Implantable Microelectronic Devices

Transcription

1 ECE 883/483 Implantable Microelectronic Devices Fall - 5 Maysam Ghovanloo (mgh@gatech.edu) School of Electrical and Computer Engineering Georgia Institute of Technology 5 Maysam Ghovanloo Outline Introduction Inductive Coupling Fundamentals Coil Design and Optimization 5 Maysam Ghovanloo

2 High Efficiency Inductive Power Transmission Advanced Bionics Inc. University of Southern California Alfred Mann Institute - USC Medtronic Corporation Battery powered devices: ow stimulus pulse rate Autonomous (after initial adjustments) Small number of stimulating sites Inductively powered devices: High current (Neuromuscular stimulators) High stimulus rate (Cochlear implants) arge number of sites (Visual prostheses) All implants should be wireless. 5 Maysam Ghovanloo 3 Reactive vs. Radiative Carrier signal wavelength: ~ : Near field /r 3 ~ : Transition zone ~ : Far field /r f = GHz = 3 cm All transcutaneous EM interactions are in near field. Near field ~ /: Reactive: Energy is stored in the field very close to the antenna and it can return back to the antenna in a regenerative fashion; Example: Inductive links (5 khz ~ 5 MHz) / ~ : Radiative: Only radiant energy and no storage; Example: MICS * Band (4~45 MHz) * Medical Implant Communication Service (MICS) P.V. Nikitin et al., Intl. Conf. on RFID, 7 5 Maysam Ghovanloo 4

3 Inductive Coupling Fundamentals 5 Maysam Ghovanloo 5 Self and Mutual Inductance Self inductance (): The ratio of the magnetic flux generated in an area enclosed by a conductor loop to the current passing through the loop. If r/r << One turn loop: R r For N turns of radii R i (i =,, N), α i,j = if i = j, and α i,j = otherwise di v dt 8R Rln, 5 Maysam Ghovanloo C.M. Zierhofer and E.S. Hochmair, TBME r i N j N Ri, r Mij Ri, R j, dr i j N, i i j Mutual inductance between two conductor loops (M ij ): Proportion of the magnetic flux generated by one loop that passes through the other loop (flux coupling). NR NR M( R, R, d) 3 ( R d ) 3

4 Non-Resonant Coupled Coils In addition to distance and geometry, alignment of the coils has a significant effect on their mutual inductance. Symmetrical-vertical coils have no mutual coupling. M d, ) M ( d,) cos( ) ( Coupling Coefficient (k ij ): M ij normalized by geometric mean of i and j. A pair of non-resonant inductively coupled coils: ( j) jm i j i i R V jm i V ( j) j R R V P R k M M. Soma et al., TBME Maysam Ghovanloo 7 Resonant Coupled Coils A pair of resonant inductively coupled coils: f res f C = C C p + C V ( j) ( j jm i R ) jc R At resonance, the resonant Ctank circuit produces a voltage that is about one order of magnitude larger than the nonresonant inductive circuit. K. Finkenzeller, RFID Handbok, Wiley 3 5 Maysam Ghovanloo 8 4

5 Calculating Power Transfer Efficiency (PTE) PTE is highly dependent on: k, Q, and Q Q = ω /R, Q = ω /R Secondary R C can be reflected onto the primary: Q = R P /ω At resonance ( = ) : Series C shorts Parallel ref C ref opens R P = Q R R = R R P P Rref k ( / ) RP kq C ref ( / )( C / k) /( k ) We are only interested in that portion of the power that is delivered from V s to R R.R. Harrison, ISCAS 7 5 Maysam Ghovanloo 9 Calculating Power Transfer Efficiency (PTE) Rref PTE R R k QQ Q. k Q Q Q Q QQ = Q +Q ref Q RP R R P = Key factors: k, Q, Q 5 Maysam Ghovanloo M.W. Baker & R. Sarpeshkar, TBioCAS 7 R For a given set of Q, Q and k values, there is an optimal load, R,PTE, which can maximize the PTE at that particular arrangement, such as the coupling distance. R,PTE Q,PTE Q, PTE Q ( k Q Q ) / For a given set of Q, Q and k values, Q the power delivered to the load (PD) Q, PD will be maximized if R = R ref : kqq 5

6 Maximizing PTE and PD in -Coil ink Parameter Sym Sim Inductance (H). Outer diameter (cm) D o 7 Fill factor Φ.8 Num. of turns n ine width (mm) w ine spacing (mm) s 35 Quality factor Q 96 Inductance (H).58 Coil diameter (cm) D o 4 Wire diameter (mm) w 5.68 Num. of turns n ine spacing (μm) s Quality factor Q 5.5 coupling distance (mm) d 5 coupling coefficient k.3 Power transfer efficiency (%) PTE 7. Power delivered to load (mw) PD 5.3 R,PTE = PTE and PD are highly dependent on R R is often predefined ) Matching circuits ) Multiple coils Impedance Transformation 5 Maysam Ghovanloo Kiani, Jow, and Ghovanloo, TBioCAS 3-Coil Inductive ink ( k3qq3)( k34q3q4 ) k4qq4 Q4 3 coil. cos( ) k Q Q A B Q A k 3QQ3 k34q3q4 k 4QQ4 B QQ3Q4 k 3k 4k tan ( B ) A 34 nd inductive link (M 34 ) provides designers with a DoF to adjust the reflected load on to 3 to be the optimal value: R,PTE. the reflected load from the (j+) th coil to the j th coil: R ref j j k, j, j jq( j) 3 R,PTE 3Q,PTE Q, PTE / ( k3qq3 ) 5 Maysam Ghovanloo Kiani, Jow, and Ghovanloo, TBioCAS Q 6

7 Maximizing PTE in 3-Coil ink By changing k 34 (and R ref,3 ), the 3-coil PTE can be kept at maximum for a wide range of R. A -coil link does not provide this flexibility, and PTE maximizes only for a specific R value. 5 Maysam Ghovanloo Kiani, Jow, and Ghovanloo, TBioCAS 3 PTE and PD in 3-Coil ink Maximizing PTE should not be at the cost of decreasing PD. Vs ( k3qq3 )( k34q3q4 ) Q4 P,3coil R ( k3qq3 k34q3q4 ) Q The optimal design maximizes both PTE and PD. k k 3, PD 34, PD / k34q3q 4 QQ3 k3qq 3 Q3Q4 / 5 Maysam Ghovanloo Kiani, Jow, and Ghovanloo, TBioCAS 4 7

8 4-Coil Inductive ink 4 coil ( [( k QQ ).( kqq )( k3q Q3 )( k34qq 3 4) Q4 k34qq 3 4) k3q Q3 ].[ k3q Q3 k34qq 3 4] Q 4-Coil link adds an additional DoF for impedance matching on the source side. If k is large, the reflected load onto increases dramatically, which helps maximize the PTE at the cost of reducing PD. Kiani, Jow, and Ghovanloo, TBioCAS 5 Maysam Ghovanloo 5 k PTE and PD in 4-Coil ink If k is large enough, 4-coil can tolerate variations in coil separation (k3) and maintain a large PTE. P,4coil s V R ( kqq )( k3qq3)( k [( k Q Q ).( k Q Q 3, PTE k 3, PD k ( k 34 3 Q Q 34 4 ) 3 Q Q ) Q Q ] 3 4 k3 Q Q.( k Q Q QQ ).( k34q3q Q Q ) ) Q Q / / Small overlap between high PTE and PD areas. Kiani, Jow, and Ghovanloo, TBioCAS 5 Maysam Ghovanloo 6 4 8

9 Effects of Driver Output Impedance, R s Available power: P av = V s /8R s 4-coil link maintains high PTE regardless of R s. PD (3-coil) >> PD (4-coil) V s = V, d 3 = cm 5 Maysam Ghovanloo Kiani, Jow, and Ghovanloo, TBioCAS 7 Comparing - 3- & 4-Coil inks for IMDs At d 3 = mm, 4-coil link provides 4.9% more PTE and times less PD than an equivalent 3-coil. At d 3 = mm, PTE difference is 7.7%, while PTE of 3-coil is 9 times larger than an equivalent 4-coil. For coils specifications, measurements, and more details: M. Kiani and M. Ghovanloo The circuit theory behind coupled-mode magnetic resonance based wireless power transmission, To appear in IEEE TCAS-I 5 Maysam Ghovanloo Kiani, Jow, and Ghovanloo, TBioCAS 8 9

10 Coil Design and Optimization 5 Maysam Ghovanloo 9 Why Printed Spiral Coils? PSCs can be fabricated on flexible substrates to conform to the outer body or brain surface curvature. PSCs occupy a small volume Planar shape suitable for implantation under the skin or within the epidural space. Wire-wound coils cannot be batch-fabricated or reduced in size without the use of sophisticated machinery. PSCs can be lithographically defined in one or more layers Skin on rigid or flexible substrates. PSCs offer more flexibility in optimizing their geometries. PSC optimization for medical Tissue implants has not been well studied. 5 Maysam Ghovanloo

11 Transcutaneous ink Power osses P B : Power drained from battery P S : Power delivered to the primary coil P : Transmitted power P T : Power passed through the tissue P : Received power P : Power delivered to the load (implanted electronics) U. Jow and M. Ghovanloo, TBioCAS 7 5 Maysam Ghovanloo Inductive Power Transfer Efficiency k QQ k Q Q Q R C R R R C Q RS Q R R S Power efficiency is a strong function of the coupling coefficient (k) and quality factor (Q) of the external and implanted coils. 5 Maysam Ghovanloo Ko et al., Med. Biol. Eng. Comput. 977

12 Modeling of Implanted Printed Spiral Coils (PSC) How to find the circuit parameters and parasitic component values for coils & based on PSC geometries? 5 Maysam Ghovanloo 3. 7 μ n d : Self Inductance An experimentally derived equation for the inductance of the square shaped coils: avg. 7 ln n: number of turns μ : permeability d avg : (d o + d i )/ φ (fill factor): (d o - d i )/(d o + d i ) d o d i Mohan et al., IEEE JSSC Maysam Ghovanloo 4

13 C p : Parallel Capacitance Parasitic parallel capacitance, C p, is determined by Spacing between conductive traces Overlapping areas Surrounding materials, such as substrates, coating, and tissue C p C ext l c C ov l c : Total length of the PSC trace Capacitance between planar wires C ov / 5 Maysam Ghovanloo S. Gevorgian et al., Microwave Mag., 3 5 d Capacitance of overlapping wires Multilayer Surrounding Environment ε r Tissue Capacitance between adjacent traces t t Tissue ε r - t Top Coating ε r - ε r ε r ε r5 Substrate ε r = C C C ε r4 ε r3 Air C ext t 5 Substrate ε r5 - ε r4 C 5 Bottom Coating ε r4 - ε r3 Considering PSC traces as coplanar striplines to model the unit length C p using conformal mapping and superposition of partial capacitances. P. Pieters et al. IEEE TMTT 5 Maysam Ghovanloo 6 t 4 C 4 t 3 Air ε r3 C 3 3

14 R s R R skin DC R eddy t ( e R s : Series Resistance t / ) t / w crit R Skin effect c DC c w t t : Metal thickness w: ine width of wire l Current flow Current crowding Increasing Magnetic Field Eddy Current Skin effect PSC Current Current crowding effect 5 Maysam Ghovanloo W.B. Kuhn & N.M. Ibrahim, TMTT 7 R p : Parallel Resistance Tissue is significantly more conductive than air and its effect should be considered. Dielectric losses, which are related to the loss tangent, tan(δ), of each material in the multilayer environment surrounding PSC affect R p. We used partial conductance technique combined with conformal mapping to find R p. 5 Maysam Ghovanloo S. Gevorgian et al., IEEE Microwave Mag

15 Optimizing PSC Geometries Iterative procedure to achieve the optimal PSC geometries:. Applying design constraints based on implantable device application and PSC fabrication process. Parameters: d o, d imin, D, f, R, w min, s min, t c,, rc, t s, rs Parameter Symbol Design Value Implanted PSC outer d o mm diameter Minimum PSC inner d imin or 8 mm diameter * PSC relative distance D mm ink operating frequency f 3.56 MHz Secondary nominal R 5 loading Minimum conductor width w min 5 m Minimum conductor s min 5 m spacing Conductor thickness t c 38 m ** Conductor material, rc ~7 nm, ~ ** properties Substrate thickness t s.5 mm Substrate dielectric constant rs 4.4 (FR4) *Depending on whether a chip or magnet is going to be placed in the center of the PSC or not. **-oz copper on FR4 printed circuit board.. Applying the initial values. Parameters: w, w,, 3. Optimizing size and fill factor of the primary PSC. Parameters: d o, 4. Optimizing fill factor and line width of the secondary PSC. Parameters:, w 5. Optimizing size and line width of the primary PSC. Parameters: d o, w 6. Is the efficiency improvement less than.%? Yes 7. Optimized design is achieved and can be validated by field solver simulation. No Jow and Ghovanloo, IEEE TBioCAS 7 5 Maysam Ghovanloo 9 Iterative Procedure for PSC Optimization Step : Applying design constraints based on the implantable device application and PSC fabrication technology. Step : Initial values for PSC geometries. Step 3: Size and fill-factor of the implanted PSC. Step 4: Fill-factor and conductor width of external PSC. Step 5: Size and conductor width of external PSC. Step 6 & 7: Check whether PCE is maximized, if not go to Step Efficiency (%) do (mm) Efficiency (%) w (mm) Efficiency (%) dot (mm).5.5 w (mm) Jow and Ghovanloo, IEEE TBioCAS 7 5 Maysam Ghovanloo 3 5

16 Effect of PSC Coating (Hermetic Packaging) ε silicone < ε tissue and tan δ silicone < tan δ tissue Thickness of the silicone coating C p and R p However, Coupling distance, d, volume of implant Optimal thickness of PSC coating Efficiency 3% For muscle 5% % 5% % 5% % 5 5 Thickness of Silicone Coating (μm) Coating material: NuSil CF6-86 Subtract Package coating Tissue Coil t d= mm t 5 Maysam Ghovanloo Jow and Ghovanloo, IEEE TBioCAS 9 3 Optimal PSC Parameters for Air and Tissue Parameter Pair- for Air Pair- for Muscle Name PSC PSC PSC PSC d o (mm) 38 4 n (turns) w (m) (H) R S () R P (k) C P (pf) Q k _cal (%) _sim (%) _meas (%) * Coupling distance d = mm at 3.56 MHz and R = 5, coated with a 3 m layer of silicone. Calculation results. Simulations results using HFSS. Measurement results. 5 Maysam Ghovanloo Jow and Ghovanloo, IEEE TBioCAS 9 3 6

17 Measurement Setup Commercial PSC fabrication process, coated with silicone. A network analyzer (R&S ZVB4) was used S-parameters Z-parameters calculate k and Q Calculate Two plastic bags (~5 μm thick) hanged from a clamp, and filled with beef to emulate implantation environment. Air Muscle 5 Maysam Ghovanloo Jow and Ghovanloo, IEEE TBioCAS 9 33 Calculated, Simulated, and Measured Q PSC (external) optimized for air % drop! Quality Factor (Q) 5 5 PSC (external) 5 optimized for muscle 5 9 5% drop Quality Factor (Q) 5 Cal. in Air Cal. in Muscle Sim. in Air Sim. in Muscle Meas. in Air Meas. in Muscle Frequency (MHz) Frequency (MHz) 3.56 MHz Cal. in Air Cal. in Muscle Sim. in Air Sim. in Muscle Meas. in Air Meas. in Muscle 5 Maysam Ghovanloo Jow and Ghovanloo, IEEE TBioCAS

18 Calculated, Simulated, and Measured Air Environment 7.% vs. 55.6% Muscle Environment.8% vs. 3.8% Efficiency Efficiency 4.3% Improvement! % 8% 6% 4% % % % 8% 6% 4% % % Coupling Distance (mm) Coupling Distance (mm) Cal. Set Sim. Set Meas. Set Cal. Set Sim. Set Meas. Set Cal. Set Sim. Set Meas. Set Cal. Set Sim. Set Meas. Set 5 Maysam Ghovanloo Jow and Ghovanloo, IEEE TBioCAS 9 35 Conclusions High performance implantable microelectronic devices (IMD), such as cochlear and retinal implants and invasive brain-computer interfaces need to be inductively powered and communicated as efficiently and reliably as possible. -, 3-, and 4-coil links can be utilized for inductive power transfer. Quality factor (Q) and coupling coefficient (k) between the coils determine the PTE and PD, both of which should be considered. 3-coil links seem to be the best choice to improve both PTE and PD when k is low. 4-coil links can offer good PTE over a long distance. However, their PD is quite low. Planar Spiral Coils (PSC) are appropriate for IMD applications due to their small volume and potentially conformal substrate. But their geometries play a significant role in the PTE and need to be optimized considering the tissue volume conductor and parasitics. Bidirectional communication link with IMD is often asymmetrical, depending on the application, and need to be designed accordingly. 5 Maysam Ghovanloo 36 8