Implantable, Bioresorbable Radio Frequency Resonant Circuits for Magnetic Resonance Imaging

Abstract

Magnetic resonance imaging (MRI) is widely used in clinical care and medical research. The signal-to-noise ratio (SNR) in the measurement affects parameters that determine the diagnostic value of the image, such as the spatial resolution, contrast, and scan time. Surgically implanted radiofrequency coils can increase SNR of subsequent MRI studies of adjacent tissues. The resulting benefits in SNR are, however, balanced by significant risks associated with surgically removing these coils or with leaving them in place permanently. As an alternative, here the authors report classes of implantable inductor-capacitor circuits made entirely of bioresorbable organic and inorganic materials. Engineering choices for the designs of an inductor and a capacitor provide the ability to select the resonant frequency of the devices to meet MRI specifications (e.g., 200 MHz at 4.7 T MRI). Such devices enhance the SNR and improve the associated imaging capabilities. These simple, small bioelectronic systems function over clinically relevant time frames (up to 1 month) at physiological conditions and then disappear completely by natural mechanisms of bioresorption, thereby eliminating the need for surgical extraction. Imaging demonstrations in a nerve phantom and a human cadaver suggest that this technology has broad potential for post-surgical monitoring/evaluation of recovery processes.

Keywords: LC‐resonant circuits; biomedical implants; bioresorbable devices; magnetic resonance imaging; radiofrequency coils.

Figure 1

Implantable, bioresorbable resonant circuits for enhanced magnetic resonance imaging. a) Schematic illustrations of a conventional MRI scanner operated with a human subject that has an implant (top), and the process of bioresorption of the device post‐surgery (bottom). b) Exploded‐view illustration of the materials and design features of the device, which consists of a single inductor (square layout; 1 turn, 7 mm diameter, 200 µm trace width) and a single capacitor (≈42 mm² area for each electrode), encapsulated in a structure of PA and edible oil. Inset, circuit diagram. c) Measured RF behavior (S₁₁) of the device. The resonant frequency (f ₀) is ≈200 MHz. d) Sequence of images that shows accelerated dissolution of a device during immersion in 1 × PBS (pH 7.4) at 75 °C for 30 days. Scale bars, 10 mm.

Figure 2

Design features and associated electromagnetic properties of bioresorbable LC‐resonant circuits. a) Experimental (dots) and simulated (dash lines) results for the dependence of the capacitance on the area of the plate and b) for the dependence of the inductance on the loop diameter. c,d) Schematic illustration (left) and equivalent circuit diagram (right) of an experimental setup to determine the f ₀ of the LC‐resonant circuits. L ₀: inductance of the readout coil; L ₁: inductance of the device coil (square layout, 1 turn); C ₁: capacitance of the parallel‐plate capacitor (metal/insulator/metal structure). Near‐field magnetic coupling between the device coil and the readout coil and enables determination of f ₀ through measurements of the impedance. e,f) Measured changes in the real part of the impedance (Re Z) for LC‐resonant circuits with different designs. The f ₀ shifts toward lower frequency with increases in the electrode area of the capacitor (i.e., higher capacitance) and the loop diameter of the inductor (i.e., higher inductance).

Figure 3

Characteristics of encapsulating materials, in isolation and as used in the BIC. a) Schematic illustration of a reactive diffusion model for the OPA encapsulating strategy applied to an electrode test structure. b) Water permeability of layers of OPA and PA. The measurements show changes in the theoretical (dash lines) and measured (dots) resistances of Mg thin traces (≈300 nm thick) encapsulated with PA (135 µm thick) and OPA (oil: 350 µm thick, PA: 135 µm thick) layers during exposure to 1 × PBS (pH 7.4) at 37 °C. c) Measured RF behavior (S₁₁) and d) drifts in the f ₀ of the BIC encapsulated with OPA (top, yellow) and PA (bottom, green) during immersion in 1 × PBS (pH 7.4) at 37 °C. Independent samples, n = 4. All error bars, standard deviation.

Figure 4

Simulations of image enhancement with a BIC designed to operate with an MRI scanner at 4.7 T. a) Schematic illustration of the specifications of a birdcage (top) and a phantom (bottom) used in the simulations. b) Simulated distribution of B ₁ ⁺ for a case without and with a BIC. c) Simulated spatial maps of SNR along the y‐direction for different depth positions. A position far from the BIC results in a decrease in SNR. ROI, circle with a radius of 10 mm. d) Simulated SNR profiles along the x‐direction for various depths of implantation of the BIC. Deep implantation results in a decrease in SNR. ROI, circle with a radius of 10 mm.

Figure 5

Simulated and experimental results for imaging capabilities enabled by a BIC. a) Schematic illustration of the configuration of a volume coil (63 mm diameter) and a phantom used for MRI. The volume coil has 16 channels, and the phantom consists of a rectangular bar as a support for the BIC and six glass capillaries (800 µm diameter) as an imaging target. Scale bar, 10 mm. b) Measured images with the phantom at three different positions along the z‐axis (z = 0 and z = ±15 mm) (bottom). Corresponding simulated distributions of SNR (top). FOV: 51.2 × 51.2 mm²; Scan time: 32 s; SNR in ROI: 47.4. c) Images acquired from the phantom with different resonant devices (top); Cu (non‐bioresorbable device as a control), Mo on Day 0 (bioresorbable device), and Mo on Day 28 (bioresorbable device). Corresponding SNRs in ROI of each device with a scan time 32 s (green) and 480 s (yellow) (bottom). Each white arrow indicates 6 GCs. FOV: 25.6 × 25.6 mm². Scale bars, 5 mm. d) Measured Ch‐B and Ch‐V images of cadaver wrist (bottom). Corresponding simulated distribution in SNR at the skin surface above the BIC (top). FOV: 51.2 × 51.2 mm²; Scan time: 32 s; SNR in ROI: 22.6 for Ch‐V and 64.1 for Ch‐B. All scale bars, 10 mm.