Evaluation of a Refined Implantable Resonator for Deep-Tissue EPR Oximetry in the Clinic

Abstract

Objectives: (1) Summarize revisions made to the implantable resonator (IR) design and results of testing to characterize biocompatibility;(2) Demonstrate safety of implantation and feasibility of deep tissue oxygenation measurement using electron paramagnetic resonance (EPR) oximetry.

Study design: In vitro testing of the revised IR and in vivo implantation in rabbit brain and leg tissues.

Methods: Revised IRs were fabricated with 1-4 OxyChips with a thin wire encapsulated with two biocompatible coatings. Biocompatibility and chemical characterization tests were performed. Rabbits were implanted with either an IR with 2 oxygen sensors or a biocompatible-control sample in both the brain and hind leg. The rabbits were implanted with IRs using a catheter-based, minimally invasive surgical procedure. EPR oximetry was performed for rabbits with IRs. Cohorts of rabbits were euthanized and tissues were obtained at 1 week, 3 months, and 9 months after implantation and examined for tissue reaction.

Results: Biocompatibility and toxicity testing of the revised IRs demonstrated no abnormal reactions. EPR oximetry from brain and leg tissues were successfully executed. Blood work and histopathological evaluations showed no significant difference between the IR and control groups.

Conclusions: IRs were functional for up to 9 months after implantation and provided deep tissue oxygen measurements using EPR oximetry. Tissues surrounding the IRs showed no more tissue reaction than tissues surrounding the control samples. This pre-clinical study demonstrates that the IRs can be safely implanted in brain and leg tissues and that repeated, non-invasive, deep-tissue oxygen measurements can be obtained using in vivo EPR oximetry.

Keywords: EPR; electron paramagnetic resonance; implantable resonator; oximetry.

Figure 1.

Drawing of the IR for human use, showing the names of the features. Note: the coupling loop, when implanted, will be placed below the skin with the flexible transmission line and sensors at an angle to the skin surface (not linear as displayed).

Figure 2.

Photographs of two different length IRs. (Top) An IR with a 50-mm transmission line and 4 sensors. (Bottom) An IR with an 18-mm transmission line and 2 sensors.

Figure 3.

IR implantation using a peel-away catheter

Figure 4.

Anatomy of the rabbit brain showing the location of sensory loop, transmission line and coupling loop of IR in the brain.

Figure 5.

Rabbit body weight measured over 9 months. No difference in weight between those rabbits implanted with IR vs Control.

Figure 6.

Representative EPR spectra measured from IRs in the brain (left) and leg (center). A rabbit positioned for brain pO2 measurement is shown on the right. Spectra shown are medians of twelve 5-second scans, with modulation amplitude set independently to 1/3 of the narrowest observed linewidth, scan range equal to 10x the full-width half-maximum linewidth, and other acquisition parameters held constant. Signal-to-noise ratios are 43 and 103, respectively, reflective of the differences in linewidth.

Figure 7.

H+E staining images obtained from tissue specimens adjacent to the IR and HDPE control implanted in the rabbit hind leg (shown at 10x and 40x magnifications)

Figure 8.

H+E staining images obtained from tissue specimens adjacent to the IR and HDPE control implanted in the rabbit hind leg (shown at 2x and 20x magnifications). At 9 months post-implantation, there is minor inflammation and foreign body reaction. Fibrosis was observed both in the control and the IR in the tissue surrounding the transmission line as well as the coupling loop (short arrows). No hemorrhage or leakage of the LiNc-BuO crystals was observed in the experimental group.

Figure 9.

Jig used in manufacturing the implantable resonator. The fully assembled jig shown here is the length used to manufacture the 5 cm implantable resonator for human use. Three additional lengths are displayed that have been used to make longer implantable resonator for some in vitro tests, e.g., MRI compatibility.