Ingelin Clausen

Development of Clinically Relevant Implantable Pressure Sensors: Perspectives and Challenges

This review describes different aspects to consider when developing implantable pressure sensor systems. Measurement of pressure is in general highly important in clinical practice and medical research. Due to the small size, light weight and low energy consumption Micro Electro Mechanical Systems (MEMS) technology represents new possibilities for monitoring of physiological parameters inside the human body. Development of clinical relevant sensors requires close collaboration between technological experts and medical clinicians. Site of operation, size restrictions, patient safety, and required measurement range and resolution, are only some conditions that must be taken into account. An implantable device has to operate under very hostile conditions. Long-term in vivo pressure measurements are particularly demanding because the pressure sensitive part of the sensor must be in direct or indirect physical contact with the medium for which we want to detect the pressure. New sensor packaging concepts are demanded and must be developed through combined effort between scientists in MEMS technology, material science, and biology. Before launching a new medical device on the market, clinical studies must be performed. Regulatory documents and international standards set the premises for how such studies shall be conducted and reported.

A miniaturized pressure sensor with inherent biofouling protection designed for in vivo applications

The design, fabrication, and measurement results for a diaphragm-based single crystal silicon sensor element of size 820 μm × 820 μm × 500 μm are presented. The sensor element is designed for in vivo applications with respect to size and measurement range. Moreover, it is optimized for longtime operation in the human body through a built-in protection preventing biofouling on the piezoresistors. The sensitivity is about 20 mV/V for a change from 500 to 1500 mbar absolute pressure. This result is comparable to conventional sized micromachined pressure sensors. The output signal is not found to be influenced by exposure to 60 °C for three hours, a normal temperature load for a typical sterilization process for medical devices (Ethylene Oxide Sterilization). The hysteresis is low; < 0.25% of full scale output signal. The sensor element withstands an overload pressure of 3000 mbar absolute pressure. Observed decrease in the output signal with temperatures and observed nonlinearity can easily be handled by traditional electronic compensation techniques.

Design and processing of a cost-effective piezoresistive MEMS cantilever sensor for medical and biomedical use

In this special section article, cost-effective methods for fabrication of a piezoresistive cantilever sensor for industrial use are focused on. The intended use of the presented cantilever is a medical application. A closer description of the cantilever design is given. The low-cost processing sequence is presented and each processing step is explained in detail. The processing sequence is also compared to other low-cost fabrication techniques. Results from the electrical probing and mechanical strength test are given. The results demonstrate that the chosen low-cost processing route results in high yield and a mechanical robust device.

Biofouling on protective coatings for implantable MEMS

Protective coatings can replace traditional packaging methods, which are often voluminous and may spoil the otherwise excellent opportunity for miniaturized implantable medical MEMS. The bio-growth on a selection of biocompatible protective coatings (TiO2, DLC and Parylene) was investigated. The model system for evaluation was a diaphragm based acoustic resonator primary designed for fish identification. By detecting the shift in resonance frequency, we wanted to highlight the following; i) does the amount of biological growth vary for the different coatings? ii) if biofouling occurs, is the growth devastating for the device characteristics? We found that the resonance frequency did not change significantly. From this we conclude that the stiffness, represented by the spring constant for the resonating structure, was not affected. This result is of major importance also for other diaphragm based in vivo devices to be, e.g. pressure sensors, ultrasonic imaging devices, and dosage pumps.

The effect of true human synovial fluid on the functionality of an in vivo pressure sensor element

This paper presents a study on the feasibility of packaging a sensor element by a thin biocompatible coating. The goal of the work was twofold; Firstly to investigate the possible impact of the coating on sensor element performance; Secondly to examine the sensor element functionality after soaking into true human synovial fluid for more than 30 days. Sensor elements with two different structures of TiO2, the amorphous and the anatase, were examined and compared to uncoated elements. The device under test was a piezoresistive pressure sensor element designed for in vivo applications. Pressure characteristics were measured before and after Atomic Layer Deposition of the TiO2 coatings. Sensor signals were examined and visual inspection of the sensor element surfaces were done after more than 30 days soaking in true human synovial fluid. Throughout the soaking period the shift in output signal was higher and varied more for uncoated elements than for coated ones. Our results indicate that a 20 nm thick TiO2 coating can provide good protection towards the harsh synovial fluid.

Investigations of TiO 2 as a protective coating on diaphragm-based in vivo sensors

The motivation for these experiments has been to investigate the influence of a biocompatible protective coating on diaphragm-based in vivo sensors. The investigated device is a resonator for fish identification. Such a diaphragm-based configuration is also commonly used for pressure sensors. Six passive ID tags with a set of acoustic resonators have been coated with a 12 nm thin TiO2 film by the atomic layer deposition (ALD) technique. The frequency response in the 200 kHz to 400 kHz range has been measured in water before and after coating. The resonance peaks can still be detected after coating, but an increase in the resonance frequencies of about 2 % is measured. The increase is explained by a thicker diaphragm due to the TiO2 film.

Measurement of Urinary Bladder Pressure: A Comparison of Methods

Pressure is an essential parameter for the normal function of almost all organs in the human body. Measurement of pressure is therefore highly important in clinical practice and medical research. In clinical practice, pressures are often measured indirectly through a fluid line where the pressure is transmitted from the organ of interest to a remote, externally localized transducer. This method has several limitations and is prone to artefacts from movements. Results from an in vitro bench study comparing the characteristics of two different sensor systems for bladder assessment are presented; a new cystometry system using a MEMS-based in-target organ sensor was compared with a conventional system using water-filled lines connected to external transducers. Robustness to measurement errors due to patient movement was investigated through response to forced vibrations. While the new cystometry system detected real changes in applied pressure for excitation frequencies ranging from 5 Hz to 25 Hz, such small and high-frequency stimuli were not transmitted through the water-filled line connected to the external transducer. The new sensor system worked well after a resilient test at frequencies up to 70 Hz. The in-target organ sensor system will offer new possibilities for long-term monitoring of in vivo pressure in general. This opens up the possibility for future personalized medical treatment and renders possible new health services and, thereby, an increased patient empowerment and quality of life.