Development and evaluation of a titanium-based planar ultrasonic scalpel for precision surgery.

This paper introduces a titanium-based planar ultrasonic microscalpel. The concept of silicon-based planar ultrasonic transducers has already been proven, but they are not yet suitable for clinical use due to material failure. The main objective of this work was to develop a smaller, lighter, and more cost-effective ultrasonic scalpel that could be used as an alternative or supplementary device to current surgical instruments. Various prototypes were fabricated and characterized, differing in bonding by three epoxy adhesives and two solder pastes as well as three variations in tip design. The instruments were designed to operate in the frequency range of commercial instruments and to generate a longitudinal displacement amplitude. The electro-mechanical characterization through impedance analysis and vibration measurements was complemented by an in vitro cutting trial and an acute in vivo animal experiment in comparison to commercial ultrasonic and electrosurgical devices. The operating frequency was around 40 kHz and 48 kHz depending on whether matched or unmatched operation was used. Unmatched operation turned out to be more suitable, achieving displacement amplitudes of 25.3 µm and associated velocity amplitudes of up to 7.9 m/s at an electrical power of 10.2 W. The cutting ability was demonstrated in vivo by successful dissection even under anticoagulation. The geometry of the instrument tip was found to have a major influence on cutting performance by affecting the resonance behaviour and tissue penetration.

Electrical Impedance to Assess Facial Nerve Proximity During Robotic Cochlear Implantation.

Reported studies pertaining to needle guidance suggest that tissue impedance available from neuromonitoring systems can be used to discriminate nerve tissue proximity. In this pilot study, the existence of a relationship between intraoperative electrical impedance and tissue density, estimated from computer tomography (CT) images, is evaluated in the mastoid bone of in vivo sheep. In five subjects, nine trajectories were drilled using an image-guided surgical robot. Per trajectory, five measurement points near the facial nerve were accessed and electrical impedance was measured (≤1 KHz) using a multipolar electrode probe. Micro-CT was used postoperatively to measure the distances from the drilled trajectories to the facial nerve. Tissue density was determined from coregistered preoperative CT images and, following sensitivity field modeling of the measuring tip, tissue resistivity was calculated. The relationship between impedance and density was determined for 29 trajectories passing or intersecting the facial nerve. A monotonic decrease in impedance magnitude was observed in all trajectories with a drill axis intersecting the facial nerve. Mean tissue densities intersecting with the facial nerve (971-1161 HU) were different (p <0.01) from those along safe trajectories passing the nerve (1194-1449 HU). However, mean resistivity values of trajectories intersecting the facial nerve (14-24 Ωm) were similar to those of safe passing trajectories (17-23 Ωm). The determined relationship between tissue density and electrical impedance during neuromonitoring of the facial nerve suggests that impedance spectroscopy may be used to increase the accuracy of tissue discrimination, and ultimately improve nerve safety distance assessment in the future.

Characterization of the electrical conductivity of bone and its correlation to osseous structure.

The interaction of osseous tissue with electric fields is an important subject. The electrical stimulation of bone promotes osteogenesis, while bone impedance has been proposed as a measure of osteoporosis, to follow fracture healing, or as a method to improve safety of surgical procedures. However, a deeper understanding of the electrical properties of bone and their relation to the architecture of osseous tissue is required to extend the range of use of electrical measurements to clinical studies. In this paper we apply electrical impedance spectroscopy to study the conductivity of fresh bovine tibia and we correlate the measured conductivities with its structural properties. Impedance was measured using a custom-made cell and a potentiostat. Bone conductivity was determined at 100 kHz, where the phase shift was negligible. A good agreement (R2 = 0.83) was found between the measured conductivity and the bone volume fraction, determined on microCT images. Based on this relationship, an equivalent circuit model was created for bone samples. The results of this ex-vivo study are comparable to previous in-vivo observations reporting bone resistivity as a function of bone density. This information can be used to construct a map of the tissue resistivity directly derived from clinical images.

In-Vivo Electrical Impedance Measurement in Mastoid Bone.

Nerve monitoring is a safety mechanism to detect the proximity between surgical instruments and important nerves during surgical bone preparation. In temporal bone, this technique is highly specific and sensitive at distances below 0.1 mm, but remains unreliable for distances above this threshold. A deeper understanding of the patient-specific bone electric properties is required to improve this range of detection. A sheep animal model has been used to characterize bone properties in vivo. Impedance measurements have been performed at low frequencies (<1 kHz) between two electrodes placed inside holes drilled into the sheep mastoid bone. An electric circuit composed of a resistor and a Fricke constant phase element was able to accurately describe the experimental measurements. Bone resistivity was shown to be linearly dependent on the inter-electrode distance and the local bone density. Based on this model, the amount of bone material between the electrodes could be predicted with an error of 0.7 mm. Our results indicate that bone could be described as an ideal resistor while the electrochemical processes at the electrode-tissue interface are characterized by a constant phase element. These results should help increasing the safety of surgical drilling procedures by better predicting the distance to critical nerve structures.

Energy harvesting through arterial wall deformation: design considerations for a magneto-hydrodynamic generator.

As the complexity of active medical implants increases, the task of embedding a life-long power supply at the time of implantation becomes more challenging. A periodic renewal of the energy source is often required. Human energy harvesting is, therefore, seen as a possible remedy. In this paper, we present a novel idea to harvest energy from the pressure-driven deformation of an artery by the principle of magneto-hydrodynamics. The generator relies on a highly electrically conductive fluid accelerated perpendicularly to a magnetic field by means of an efficient lever arm mechanism. An artery with 10 mm inner diameter is chosen as a potential implantation site and its ability to drive the generator is established. Three analytical models are proposed to investigate the relevant design parameters and to determine the existence of an optimal configuration. The predicted output power reaches 65 µW according to the first two models and 135 µW according to the third model. It is found that the generator, designed as a circular structure encompassing the artery, should not exceed a total volume of 3 cm³.

Computer assisted optimization of an electromagnetic transducer design for implantable hearing aids.

A simple, contactless electromagnetic transducer design for implantable hearing aids is investigated. It consists of a coil and a permanent magnet, both of which are intended for implantation in the middle ear. The transducer is modeled and optimized using computer simulations, followed by experimental verification. It is shown that the proposed transducer design can, because of its size and geometry, allow implantation through the external auditory canal, and provide a sufficiently high acoustic output corresponding to approximately 120 dB sound pressure level. It can be optimized to be tolerant of radial displacements between coil and magnet of up to 1 mm.