen saturation as well as other physiological measurements. Rather, the various embodiments may cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the following appended claims.