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A mm-Sized Wirelessly Powered and RemotelyControlled Locomotive Implant

Daniel Pivonka, Anatoly Yakovlev, Ada Poon, Senior Member, IEEE, and Teresa Meng, Fellow, IEEE

Abstract—A wirelessly powered and controlled implantabledevice capable of locomotion in a fluid medium is presented.Two scalable low-power propulsion methods are described thatachieve roughly an order of magnitude better performance thanexisting methods in terms of thrust conversion efficiency. Thewireless prototype occupies 0.6mm×1mm in 65 nm CMOS withan external 2mm×2mm receive antenna. The IC consists of amatching network, a rectifier, a bandgap reference, a regulator,a demodulator, a digital controller, and high-current drivers thatinterface directly with the propulsion system. It receives 500µW from a 2 W 1.86 GHz power signal at a distance of 5 cm.Asynchronous pulse-width modulation on the carrier allows fordata rates from 2.5–25 Mbps with energy efficiency of 0.5 pJ/bat 10 Mbps. The received data configures the propulsion systemdrivers, which are capable of driving up to 2 mA at 0.2 V andcan achieve speed of 0.53 cm/sec in a 0.06 T magnetic field.

Index Terms—Biomedical telemetry, drug delivery, implantablebiomedical devices, low power, micro-scale fluid propulsion,noninvasive, wireless health monitoring, wireless powering.

I. INTRODUCTION

IMPLANTABLE devices capable of in vivo controlledmotion can serve a variety of existing applications, and

they also open up new possibilities for emerging noninvasivemedical technologies. Drug delivery is an especially attractiveapplication, as drugs can be precisely targeted to problematicregions with minimal disturbance to the rest of the body.Additionally, precision guidance through fluid cavities couldenhance both endoscopic and cardiac procedures that currentlyrely on catheter systems, such as angioplasty and coronarystent treatments, cardiac arrhythmia ablation surgeries, anddiagnostic techniques like endomyocaridial biopsies. Heartdisease is the leading cause of death, and this technologycould improve the effectiveness of these procedures as well asreducing costs. With these enhancements and cost reductions,it is possible to develop new noninvasive procedures forcardiac care or endoscopy that can aid in prevention, earlydetection, and treatment of a variety of conditions.

The key challenge in designing an effective locomotive im-plant is the limited power budget, as existing fluid propulsionmethods have significant power requirements and batteries hin-der the potential for miniaturization. Existing wireless devices

Manuscript received May 21st , 2012. This work was supported in part bythe C2S2 Focus Center, one of six research centers funded under teh FocusCenter Research Program (FCRP), a Semiconductor Research Corporationentity. Additional support was provided by Olympus Corp and the StanfordGraduate Fellowship.

D. Pivonka, A. Yakovlev, A. Poon, T. Meng are with the ElectricalEngineering Department, Stanford University, Stanford, CA 94305, USA (e-mail: pivonka@stanford.edu, yakovlev@stanford.edu, adapoon@stanford.edu,yhm@stanford.edu).

rely on batteries and have very limited or no motion control.These devices are not small enough to travel through the circu-latory system, and are most widely used in the gastrointestinaltract for endoscopy [1]. There are many proposed systemsbeing researched that attempt to enhance motion either withmechanical actuation or by manipulating passive magneticstructures with external fields [2], [3]. Mechanical techniquestend to require significant power, have complex designs andmoving parts, and have very low thrust efficiency as they arescaled down. Devices relying on passive magnetic fields re-quire complex external equipment have limited controllabilityin small regions (typically a few cm3), and move very slowlyas they are scaled down [4]. Because of the challenges indeveloping a highly efficient and scalable propulsion method,wireless locomotive devices have not been possible.

An alternative to both the mechanical and passive magneticpropulsion techniques can be accomplished with the manip-ulation of Lorentz forces on current-carrying wires. Theseforces require a simple static magnetic field, and are powerefficient and controllable. In our prior work, we developedtwo mechanisms for generating propulsion with these forces[5], [6]. The first is based on magnetohydrodynamics (MHD),in which current is driven directly through a conductive fluid.This current flowing through the fluid experiences a force inthe magnetic field, and the device experiences an equal andopposite force that propels it forward. The second methodrelies on asymmetries in fluid drag forces experienced byan oscillating structure. The oscillations are achieved withalternating currents flowing through a loop of wire, whichexperiences torques in the magnetic field. With loops indifferent orientations, the structure can be oscillated to create”swimming” motions similar to fins on a fish. Both of thesemethods have significant potential for operating efficiently atvery small sizes, and can enable fully wireless locomotiveimplants.

In this paper, we will present a fully wireless device capableof controlled motion in water. An external transmitter powersthe device wirelessly with data modulated on the powercarrier. Propulsion is achieved via the described methods basedon the manipulation of Lorentz forces, and the prototypeoperates with either method. Fig. 1 shows the conceptualoperation of the prototype travelling through the bloodstreamwith MHD propulsion. The constructed device is comprisedof a 2mm×2mm receive antenna and an integrated circuitthat includes a matching network, a rectifier, a regulator, ademodulator, a digital controller, and high-current drivers thatinterface with the propulsion system. This prototype can travelthrough any fluid and can potentially be navigated through

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Fig. 1. Conceptual operation of the device in the bloodstream.

the circulatory system, enabling a variety of new medicalprocedures.

The organization of the paper is as follows. Section IIpresents the analysis and simulation of the fluid propulsionmethods based on Lorentz forces. Section III describes thedesign of the wireless power transmission system as well asthe data receiving architecture. The circuit implementation ispresented in section IV, and section V discusses the experi-mental results and summarizes performance. Section VI thenconcludes the paper.

II. ELECTROMAGNETIC PROPULSION

Propulsion for implantable devices has not been possiblebecause of the high power requirement for mechanical de-signs, and the high complexity of passive magnetic designs.Our prior work based on Lorentz forces demonstrates twomethods with significant advantages over existing techniquesin terms of power efficiency, scalability, and controllability[5], [6]. The first method drives current directly through thefluid using magnetohydrodynamics (MHD), and the secondswitches current in a loop of wire to oscillate the device, whichexperiences asymmetric drag fluid forces. In both methods,the force is proportional to current, and therefore maximizingcurrent will maximize the speed.

The thrust forces work against fluid drag forces, which arevelocity dependent. This dependence varies with the Reynoldsnumber of the fluid flow. The Reynolds number is a dimen-sionless representation of the ratio of the inertial forces to theviscous forces, and is given by

Re =ρv2D

µ(1)

where ρf is the density of the fluid, v is the velocity, D is acharacteristic dimension, and µ is the fluid viscosity. For highReynolds numbers (>1000), the drag force is given as

D =12ρv2CDA (2)

where Af is the frontal area of the device, and CD is the shapefactor. These forces scale with area, and as will be shown,the thrust forces for both propulsion methods scale linearlywith length. This means that in the high Reynolds regime, lesscurrent is needed to maintain a constant speed as the device

Fig. 2. Operation of magnetohydrodynamic (MHD) propulsion.

is scaled. As the Reynolds number decreases, viscous forcesbecome dominant. For extremely low Reynolds numbers (<1),the drag force scales linearly with the size of the deviceas predicted by Stokes Law. In the low Reynolds regime,the current must be kept constant as the device is scaled tomaintain a constant speed. For mm-sized devices moving atcm/sec speeds in water, the Reynolds number ranges fromroughly 10–100, so numerical fluid simulations are necessaryfor an accurate analysis of the fluid drag forces.

A. Magnetohydrodyanmic (MHD) Propulsion

MHD propulsion drives electric currents through fluids, sothe efficiency of this method depends on the fluid conductivity.The basic principle of motion is described in Fig. 2. Theconductivity of human blood varies approximately from 0.2S/m to 1.5 S/m depending on the concentration of bloodcells [7]. This translates to a load of less than 300 Ω atthe device, which varies with the size, shape, and distancebetween the electrodes as well as the temperature and appliedvoltage. Stomach acids tend to have higher conductivities butalso vary significantly with normal biological processes. Inthe following analysis, the required current for a given speedwill be estimated as a function of the size of the device andthe background magnetic field. This will give insight into thescalability of the propulsion method and also provide a designtarget for the circuitry.

The thrust force for MHD propulsion is the Lorentz force onthe current flowing through the fluid. These forces are givenin the equation below, where I is the current in the wire, L isa vector denoting the length and direction of the wire, and Bis the background magnetic field:

F = I L×B (3)

These forces scale linearly with the length of the wire L,which allows for the operation of very small devices. Itscales more slowly than high Reynolds drag forces, whichmeans that for smaller devices constant current scaling resultsin higher speeds; and it scales evenly with low Reynoldsdrag forces, which means that constant current scaling resultsin a constant speed. Additionally, the amount of force islinearly proportional to the background magnetic field, so theperformance of this method improves with stronger magneticfields. To accurately estimate the speed, numerical simulations

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Fig. 3. Simulated performance of MHD propulsion.

of the fluid mechanics are performed. Fluid simulations basedon incompressible Navier-Stokes flows predict the fluid dragforces, and from these forces the steady-state velocity can beextracted. In Fig. 3, the required current is estimated for agiven speed as a function of the size of the device with abackground magnetic field of 0.1 T, which can be generatedwith permanent magnets. This analysis shows that mm-sizeddevices should be able to achieve speeds on the order of cm/secwith approximately 1 mA of current.

The amount of current that can be driven is a strong functionof the fluid conductivity, and has significant nonlinear varia-tions with electrode area, electrode materials, applied voltage,and the types of ions in the fluid. To drive 1 mA through blood(which has the lowest conductivity of the targeted fluids),roughly 300 mV is required, resulting in a power consump-tion of around 300 µW. As the fluid conductivity increases,the required power decreases. These power requirements arewithin the bounds of optimized wireless powering techniquesthrough tissue, so miniaturized locomotive implantable devicesare possible with this method.

B. Asymmetric Fluid Drag Propulsion

The second fluid propulsion method relies on asymmetriesin fluid drag created by an oscillating asymmetric structure.The structure is oscillated by alternating currents in a loopof wire that is placed in a background magnetic field. The

Fig. 4. Principle of operation for asymmetric fluid drag propulsion.

basic principle of operation is described in Fig. 4. The forcesgenerated with this method are a function of the fluid viscosity,which for most bodily fluids is similar to water. The perfor-mance of this method is enhanced as the number of loopsis increased, and the amount of current that can be drivenis limited by the internal resistance of the circuitry and theamount of power delivered through the antenna. The followinganalysis estimates the required current as a function of the sizeof the device and the desired speed. This analysis predicts thedevice scalability and also specifies the requirements on thecircuitry.

The thrust forces result from asymmetric fluid drag on astructure that oscillates with electromagnetic torque of

τem = IL2B (4)

where I is the current on the loop, L is the length of thewire, and B is the background magnetic field. The asymmetryin the fluid drag is represented by the shape factor, CD. Byintegrating the fluid drag along one side of the device, the netforce can be represented as

Fp ∝ (CD,H − CD,L)L4ω2 (5)

where CD,H and CD,L represent the different shape factorsdue to the asymmetry, L is a side length of device, andω is the rotation frequency. Assuming small angle rotationsand constant angular acceleration, which is true when theelectromagnetic torque dominates the fluid drag torque, theaverage angular velocity over a half-cycle is

ωavg =√θτem/4Iint (6)

where θ is the angle of rotation and Iint is the momentof inertia. Realizing that τem ∝ L2 and Iint ∝ L5, constantcurrent scaling results in the average angular velocity scalingas ω ∝ L(−3/2). Using this result in the equation for the netforce, we again find that these thrust forces scale linearly withL. This method scales in the same way as MHD propulsionand allows for the operation of very small devices. As theReynolds number decreases, the fluid drag becomes muchmore shape dependent, which complicates analytical analysis.For accurate estimations of the forces on these devices, weagain rely on numerical simulations of the fluid mechanics.

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Fig. 5. Fluid simulations of drag torque and resulting net force

For this propulsion method, the simulations predict boththe average fluid drag torque and the average net force over acycle as a function of the rotation frequency and the size of thedevice. The fluid drag torque and the average force are shownin Fig. 5. These simulations agree with the predicted scalingbehavior in terms of size and rotation frequency. From the fluiddrag torque simulation, the current required to achieve a givenrotation frequency can be estimated. The simulated net forcescan then predict the speed, which relates to the current shownin Fig. 6. From these simulated results, mm-sized devices witha single loop of wire require currents of approximately 1 mAto achieve cm/sec speeds in water with a 0.1 T magnetic field.Additional loops of wire enhance the performance, essentiallymultiplying the current experiencing a force.

III. WIRELESS CHIP ARCHITECTURE

The purpose of the chip was to create a wireless prototypethat demonstrates the effectiveness of the propulsion systemat the mm-scale. The specifications were derived from the

Fig. 6. Estimated performance of asymmetric fluid drag propoulsion.

requirements of the propulsion methods, which need approx-imately 1 mA of current for cm/sec speeds. The integratedcircuit (IC) must receive both power and data from the externalreceiver to propel and navigate the device, and must operatewith a limited power budget. The chip architecture is shownin Fig. 7, and the IC consists of a matching network, a charge-pump connected rectifier, a regulator, a bandgap referencecircuit, a demodulator, a digital controller, and configurableelectrode drivers. There are no external components exceptfor the receiving antenna. The key challenge in this designis driving the high-current propulsion system efficiently andcontrollably while continuously harvesting RF energy. Poweris the primary limitation, and minimizing power consumptionwas critical for the design.

The non-linear electrode-fluid resistance limits the mini-mum voltage required to drive the current, and is estimated atapproximately 200–300 mV. The propulsion system dominatesthe power budget consuming over 90% of the total deliveredpower to the chip. The required 1 mA of current for propulsion

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Fig. 7. Integrated circuit architecture.

needs to be sourced from no more than 300 mV while theactive circuitry requires a regulated voltage of 700 mV anddraws approximately 15 µA. Using a linear regulator for thepropulsion system is inefficient, and a switching regulatorrequires large passive components, accurate on-chip clock, andcomplex controllers [8]. Therefore, the chip was designed todrive the propulsion system from the first rectifier stage, whichprovides an unregulated 200–300 mV supply depending on thereceived power and can source the required current. Becausethe loading from the propulsion system varies with navigation,an adaptive loading network is also necessary to maintaineffective matching at the antenna. The first rectifier stage isfollowed by three additional stages to boost the voltage, whichis then regulated for the analog and digital circuits.

The size requirements prohibit the use of external energystorage components, so power must be continuously trans-mitted to the device. Power transmission must adhere toFDA safety regulations for tissue heating. From prior work,mm-sized antennas can receive approximately 200–300 µWat low-GHz frequencies safely [9]. A 2mm×2mm antennaprovides sufficient power for this design, and performing afrequency sweep with the antennas yields an optimal frequencyof 1.86GHz. It is important that the modulation schememinimally affects the power transfer to the device because ofthe limited power budget. Frequency-shift keying (FSK) andphase-shift keying (PSK) operate with a constant envelope, butthe demodulator requires either a frequency or phase-lockedloop for carrier synchronization, which consumes significantpower at high frequency [10]. Amplitude modulation doesnot require carrier synchronization, and the modulation depthand duty cycle can be designed to minimize the impact onpower delivery. For this reason, we implemented amplitudeshift keying (ASK) with low modulation depth (minimum of9%), and the pulse width (PW) encodes the data allowing forasynchronous clock and data recovery with simple circuitry.

A high-level description of the data receiver is shown inFig. 8. The demodulator provides both the clock signal for thedigital controller and decodes incoming data. The demodulatorinterface with the matching network uses two rectifiers: thefirst has a small time constant and tracks the envelope, and thesecond has a large time constant and approximates the averageof the envelope. These two signals are input to a comparatorto generate the digital signal Vout1. This signal is buffered toproduce a digital clock. Vout1 is also integrated and comparedto a threshold to decode the data. With this implementation,

Fig. 8. Demodulator and description of operation.

long pulses produce high output and short pulses produce lowoutput. The demodulated data is captured on the falling edge ofthe clock by a low-power digital controller, which configuresthe high-current electrodes for driving the propulsion system.

IV. CIRCUIT IMPLEMENTATION

A. Antenna and Matching Network

The antenna dominates the size of the prototype, and isimplemented with a 2mm×2mm loop on a PCB using Rogers4350 substrate. External components are not possible due tosize constraints, so a balanced L-match consisting of onlycapacitors was implemented because on-chip inductors havesignificant loss and occupy large area [11]. The total qualityfactor of the antenna and the matching network in air isestimated at 39. The chip input impedance is dominated bythe propulsion system, and loading varies significantly duringnormal operation. Therefore, an adaptive loading network wasimplemented to maintain an effective match. When the chipis powered on and before the controller is reset, the gateof transistor Mn4 in Fig. 9 is weakly pulled up by Vunreg,which shunts the first rectifier stage with an internal 200 Ωresistor. After the digital supply is enabled, the weak pull-down transistor Mn3 slowly turns off the shunt resistor. Oncethe power-on reset (POR) signal has been issued, the digitalcontroller is reset and takes control of the network, adjustingthe resistance based on incoming data.

B. Start-up and Power-on Reset Circuits

Start-up circuitry for the initial power-on is necessary toensure that the antenna impedance maintains a match and thatthe chip enters a known state. A start-up network that turnson a pass transistor for the digital supply voltage is shownin Fig. 9. The cross-coupled inverters are skewed in oppositedirections to prevent metastability, and the delay is controlled

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Fig. 9. Power-on shunting and Vdd enable circuit.

Fig. 10. Power-on reset signal generation circuit.

by a capacitor at the supply of the cross-coupled invertersthat slowly charges through a weak current source. This delayensures that the analog supply voltage has reached a stable 700mV before powering on the digital circuits. Once the digitalsupply is enabled, a POR pulse is issued after an additionaldelay. This pulse generation is shown in Fig. 10, and is similarto the design used in [12]. The pulse width is set by the delayof a capacitively loaded inverter chain that provides a sufficientduration pulse to reset the controller.

C. Power Management

When the antenna receives 500 µW, the RF input voltageto the rectifier is 350 mV. Conventional diode-capacitor ladderrectifiers suffer from low efficiency at low input voltage.Therefore, charge-pump connected self-driven synchronousrectifiers (SDSR) based on [13] are used with low-Vt devices.The first stage of the rectifier is sized 10 times larger than theconsecutive stages because the propulsion system is drivendirectly from this first stage. It outputs an unregulated 200–300 mV and drives roughly 1 mA of current. The remainingthree stages are all sized the same and output 0.9–1.2 V whiledriving 15 µA. The pump capacitance between these threestages is 5 pF. The simulated efficiency of the rectifier isapproximately 55%.

The unregulated supply voltage fluctuates significantly withvariations in available power due to varying link gain as thedevice moves, propulsion driver strength, and switching noisefrom the digital circuits. The device must also be insensitive totemperature variations. To create a stable 700 mV supply for

Fig. 11. Regulator circuit with both analog and digital supply.

the active circuitry, we implemented a low drop-out voltageregulator that relies on a bandgap reference circuit similar to[14]. A total of 86 pF of smoothing capacitance was usedto maintain stable voltage at the supply. The schematic ofthe regulator is shown in Fig. 11. The regulated voltage issampled via a resistive voltage divider and is compared to thebandgap reference output voltage of 525 mV. The resistivedivider also outputs a voltage of Vdd/2, providing a referencefor the demodulator. Capacitor C1 is added to help stabilize thefeedback loop. The regulator has an overall efficiency of 58%.However, the dissipated power due to the rectifier inefficiencyis only

ηdegradation =Plost

Ptotal=

(Vunreg − Vreg)Ireg

Ppropulsion + Pcircuits= 3% (7)

of the total power consumption because the unregulatedpropulsion system dominates power usage.

D. Clock and Data Recovery

The low modulation depth and fluctuating input power makeit impossible to use a fixed reference voltage for the ASKthreshold detector. Instead, a dynamic reference voltage isgenerated concurrently with envelope detection. The schematicof the envelope detector and dynamic reference generator isshown in Fig. 12. Both circuits use cross-coupled PMOStransistors to achieve full-wave rectification. The envelopedetector RC time constant filters out the carrier and passes thedata. In the dynamic reference generator, the RF input voltageis resistively divided to weakly turn on the cross-coupled tran-sistors. The higher on-resistance and larger load capacitanceform a large RC time constant, which effectively averages theenvelope. The resistor at the output of envelope detector alignsthe average of the envelope with the dynamically generatedreference voltage.

Clock and data signals are recovered from the envelope andthe dynamic reference, which are first input to a comparator togenerate the full-swing digital signal Vout1. This comparatorconsists of two differential amplifier stages followed by aSchmitt-trigger inverter as shown in Fig. 13. Two low-powerdifferential amplifiers ensure that the gain remains high for a

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Fig. 12. Envelope detection and dyanmic reference voltage generationcircuits.

wide range of common-mode input voltages, which vary de-pending on input power. The Schmitt-trigger inverter reducesthe crowbar current due to slow transitions of the amplifieroutput, and it also decreases sensitivity to noise [15]. Theresulting digital signal is both buffered to generate the clockand integrated to decode the data as described in Fig. 14. Theintegrator consists of a skewed inverter with a capacitive loadto provide slow rising and fast falling edges. This capacitancedefines the pulse width that causes the data to transition fromlow to high, and therefore sets the minimum and maximumdata rates. On the falling edge of each incoming pulse, data iscaptured from a comparator that compares the integrated resultwith a fixed reference at Vdd/2. This comparator consists of asingle differential pair followed by a Schmitt-trigger inverter.The entire demodulation system draws a current of 5 µA.

E. Controller

The digital controller receives data and clock signals fromthe demodulator, and configures the propulsion system driversand the adaptive loading network. Data transmission beginswith a 5-bit prefix that, when received, enables a shift registerto begin accepting the 55-bit data packet. While data isbeing shifted into the register, the prefix detection circuitry isdisabled. Once the entire packet is received, the shift registerpushes all the data to a memory register that stores it untilthe next valid transmission. By only enabling the necessarycircuitry in each stage of data reception, power consumption isminimized. Because the clock is derived from the data signal,when no data is being received the only current drawn is dueto leakage. The estimated average power consumption of thedigital controller while receiving data is 2 µW, and it occupies0.009 m2.

F. Configurable High-Current Drivers

The chip has 6 high-current electrode drivers with config-urable strength to accommodate both propulsion mechanisms.Each of the drivers can be independently set to Vpropulsion fromthe first rectifier, ground, or left floating. Additionally, thedriver strength can be controlled with 4 parallel transistors,and ranges from 20–1000 Ω. This configurability is necessaryto adapt to uncertainty in electrode-fluid resistance and toenable speed and steering control. Data in the memory registerdirectly controls the electrode driver state and strength.

Fig. 13. First comparator that converts the envelope into digital signal.

Fig. 14. Integrator and second comparator for data decoding.

V. EXPERIMENTAL VERIFICATION

Experimental tests verified all the elements of the designincluding wireless power transmission, the ASK-PWM datatransfer, the analog and digital circuitry, and the two propul-sion schemes. Independent tests evaluated the wireless link andthe circuit performance, and testing of the complete systemdemonstrated navigation and propulsion through fluids. Eachexperiment will be described in detail in this section. Theoverall circuit performance is summarized in Table I, and theannotated chip micrograph is shown in Fig. 20.

A. Wireless Power Transmission

The transmitter consists of a signal generator, a high-frequency amplitude modulator, a power amplifier, and a4cm×4cm loop antenna fabricated on PCB. The IC was wirebonded to a 2mm×2mm antenna fabricated on a Rogers 4350substrate to minimize RF losses. A frequency sweep of thelink gain was tested at a separation distance of 5 cm both inair and with the device placed on the surface of water. Themeasurements are shown in Fig. 15. From this plot, the qualityfactor in air is 39 for the antenna including the matchingnetwork. The rectified output voltage and the regulated voltageare plotted as a function of input power in Fig. 16, showingthat the device first powers on with roughly –7 dBm. With arectifier efficiency of approximately 55%, roughly 2 W mustbe transmitted to receive 500 µW, resulting in approximately250 µW of usable power after rectification.

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Fig. 15. Measured link gain in air and water.

B. ASK-PWM Data Transfer

Data modulation was designed to minimize impact onpower delivery with a low power circuit implementation. Toaccomplish this, an asynchronous design was implemented thatoperates with minimal modulation depth and without carriersynchronization circuitry. This method allows for variable datarates and modulation depths. In order to test the range of op-eration, a versatile high-frequency modulator was constructed.The data signal was generated from an FPGA and input tothe modulator, which modulates the output from the signalgenerator at an adjustable depth from 0–100%. The FPGAwas able to stream data at up to 25 Mbps, and the chipproperly received data from 2.5–25 Mbps. Additionally, thechip functioned with as low as 9% modulation depth. Thespectrum of the carrier modulated at 9% with an 8.3 MHzclock is shown in Fig. 17, and the received clock and datasignals on chip are shown in Fig. 18. The power consumptionof the demodulating circuitry is approximately 5 µW at 10Mbps, resulting in energy efficiency of 0.5 pJ/bit.

C. Fluid Propulsion

The IC was designed to function with either of the describedfluid propulsion mechanisms. The chip and receive antenna areencapsulated in RF-transparent epoxy to protect them from thefluid. The leads from the electrodes are exposed to adapt thedevice for use with either of the fluid propulsion methods. ForMHD propulsion, these leads are positioned to directly connectto a conductive fluid, and salt water was used for testing.For the method relying on asymmetric fluid drag forces, theelectrodes are connected to loops of wire that oscillate thedevice. In both test cases, the device floats on the surfaceof the water with a neodymium magnet placed next to thefluid to provide a magnetic field. Even though testing wasperformed on floating devices, both propulsion methods canfunction when fully submerged.

Fig. 16. Rectified output and regulated supply voltage.

Fig. 17. Spectrum of the 1.86 GHz carrier modulated at 9% depth with an8.3 MHz clock.

The experimental setup for MHD propulsion is shown inFig. 19. During propulsion testing, the external antenna trackedthe device at a distance ranging from 2 to 5 cm. Data is contin-uously transmitted with commands to control the motion. Thedevice achieves speeds of up to 0.53 cm/sec in a 0.06 T fieldwith approximately 1 mA, and can be navigated successfullyalong the surface of the water. Performance improves as themagnetic field is increased, so MRI systems will generateapproximately 100 times as much propulsion force.

The setup for asymmetric fluid drag propulsion is verysimilar to MHD propulsion. The device is connected to 40loops of wire, which are oriented to oscillate it. The prototypehas an attached fin that experiences asymmetric fluid dragwhen oscillating. By changing the orientation of the magneticfield, the device can oscillate along the surface of the water, orinto and out of the water. The external antenna is again placed

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Fig. 18. Measured waveforms of data and clock signals at the output of thedemodulator.

Fig. 19. MHD propulsion test setup. The device is navigated to the targetdestination.

above the device and continuously transmits data. The forceson the device are much stronger for this method because theadditional loops and smaller load; however propulsion is muchmore difficult to control. This method is also more sensitiveto non-uniformities in the magnetic field. Additionally, theantenna link degrades as the device rotates, causing frequenterrors in data reception. For this method to operate effectively,a new antenna link and a feedback controller are necessary.

VI. CONCLUSION

In this work, we have demonstrated a fully wireless3mm×4mm prototype capable of controlled motion in a fluidenvironment, requiring only a static background magnetic fieldgenerated from permanent magnets. The device is wirelesslypowered and operates with approximately 250 µW, and travelscontrollably at 0.53 cm/sec in a 0.06 T field. Additionally,data transfer is fast and efficient, achieving rates of 25 Mbpsand consuming only 0.5 pJ/bit at 10 Mbps. These devices canserve as a versatile tool for a variety of medical treatments thatrequire precise guidance including drug delivery, diagnostics,and cardiac catheter treatments.

Two propulsion methods have been demonstrated, and bothcan operate at sub-mm sizes. MHD propulsion is simple tocontrol with high efficiency, though it requires a conductivefluid. The second method relies on asymmetric drag forces

TABLE IPERFORMANCE SUMMARY

Fig. 20. Chip layout.

experienced by an oscillating structure. This method has higherforces and can function in any fluid, but is more sensitive tofield non-uniformities and is more difficult to control. Futurework can improve the robustness and controllability of thesemethods, and can further optimize wireless power transfer todesensitize it to the motion of the device.

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Daniel Pivonka (S’09) received his B.S. in engineer-ing from Harvey Mudd College in 2007, his M.S.in electrical engineering from Stanford Universityin 2009, is currently working at Stanford Universitytoward his Ph.D. degree in electrical engineering.

He interned at the Beckman Laser Institute in2005, at the Southwest Research Institute in 2006,and at ViaSat in 2007. His current research interestsis in wireless implantable devices capable of loco-motion through fluid cavities.

Daniel Pivonka is a recipient of the StanfordGraduate Fellowship and the Clay/Wolkin Fellowship.

Anatoly Yakovlev (S’04) received his B.S. degreein electrical engineering from Cal Poly Pomonain 2007, where he graduated valedictorian of theSchool of Engineering. In 2009, he received hisM.S. degree in electrical engineering with emphasisin analog/RF circuits from Stanford University andis now continuing to pursue his Ph.D. degree. HisPh.D. work focuses on wireless powering and datacommunication for biomedical devices.

Between 2007 and 2008 he was with the Ad-vanced Channel Architecture team at Western Dig-

ital. In summer 2011, he was with VLSI Group at Oracle Labs working onclock and data recovery circuits for optical interconnects.

Ada Poon Ada S. Y. Poon (S’98–M’04–SM’10)was born in Hong Kong. She received the B.Engand M.Phil. degrees in Electrical and ElectronicEngineering from the University of Hong Kong, andreceived the M.S. and Ph.D. degrees in ElectricalEngineering and Computer Sciences from the Uni-versity of California at Berkeley.

In 2004, she was a senior research scientist at IntelCorporation, Santa Clara, CA. In 2005, she was asenior technical fellow at SiBeam, Inc., Fremont,CA. In 2006–2007, she was an assistant professor

at the Department of Electrical and Computer Engineering in the Universityof Illinois at Urbana-Champaign. Since 2008, she has been at the Departmentof Electrical Engineering in Stanford University, where she is currentlyan assistant professor. Her research focuses on applications of wirelesscommunication and integrated circuit technologies to biomedicine and healthcare.

Teresa Meng (S’82-M’83-SM’93-F’99) received thePh.D. degree in electrical engineering and computerscience from the University of California,Berkeley,in 1988.

She is the Reid Weaver Dennis Professor of Elec-trical Engineering at Stanford University, Stanford,CA. Her current research interests focus on neu-ral signal processing and bioimplant technologies.In 1999, she left Stanford University and foundedAtheros Communications (NASDQ: ATHR), whichis a leading developer of semiconductor system

solutions for wireless communications products. She returned to StanfordUniversity in 2000 to continue her research and teaching.

Dr. Meng received the 2009 IEEE Donald O. Pederson Award, the DEMOLifetime Achievement Award, the McKnight Technological Innovations inNeurosciences Award in 2007, the Distinguished Lecturer Award from theIEEE Signal Processing Society in 2004, the Bosch Faculty Scholar Awardin 2003,the Innovator of the Year Award by MIT Sloan School eBA in2002, and the CIO 20/20 Vision Award, a Best Paper Award from the IEEESignal Processing Society, a National Science Foundation Presidential YoungInvestigator Award, an ONR Young Investigator Award, and an IBM FacultyDevelopment Award, all in 1989. In 2002, she was named one of the Top10 Entrepreneurs by Red Herring for 2001. Dr. Meng is a member of theNational Academy of Engineering.