Transdermal power transfer for recharging implanted drug delivery devices via the refill port
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- Evans, A.T., Chiravuri, S. & Gianchandani, Y.B. Biomed Microdevices (2010) 12: 179. doi:10.1007/s10544-009-9371-z
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This paper describes a system for transferring power across a transdermal needle into a smart refill port for recharging implantable drug delivery systems. The device uses a modified 26 gauge (0.46 mm outer diameter) Huber needle with multiple conductive elements designed to couple with mechanical springs in the septum of the refill port of a drug delivery device to form an electrical connection that can sustain the current required to recharge a battery during a reservoir refill session. The needle is fabricated from stainless steel coated with Parylene, and the refill port septum is made from micromachined stainless steel contact springs and polydimethylsiloxane. The device properties were characterized with dry and wet ambient conditions. The needle and port pair had an average contact resistance of less than 2 Ω when mated in either environment. Electrical isolation between the system, the liquid in the needle lumen, and surrounding material has been demonstrated. The device was used to recharge a NiMH battery with currents up to 500 mA with less than 15°C of resistive heating. The system was punctured 100 times to provide preliminary information with regard to device longevity, and exhibited about 1 Ω variation in contact resistance. The results suggest that this needle and refill port system can be used in an implant to enable battery recharging. This allows for smaller batteries to be used and ultimately increases the volume efficiency of an implantable drug delivery device.
KeywordsDrug deliveryIntrathecalPower transferImplantMEMS
Chronic pain, at a level that causes partial or total disability, is a medical condition that afflicts an estimated 100 million people in the United States (Rainov and Heidecke 2007; Joint Committee 1999). Chronic pain arises from a variety of causes including traumatic injury, various diseases (like arthritis), surgery, or nerve damage (Phillips 2003). One treatment method for severe chronic pain or spasticity is to implant an intrathecal pump that delivers medication directly into the spinal canal (Erdine and De Andres 2006; Winkelmuller and Winkelmuller 1996; Wermeling 2005; Schug et al. 2006; Rauck et al. 2003; Deer et al. 2004). These pumps are highly effective because they have a direct path into the cerebrospinal fluid (CSF). This requires precise dosing, but it offers several benefits. It reduces drug-related side effects by decreasing the dose required to achieve a certain level of analgesia, reduces the need for oral medications, and enhances quality of life in a segment of chronic pain patients whose pain has not been controlled with more conservative therapies (Likar et al. 2006).
Implantable pain therapy pumps work by delivering medication into the CSF that is in the intrathecal space. Several companies have developed intrathecal implantable pumps (Medtronic 2008). Current pumps are generally disc shaped, 180 to 220 cc in total volume, contain a single reservoir, are implanted beneath the skin of the abdomen, and are typically refilled every 12–24 weeks via a subcutaneous refill port. Active devices typically use a peristaltic pump controlled by a microprocessor that can be programmed for the infusion mode (e.g. bolus, multi-step bolus, continuous, etc.) and delivery rate depending on patient needs. The battery size of these devices is typically 25–50% of total device volume because it must continuously operate for the implant lifetime (5–10 years). This research is motivated by the consideration that the overall volume efficiency of an implant, which is critical to its placement and usability (particularly in pediatric cases), can be improved substantially if the conventional battery is replaced with a smaller battery that is recharged.
Implantable batteries can be recharged through a direct physical connection, a wireless radio frequency link, or via a wireless inductive link. While wireless power transfer is possible for very low-power applications (Boveja and Widhany 2008), DC recharge capability offers higher current levels and may be more suitable for implantable drug delivery devices (Vipul 2007). The only connection made between the external environment and the implanted drug delivery device occurs when a needle is inserted into the reservoir port during a reservoir refill session. Refill ports typically consist of an external biocompatible housing, a re-sealing silicone septum, a metal base plate that limits needle penetration, and a gap between the septum and the base plate with an exit channel through which the fluid enters the reservoir (Andrews et al. 1990; Strum et al. 1986). The refill port is typically inset within the drug pump housing in a position in which the rounded rim protrudes just above the wall of the housing (Reynaerts et al. 1996). A normal refill session begins with the puncturing of the septum using a non-coring Huber needle. The needle tip is then advanced until it presses against the base plate (The reservoir is first emptied before administering fresh drug). Medication is then driven into the device from an external syringe (Morris et al. 1992). Refills generally require 10–20 min and occur every 12–24 weeks.
Recharging the battery through an electrical coupling within the refill port may provide efficiency and convenience. It would permit implantable device designs that require significantly smaller batteries. Architectures taking advantage of this reduced battery size may be able to achieve a greater volume efficiency than traditional devices. This is particularly true for devices that have long implant lifetimes and relatively high rates of power consumption (Carmichael 2007).
2 Component design and fabrication
The most important aspects of the design are power handling capability, isolation of the drug and tissue from electrical current, minimal tissue and drug heating, and ease of alignment between the needle and the port. Alan comment on prevention of tissue heating during this refill process as it would be critical to show it is safe. How much heat transfer is likely to happen, have you measured it? In order to transfer DC power, the needle should be composed of at least two conductors, or poles, which must mate with corresponding poles in the refill port. The conductive path should also be electrically isolated. (The isolation is particularly important if the needle is being used to refill the drug reservoir at the same time the battery is being recharged. While this capability is not fundamentally required, it can improve efficiency and convenience.) Structural options for providing multiple conductive paths in a single needle include the use of multiple conductors within the lumen, the use of concentric isolated conductors, or splitting the needle longitudinally and isolating the halves. The split needle allows for simple alignment because it provides access to both conductors on the exterior of the needle. Sub-section 2.1 outlines the power transfer design constraints of the needle; sub-section 2.2 outlines the design of the mating mechanism and the port for use with the multi-pole needle; sub-section 2.3 describes the assembly procedure for the needle and the refill port.
2.1 Power transfer and needle design
While it is relatively easy to transfer data across most electrical connections, it is more difficult to transfer current at levels of hundreds of milliamps as required to recharge a battery (Soria et al. 2001). One challenge is resistive heating in the conductors. For a given conductor, this requires the use of a conductive path with the largest possible cross sectional area. The two methods of creating conductors with the largest cross sectional area are either using the needle itself or filling the needle with a conductor. Using the needle itself is preferable to other methods because the lumen remains unobstructed. Typical refill needles used in implantable drug delivery devices range in size from 22 gauge to as narrow as 28 gauge. The ratio of the inner diameter (r) to outer diameter (R) ranges from 0.55 to 0.6 across this needle range.
2.2 Refill port design
The refill ports of most implantable drug delivery devices are composed of a polymer septum through which the needle enters the device (Medtronic 2008). The polymer is relatively thick (2–5 mm), and is designed to reseal itself after the refill needle is removed from the device. Below the septum is a small open volume that is connected to the reservoir of the implant. The thickness and insulating properties of the septum make it an appealing candidate for modifications that would allow power transfer.
Fluid ports in conventional implants are typically accessed by non-coring Huber needles. In our design, the power transfer system mates when a multi-pole, non-coring needle punctures the polydimethylsiloxane (PDMS) septum of the refill port and is advanced until the tip of the needle reaches the metal base plate at the bottom of the port. (This metal base plate is electrically floating.) Each longitudinal half of the needle is exposed at a “window” on its exterior; the window for each half is at a different point along its length. The location of metal contact springs that are embedded within the septum and the exposed windows in the insulation of the needle are designed so the windows align with the mating regions when the needle is fully inserted. This occurs upon every insertion because the tip of the needle presses against the bottom base plate of the refill port. Since two separate springs are located at different heights, rotational alignment of the needle is not necessary to make electrical contact. This prevents the need to twist the needle upon insertion, and it also prevents mating the incorrect conductors to the springs. It should be noted that the needle and the metal springs in the septum are electrically isolated from the casing of the port, the surrounding tissue, and the drug being refilled. The metal contact springs could potentially be replaced by conductive layers that are composed of specialized polymers (Gerard et al. 2002) or conductive fibers in a weave (Tajima et al. 2002). The most important criteria for determining the structure of the mating springs are the formation of a low resistance contact and the ability to maintain functionality after repeated needle insertions.
2.3 Fabrication and assembly
The springs press against the needle as it is inserted. The pressure forces the needle toward the middle of the refill port, and it also improves the lead transfer conductance by maintaining pressure at the spring/needle junction. The symmetrical nature of the contact springs prevents the need for rotational needle alignment. Additionally, the springs are supported by the PDMS used as the septum polymer and return to their initial positions after the needle is removed. This allows for multiple recharging sessions to occur using a single port.
3 Experimental results
Contact resistance, in both dry and wet ambients, can provide an indication of the integrity and power handling capability between the needle and the port. Higher transfer currents can alter the contact properties of the mating pair, and these changes can be monitored while recharging batteries. Long term viability can be determined by puncturing the septum multiple times and monitoring the transient changes in resistance and septum deformation. Proper testing allows for determination of both short term and long term properties of the mating pair.
Saline is often used as the carrier agent for medication. Saline can also be used to approximate the in vivo electrical conditions experienced by implantable devices. In a separate set of tests, saline was introduced into the needle lumen and the port cavity. The exterior of the port was also immersed in saline. Insertion tests, similar to those conducted in a dry environment, were conducted in this wet ambient. As shown in Fig. 7, resistances from A to C and B to D were low (less than 2 Ω), and electrical isolation was maintained from B to C and A to E (greater than 2 MΩ). This suggests that the isolation techniques used in the system are effective at isolating the conductive paths from both the medication and the surrounding environment. Additionally, no electrolysis was observed in either the needle or in the mated port during characterization.
In addition to forming good electrical connections and limiting heat generation during battery recharging, this type of power transfer system needs to be reliable over many refill sessions. This is particularly true for mechanical springs because of potential plastic deformation. Typically refill sessions occur once every 6–8 weeks, and device lifetimes range from 5–8 years (Grabow et al. 2001). Assuming the device is refilled every 6 weeks for 8 years, the refill port could be punctured as many as 70 times. Puncturing the septum many times acts as an approximate simulation of the effect of accumulated refills on the springs, silicone, and device connectivity.
This effort explored a method for transferring electrical power across the needle through the refill port of an implantable drug delivery device. The method is intended for current levels up to 500 mA and voltage levels up to 3.3 V, as needed to rapidly charge batteries. The approach utilizes a longitudinally split, two pole Huber needle and a mating port with spring-loaded connections. The refill port springs self-align with the needle and make ohmic contact when the needle is fully inserted without additional alignment. The electrical contact and insulation perform well in both wet and dry ambients. The mechanical properties of the refill port remain functional for repeated needle insertions. The modest increases in temperature for even the highest current levels indicate that this recharging mechanism is promising. The possibility of recharging the battery of an implantable drug delivery device during a drug refill session could result in an implantable drug delivery device size reduction of up to 40%. With more sophisticated needle and port designs, high speed parallel data transfer may also be possible.