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AUTONOMOUS IMPLANTABLE DEVICE FOR APPLICATION IN LATEPHASE HEMORRHAGIC SHOCK PREVENTION
Vlad Oncescu1, Seoho Lee1, Abdurrahman Gumus2, Kolbeinn Karlsson2, David Erickson1*
1
Sibley School of Mechanical and Aerospace Engineering, Cornell University, USA
2
Electrical and Computer Engineering, Cornell University, USA
ABSTRACT
Hemorrhagic shock (HS) is the leading cause of death for people with traumatic injuries. The onset of HS is correlated with changes in plasma vasopressin levels and some studies indicate that administrating vasopressin in the bloodstream
can stabilize the situation. This situation calls for the use of implantable devices for both the monitoring and treatment of
HS. In this work, we present a self-powered hemorrhagic-shock autonomous integrated device (hemoAID) that continuously monitors vasopressin levels and releases vasopressin automatically when levels drop below a certain threshold, thus
paving the way towards the development of an autonomous implantable device for HS prevention.
KEYWORDS: autonomous, implantable, biosensor, hemorrhagic shock
INTRODUCTION
Over the past decade, implantable autonomous microsystems have been developed to counter life-threatening medical
conditions and to improve patient care by replacing cumbersome treatment procedures.[1-3] Although, independently
performing sensing[4, 5] and drug delivery[6] has been demonstrated, developing implantable medical devices capable of
achieving both of those functions simultaneously would be suitable for a number of applications.[7] One such case is the
detection and treatment of traumatic injuries such as hemorrhagic shock (HS). Studies have correlated the onset of HS
with marked changes in the plasma vasopressin levels.[8, 9] Morales et al.[8] has reported a set of human cases and canine experiments where vasopressin levels showed a marked increase at the onset of hemorrhage (to as much as
319pg/mL) followed by a fall to well below the normal physiological level (29pg/mL) as hemorrhage progressed. In addition, evidence suggests that replenishment of vasopressin at this stage can reverse hypotension in patients.[10, 11] Therefore, vasopressin can serve as both a biomarker indicative of a critical injury state and a therapeutic agent for treating it.
Developing an implantable autonomous device that can provide initial medical treatment when physician care is not
readily available is highly suitable in this case. In this paper, we present a self-powered hemorrhagic-shock prevention autonomous integrated device (hemoAID) that continuously monitors vasopressin levels in solution and releases vasopressin automatically when vasopressin levels drop below a certain threshold. We demonstrate that the device works at physiological concentrations of vasopressin in sheep serum. This represents the first steps towards the development of an
implantable, self-powered microfluidic device for late-phase HS prevention. In the theory section, we present the integrated device. In the following sections, we present characterization experiments for the sensor and then demonstrate the
operation of the entire system in the physiological range of vasopressin concentrations in which we expect the device to
operate.
THEORY
The hemoAID shown in Fig 1a consists of a nanowire biosensor for detecting changes in vasopressin levels, an onboard electrochemically driven drug delivery system
for administration of vasopressin, and a nonenzymatic glucose fuel cell for powering the system.
Those three main functional components are integrated inside a 2cm3 package. In this prototype version the electronic control system is external to the
system and uses the circuit shown in Fig. 1b. External housing of the circuitry was done to facilitate
prototype development and could be integrated into
the system in the future. The fabrication procedures
for the glucose biosensor[12], fuel cell[13] and drug
delivery[6] units have been described in detail elseFigure 1: a) Integrated device consisting of a biosensor,
where. Here we focus on demonstrating the ability of
drug delivery and fuel cell unit b) schematic of the electrical
the biosensor to function at physiologically relevant
connections between the different components of the
vasopressin concentrations and the integration of
hemoAID
these components in an autonomous device.
The non-enzymatic glucose fuel cell is used to continuously charge a small rechargeable battery (Thinergy MEC225)
that is then used to power a low power microcontroller (MSP430F2012, Texas Instruments, TX, USA). This set-up allows the device to operate steadily even with significant fluctuations in the power output of the fuel cell unit. During device operation, the sensor is driven by an adjustable 2-terminal current source (LT3092, Linear Technology, Milpitas,
978-0-9798064-6-9/µTAS 2013/$20©13CBMS-0001
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17th International Conference on Miniaturized
Systems for Chemistry and Life Sciences
27-31 October 2013, Freiburg, Germany
CA, USA) at 60 mA. The resistivity of the sensor changes with the amount vasopressin in solution, which results in detectable voltage change over the biosensor. In order to improve sensitivity, the voltage is amplified using a non-inverting
amplifier (Maxim Integrated, CA) before being detected using a low power microcontroller’s analog to digital converter
(ADC). If the voltage is lower than the pre-determined threshold, the microcontroller enables the corresponding output
pin to activate the drug delivery system. In order to increase the discharge rate of vasopressin in the drug delivery device,
the base voltage (3V) is converted to 35V with a micropower step-up DC/DC converter (LT3464, Linear Technology,
Milpitas, CA, USA).
EXPERIMENTAL
The nanowire biosensor we have developed and integrated in the hemoAID allows for the detection of
pM changes in vasopressin concentration. Immobilized
aptamer molecules on the nanowire biosensor act as
electrical bioreceptors for the surrounding vasopressin.
In order to demonstrate the ability of the biosensor
(shown if Fig. 2a) to detect gradual changes in vasopressin levels we have performed several characterization experiments. First we show the reversibility of our
sensor’s detection mechanism by monitoring the sensor response to alternating vasopressin concentrations
in a single experiment. The biosensor was placed into
its custom packaging with micro-channels as shown if
Fig. 2b and 0.01M PBS solution was introduced
through one inlet until the output current was stabilized. The PBS injection was stopped and replaced
Figure 2: a) Schematic of the nanowire biosensor and picture
with 10µM vasopressin injection through the other inof the flow through system b) demonstration of sensor reverslet, which was again replaced by a PBS injection after
ibility c) current change due to change in vasopressin con30s. As shown in Fig. 2c, the initial introduction of
centration in sheep serum. At t=0 100pM vasopressin solu10µM vasopressin results in a current decrease through
tion is introduces followed by 50 pM, 10pM and sheep serum
the sensor almost instantaneously which is a consewith no additional vasopressin d) Percentage decrease in
quence of depletion of charge carriers on the carbon
current from the initial no vasopressin case due to different
nanotubes due to vasopressin to aptamer binding. The
vasopressin concentration in sheep serum (average of 5 exrecovery of current upon PBS injection at t= 50s sugperiments)
gests the release of vasopressin molecules from the
immobilized aptamer which demonstrates the reversible nature of the binding events. The slower rate of current change
during the dilution could be attributed to the low dissociation constant of aptamer-vasopressin complex, confirming the
high affinity of aptamer molecules to vasopressin.[14] As another extension of our previous work which monitored vasopressin in serum samples ranging from 100nM to 1mM, the sensor in this work was tested in serum with more physiologically relevant range of 10 to 100pM. As shown in Fig. 2d, the biosensor was subjected to sheep serum (Valley Biomedicals Inc.) with 100pM, 50pM, 10pM and no added vasopressin in the respective order. Duration for each injection was
150s. With each dilution, the current through the sensor increased as expected from vasopressin releasing from the aptamer. The sensor was not cleaned in between different serum injections in order to mimic the intended operation conditions. This however signifies that the actual vasopressin concentration in exposure was the weighted sum of those of previous injections.
RESULTS AND DISCUSSION
The biosensor, drug delivery unit and
fuel cell stack were assembled into a single device. The integrated device was
placed inside a closed chamber within a
fluid network for the integrated testing
which was designed to resemble the physiological vasopressin states of late-phase
HS patients. According to the clinical
study by Jochberger et al.[15] vasopressin concentration for patients with hemorrhagic shock rises from the normal
6±3pM range to 52±30pM as the body
tries to reverse hypotension by releasing
vasopressin. Late-phase HS is correlated
Figure 3: a) setup for integrated device testing b) experiment showing voltage
decrease due to decrease in vasopressin levels (with 100pM initial concentration and gradual decrease due to flow of 10pM solution). The inset shows the
voltage drop during the initial 720s before the drug delivery is activated
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to the decrease of vasopressin from this initially high value which indicates the failure for reversal. Therefore in the final
experiment to demonstrate the operation of the hemoAID at physiologically relevant vasopressin concentrations, the
chamber housing the integrated device (Fig. 3a) was initially filled with 100pM vasopressin solution which is in the order
of the high initial value (52±30pM) for patients. The vasopressin concentration was then lowered by introducing 10pM
solution into the chamber using a VWR Mini-Pump Variable Flow. The flow-rate of 0.115mL/s was in balance with the
rate of gravity-driven discharge and maintained the solution level in the chamber relatively constant. As shown in Fig. 3b,
the sensor’s voltage decreased over the 14 minutes in response to the decrease in vasopressin levels. It is expected that the
release of vasopressin from the aptamer by dilution increased the conductivity of the CNT platform (decreased the resistivity), causing the sensor’s voltage output to decrease at constant current. Upon reaching the pre-determined threshold of
0.25% voltage decrease ratio for more than 10s, the drug delivery was activated autonomously by 35V application between the top and bottom electrodes. 16μL of highly concentrated vasopressin at 200μM was released, which was detected successfully by the sensor with 3.88% voltage ratio increase.
CONCLUSION
In this paper, we have demonstrated the operation of an integrated autonomous device for the monitoring and delivery
of vasopressin. The final integrated experiment shows how the hemoAID device can be used to detect a gradual drop in
vasopressin between physiologically relevant concentrations (from 100pM to 10pM) and deliver a highly concentrated
does of vasopressin to stabilize the situation. In addition we have shown the sensor’s ability to operate in a complex medium such as sheep serum thus opening the door to the development of an implantable device for late phase hemorrhagic
shock prevention.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the support of the Office of Naval Research (ONR) under award number
N000141010115 “Autonomous Microfluidic Devices for Battlefield Health Assessment and Treatment”. In addition,
V.O. wishes to acknowledge the support of the Canadian institution NSERC through a fellowship. Some elements of the
integration were also carried out with the support of NSF grant #1014891. The fabrication steps described in this paper
were carried out at the Cornell Nanoscale Facility (CNF) and the Nanobiotechnology Center at Cornell University
(NBTC).
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CONTACT
* D. Erickson; tel: (607) 255-4861; [email protected]
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