Patent Application
20080083744
The present disclosure provides methods and systems for creating and maintaining highly localized, isothermal heating.
As shown,
Substrate 12 may be any suitable material or combination of materials including, for example, fused silica, quartz glass, plastic, or ceramic. An example of a suitable plastic is Polyethylene terephthalate (PET). Examples of suitable ceramics include Borosilicate glass and Macor® ceramic glass (available from Corning, Inc., Corning, N.Y.).
In the embodiment depicted in
For the purposes of the present description, the term “target area” shall refer to an area that must be heated by the heating element in order to bring about an intended effect. For example, one possible application of the heating element of the present disclosure is for use in specialty injection devices such as those described in co-pending U.S. patent application Ser. No. 11/001,367 and in co-pending U.S. patent application Ser. No. 11/495,359, each of which is hereby incorporated by reference in its entirety for all purposes. According to some applications, specialty injection devices include a plurality of chambers, each of which is associated with an individually addressable heating element. When it is desired to effect expulsion of a material, such as a drug, injectate, fluid, or other substance, from the chamber, the heating element associated with a specific chamber is activated and the chamber (or some other structure, depending on the specific mechanism being used) is heated to a threshold temperature. Heating of the chamber (or other structure) to the threshold temperature effects expulsion of the material through an associated orifice, such as a microneedle, and into the recipient, which may, for example, be a body, apparatus, chemical system, etc. Accordingly, if the heating element shown in
Accordingly, the target area may be a flat surface, well, chamber, or the like and the heating element is typically attached or otherwise thermally connected to the target area. Depending on the intended use and desired design, none, some, or all of the target area may be formed by substrate 12.
Still referring to
Turning now to
As also shown in
Heating element 10 may further include a diffusive layer 28. The diffusive layer acts as a heat spreader and may be formed as a blanket layer extending across the entire substrate. However, the diffusive layer 28 may be formed only over the target area or the portions thereof where heat is required to be spread uniformly. Those of skill in the art will be familiar with materials that are used to form diffusive layers in heating elements. Examples of suitable materials that may be used to form a diffusive layer include silicon nitrides, silicon oxides, silicon carbides, and silicon oxynitrides.
Of course,
According to some embodiments, the heating element may include one or more vias. For example, a conductive layer may contact a resistive layer through the passivation layer by way of a via. Alternatively or additionally, the conductor traces may contact the resistor traces through vias through the substrate.
It should be appreciated that the heating elements described in the present disclosure may be used in a wide variety of applications including, for example, MEMS, MRAM and the like. According to some of these applications, the target area may need to reach or exceed a certain temperature defined as the “threshold temperature”) before the device in which the heating element is used is able to bring about the intended result (e.g. allow a specific chemical reaction to take place, effect injection, etc.). Similarly, it will be understood that the target areas will typically have a desired baseline temperature. The baseline temperature is typically the temperature at which the target areas are maintained until the heating element is activated. This temperature will vary with the intended application of the device in which the heating element is incorporated and may be, for example, at or around room temperature (˜22° C.), at or around body temperature (˜37° C.) or significantly above or below these temperatures, depending on the desired application. Accordingly, a device will typically have an acceptable temperature range that spans from the baseline temperature to at least the threshold temperature. Moreover, according to some embodiments, the heating device of the present disclosure may be used to maintain an artificial baseline temperature.
According to one embodiment, the physical characteristics of the heating element including, but not necessarily limited to, the materials used to form the various components (e.g the resistor trace, the substrate, the connector traces) and the specific shapes thereof, are selected such that the heating element produces an isothermal temperature that is localized to a specific target area.
In general, an area is considered to be isothermal when the temperature distribution in the area is uniform. Of course, it will be appreciated that it may not be possible to achieve 100% uniformity across the entire area when it is also desirable to provide a localized heated temperature. Accordingly, for the purposes of the present disclosure, the term “substantially isothermal temperature” is meant to mean that at least 90% of the heated target area varies in temperature by less than 10% of the acceptable temperature range and 99% of the region varies in temperature by less than 20% of the acceptable temperature range and 100% of the target area varies in temperature by less than 50% of the acceptable temperature range.
The depicted heating element may be suitable, for example, in a drug delivery application wherein the baseline temperature is around 27° C. and the threshold temperature is around 95° C. As shown by the various cross-hatching patterns in
Of course it will be appreciated that the heating element of the present invention may be suitable for use in devices having a wide range of baseline and threshold temperatures and that nothing in the above example is intended to limit the temperature range capabilities of the presently-described heating element. Furthermore, it should be understood that the heating element shown in
In the example shown in
It should be appreciated, however, that it may be possible to achieve an isothermal temperature profile by using connector and resistor traces formed from different materials and having cross-sectional areas that differ only in thickness, only in width, or in both width and thickness.
Furthermore, while the depicted target areas are shown as being square, it should be understood that a given target area may be any desired shape or size, including for example, circular or rectangular. Moreover, it should be understood that arrangements for the resistor trace other than the depicted serpentine design may be utilized and that the appropriate resistor trace arrangement, whether serpentine or not, may be dependant upon a variety of factors including, for example, the size and shape of the target area, the desired heating pattern, the number and size of heating elements used to heat the target area, and any other suitable factors.
For example, it is believed that the isothermal nature of the heating profile in
As stated above, the temperature profile of one embodiment of a heating element of the present disclosure produces not only an isothermal temperature, but also a localized temperature. For the purposes of the present disclosure, the term “localized” is used to mean that the temperature of the areas adjacent to the target area being heated maintain a temperature that is substantially closer to the baseline temperature of the device than the threshold temperature and does not substantially extend from those areas.
Still viewing
According to one embodiment, under normal operating conditions, when a first heating element as described herein is activated in order to heat a first target region, the temperature of any adjacent target regions is not adversely affected by heat generated from the activated heating element. For the purposes of the present disclosure, a target region is “adversely affected” if the target region, or any matter contained within the target region, is rendered unsuitable for its intended purpose or subjected to any unintended action or result (e.g. unintended ejection from a drug delivery device, alteration of chemical properties, etc.)
For comparison, the temperature profile of a heating element that is not producing a localized temperature profile is shown in
According to one embodiment, the heating device of the present disclosure is configured such that, when activated, the heat generated by the microheater associated with a first target area will not substantially effect the temperature profile of a second target area, where the second target area is separated from the first target area by a distance that is as least two times the width of the target areas. For example, if each of the target areas is approximately 1 mm wide, and the two target areas are separated by a 2 mm gap, the heat generated by a heating device that is associated with the first target area should not substantially effect the temperature profile of the second target area.
Returning to
Alternatively or additionally, substrate 12 may include thermal barriers that are configured to reduce or eliminate thermal transfer from one section of the substrate (e.g. from one target area) to another. It should be appreciated that substrate 12 may be a monolayer of material or may be formed from several layers of the same or different materials. Furthermore, substrate 12 may have any desired thickness. For example, substrate 12 may be between 50 and 1000 μm. As described above, substrate 12 may further retain properties or include layers which allow the substrate to act as a passivation layer and/or a diffusive layer. As a specific example, the heating element that produced the temperature profile shown in
Resistor trace 14 is typically formed from a resistive, thermally conductive material such as a metal or a conductive polymer. According to one embodiment, resistor trace 14 is formed from a material having a resistance between 1 E-08 Ω-m and 1 E-09 Ω-m and having a thermal conductivity between 100 W/m-K and 200 W/m-K and is more preferably formed from a material having an electrical resistivity between 4 E-08 Ω-m and 7 E-08 Ω-m and a thermal conductivity between 125 W/m-K and 175 W/m-K. For example, the heating element that produced the temperature profile shown in
Resistor trace 14 may have any suitable cross-sectional aspect ratio. For example, the heating element that produced the temperature profile shown in
Connector traces 20 are typically formed such that they possess a lower electrical resistivity and higher thermal conductivity than the resistor trace. This may be achieved by the use of different materials and/or different geometrical formations. For example, the heating element that produced the temperature profile shown in
The distal end of each connector trace 20 may lead to a contact pad. For example, the heating element that produced the temperature profile shown in
As stated above, according to some embodiments, the cross sectional area of connector traces 20 is significantly larger than the cross sectional area of the resistor trace 14. In the embodiment depicted in
It should be understood that the desired differential between the resistor trace cross-sectional area and the connector trace cross-sectional area will depend upon the specific materials used and desired heating parameters. However, it is believed that cross-sectional area ratios of between 1:5 and 1:10 (resistor trace: connector trace) may be within a suitable range for numerous applications.
While the embodiments depicted in
According to one embodiment, contrary to the types of microheaters that are typically used in thermal ink jet applications which produce a rapid temperature rise and then cool quickly (i.e. on the order of microseconds or even lower), the heating elements of the present disclosure may be used to produce a slow, sustainable, temperature rise with a sustainable peak temperature. For the purposes of the present disclosure, a slow temperature rise is considered one in which the threshold temperature is achieved in the order of 0.1-5 minutes. Similarly, a sustainable temperature is a temperature that can be maintained for at least one minute and which can be maintained for many minutes, without adversely affecting the localized nature of the heating profile. A slowly achieved sustainable temperature may be better suited for certain types of applications including some microinjection devices, biological heaters (smart Petri dishes), polymerization procedures and drug delivery applications.
It will be appreciated that the heating device of the present disclosure can use a multiple of heating profiles to accomplish the desired result. For example, the device can change the delivery voltage to change the time required to reach the thresh-hold temperature. Alternatively or additionally, the device can have an on/off/on profile, or a stepwise profile. Moreover, using lower voltages increases the delivery time, and decreases the delivery power. This may be advantageous when a slow delivery is desired.
While the invention has been described with reference to the exemplary embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments without departing from the true spirit and scope of the disclosure. Accordingly, the terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations.
