Session: 12-21-02: General: Mechanics of Solids, Structures and Fluids

Paper Number: 99271

99271 - Implantable, Wireless, Self-Fixing Thermal Sensors for Continuous Measurements of Microvascular Blood Flow in Flaps and Organ Grafts 

Vascularized free tissue auto- or allotransplantation is the ultimate means to treat the loss or dysfunction of a vital body part. For example, autologous free flaps replace areas of missing tissue after traumatic injury or oncologic resection, while allotransplanted kidneys and livers can be lifesaving in cases of total organ failure. Each of these reconstructive procedures depends upon the successful transfer of tissue from one site to another, with re-establishment of tissue circulation by repair of the critical artery and vein(s) at the recipient site. Thrombosis (blood clotting) at the site of these critical vascular anastomoses is one of the most common modes of early post-operative failure. Clinically convenient and cost-effective methods for rapid detection of thrombosis throughout the critical risk period could greatly improve the care of the patients. Recently, strategies for continuous monitoring have been developed including implantable Doppler probes which wrap around the pedicle vessels, and near-infrared spectroscopy (NIRS) techniques that detect cutaneous tissue oxygen saturation (StO2). While these continuous strategies have facilitated expeditious identification of flap thrombosis and improved salvage rates, each remains substantially limited. Implantable Doppler devices may yield results subject to inconsistent interpretation due to mounting methods; the wired connection between the delicate flap pedicle vessels and the external readout system also risks iatrogenic vascular disruption or kinking in the post-operative period as well as during removal. Peripherally mounted cutaneous NIRS probes obviate such risks, yet the mechanisms for fixing such probes to vascular or skeletal structures may disrupt the normal blood flow or cause unnecessary tissue damage. Requirements for wired connections to benchtop readout systems increase costs, complicate clinical care and constrain movements of the patient. Such devices are also inapplicable to muscle flaps and buried flaps which lack the required skin paddle. In this talk, we report a wireless, miniaturized flow sensing system that exploits sub-millimeter scale, multi-nodal thermal probes. An analytical model is proposed to first account for both heat conduction (in the tissue) and convection (due to fluid flow) simultaneously in the 3D heat transfer equation. This model is validated by the in-vitro experiments without parameter fitting and shows the effect of encapsulation layer and metal wires in the experiments. The model is useful to determine the flow velocity and perfusion in in-vivo experiments if the blood content is known, which is convenient for clinical applications.  The limit of zero flow velocity or blood content provides a way to determine the effective thermal conductivity.

Presenting Author: Shupeng Li Northwestern University

Presenting Author Biography: Shupeng Li is a Ph.D. candidate in mechanical engineering at Northwestern University. He received a B.S. in engineering mechanics from Tsinghua University. His current research focuses on the stretchable electronics. He is currently a Ryan fellow.

Authors:

Shupeng Li Northwestern University
Yonggang Huang Northwestern University