Safe Temperature Limits in Wireless Power Transfer for Ventricular Assist Devices

April 11, 2018 - Poster Presentation at ISHLT 38th Annual Meeting and Scientific Sessions, Nice, France



Lori Lucke, Aaron Bartnik – Minnetronix Inc., Saint Paul, MN, William Weiss, John Reibson, Bradley Doxtater – Penn State Hershey Medical Center, Hershey, PA

Introduction

  • Ventricular assist devices (VADs) require a percutaneous driveline
  • TETS reduces risk of infection however, tissue heating must be managed
  • Previous studies correlated power loss, (heat flux) to tissue heating with deep implants [1,2,3]
  • An in vivo swine model was used to correlate heat flux with tissue heating for shallow implant (just under the skin)
  • Power and drive frequencies were varied to determine the impact to tissue heating.

TETS System Design (Figures 1-2)

  • Consists of Primary (external electronics, coil) and Secondary (Implant coil and implant electronics)
  • Provides power for implants from 6W – 12W (capable of up to 80W), suitable for LVADs [4,5]
  • Designed for high efficiency

figure 1 transcutaneous energy transfer system
figure 2 efficiency vs output power for varying coil spacing chart

Finite Element Simulation (Figure 3)

  • Used to predict tissue heating in the sub-clavicular region
  • TETS coil implanted under the skin
  • Skin, subcutaneous fat, and muscle were incorporated into the model
  • Perfusion characteristics and convective heat loss at skin included in the model
  • Prediction of heat flux and temperature rise of surrounding tissue as power is transferred to the implant coil – heating occurs most significantly nearest to the implant coil
  • Correlated to animal implant studies

figure 3a graph of tets tissue model
figure 3b graph of tets tissue model

System Designed for Animal Studies (Figure 4)

  • TETS system designed to correlate heat flux to tissue heating
  • TETS coil designed for implant under the skin equivalent to human sizes [7]
  • Secondary electronics potted for implant
  • Implantable heat blanket used to simulate LVAD load
  • Temperature collection system using thermistors designed to collect real time data with temperature collected nearest to the coil on both the top and bottom (not shown) of the coil as well as in nearby tissue
  • Temperature data communicated wirelessly to external data collection system

figure 4a placement of components for in vivo testing

In-Vivo Testing (Figures 5-6)

  • Utilized a swine model as it provided a good proxy to shallow depth human implant
  • Data collected across a range of parameters: Load (power transferred), heat flux, and frequency of signals used for transfer
  • Method was used to vary the heat flux generated by the coil
  • Temperature sampled frequently
  • Temperature Data collected at 5 sample points
  • Center proximal to the coil, center distal to the coil
  • Offset proximal to the coil, offset distal to the coil
  • 5 cm from coil in nearby subcutaneous tissue
  • Temperature collected from resistive load for comparison
  • Completed 30 day biocompatibility study

figure 5a showing in vivo implantation
figure 5b in vivo temperature data collection

Results (Figure 7)

  • Linear correlation between heat flux and tissue temperature rise
  • Independent of frequency of signal used for power transfer from 200 kHz to 1 MHz
  • Independent of load power from 6-12W
  • Temperatures on the implant increased from 1.4°C to 3.2°C from 4 to 8.5 mW/cm2 for implants just below the skin
  • Heat flux of 5.5 mW/cm2 correlates to 2°C tissue temperature rise much lower than for deep implants
  • Absolute tissue temperature rise during power transfer ranged from 37.6 to 39.5°C at the implant coil
  • Determined that the coil packaging did not cause biocompatibility issues in a chronic 30 day study based on histology results.
figure 7 temperature rise vs heat flux chart

Conclusion

  • Heat flux necessary to heat tissue for implants just under the skin much lower than previously published studies for deep implants.
  • Developed a model for simulation for tissue heating
  • Heat flux of 5.5mW/cm2 correlates to 2°C tissue temperature rise
  • Coil heat flux, specifically I2R heating across the surface of the coil, is the primary contributor to tissue heating, where I is the RMS current through the implant coil and R is the coil impedance at the drive frequency
  • Negligible heating from secondary concerns such as frequency of signal [6] or amount of power transferred
  • Power sufficient for LVADs can be transferred using TETS system when heat flux is limited to safe levels.

References
[1] C. Davies, “Adaptation of tissue to a chronic heat load”, ASAIO Journal 40(3)., 1994

 [2] D. Prutchi, “Analysis of Temperature Increase at the Device/Tissue Interface for Implantable Medical Devices Dissipating Endogenous Heat”,Impulse Dynamics, 2013

[3] Bossetti, Chad. Design and evaluation of a transcutaneous energy transfer system. Diss. Duke University, 2009

[4] V.Bluvshtein, L.E. Lucke, W.J. Weiss, “Wireless Power Transmission for Ventricular Assist Devices”, ASAIO Annual Conference, June 2013

[5] L. Lucke, J. Mondry, S. Scott, W. Weiss, “A Totally Implantable Controller for Use with Rotary LVADs”, June 2014

[6] L. Lucke, V. Bluvshtein, “Safety Considerations for Wireless Delivery fo Continuous Power to Implanted Medical Devices”, IEEE EMBC Conference, Aug. 2014

[7] L. Lucke, V. Bluvshtein, M. Stoll, W. Weiss, “User Studies and The Design of a Completely Implantable VAD System”, ASAIO Annual Conference, June 2015

Relevant Financial Relationship Disclosure Statement

I will not discuss off label use and/or investigational use of any drugs/devices.

The following relevant financial relationships exist related to this presentation:

L. Lucke: Minnetronix, Inc.; Employee. A. Bartnik: Minnetronix, Inc.; Employee. W. Weiss: Penn State Hershey Medical Center; Employee. B. Doxtater: Penn State Hershey Medical Center; Employee. J. Reibson: Penn State Hershey Medical Center; Employee. A. McCabe: Minnetronix, Inc.; Employee