Microwave Activation of Drug Release.

Abstract

Due to current limitations in control of pharmaceutical drug release in the body along with increasing medicine use, methods of externally-controlled drug release are of high interest. In this thesis, the use of microwaves is proposed as a technique with the purpose of externally activating pharmaceutical drug capsules, in order to release drugs at a pre-determined location at a pre-determined time. The concept is, to use an array of transmitting sources that add together in phase to produce a constructive interference at a certain focus point inside the human body. To this end, an experimental setup, called the microwave activation system has been developed and tested on a body phantom that emulates the human torso. The system presented in this thesis, operates unobtrusively, i.e. without physically interfering with the target (patient). The torso phantom is a simple dual-layered cylindrical structure that contains fat and muscle tissue mimicking media. The core of the system consists of a single submerged antenna, four external antennas, four transmitters and four receivers, all designed to operate within the ISM-band around 2.45 GHz with a bandwidth of 100 MHz. The wave behaviour inside the phantom is of interest for disclosing essential information about the limitations of the concept, the phantom and the system. For these purposes, a twofold operation of the microwave activation system was performed, which are reciprocal of each other. In the first operation phase, named mapping, microwaves were transmitted from within the phantom and were received externally to the phantom. With this setup, the amplitudes and phases of the transmitted signal were measured as the submerged source was moved around, inside the phantom. The measurement results reveal a significant influence of the so-called creeping waves, on the measured signal. If the submerged source was at a certain offset from the centre of the phantom, the receiver furthest away from the submerged source, measured the contribution from the creeping waves instead of the contribution from the direct path. These creeping waves (diffracted waves) originated from the face of the phantom from which the submerged source was closest. Most of the power of the transmitted wave, exits at that face and followed the curvature of the phantom, on both sides, and was ultimately received on the other side of the phantom, by the receiver farthest away from the submerged source. iv In the second operation phase of the microwave activation system, named focusing, four transmitters, external to the phantom, transmitted microwaves at the phantom. The phases and amplitudes of each of the transmitters were controlled to provide a constructive interference at a pre-determined focus point. Focusing microwaves inside the torso phantom was partly accomplished close to the centre of the phantom. Outside a certain radius from the centre, the effect of creeping waves is believed to be responsible for the limitations of focusing. An experiment was performed to verify the presence of creeping waves. Due to the inherent high wave attenuation in biological tissues, such as muscles at microwave frequencies, sensitive receiving structures are suggested to be integrated on a drug capsule. The capsules are meant to contain the pharmaceutical drugs and the receiving structure is presented to efficiently utilize the available power, to be present at the focusing location. Split-ring resonators are proposed to be integrated on the lid of the capsules which concentrate their acquired power to high-amplitude electric fields across the gaps of the split-ring resonators, at the resonance frequency. An optimal conductivity for the lossy dielectric lid of the capsule is suggested in this work. The specific conductivity property of the lid that the split-ring resonators are suggested to be integrated on is, to ensure maximum temperature increase in the lid. The temperature increase is proposed to be used to melt an adhesive layer, between the container and its lid, consequently releasing the drug. Experiments were performed to determine the optimal orientation of the split-ring resonators, in respect to the polarization of the exciting wave.