Tactile sensor for medical applications
Catheters and guidewires are used in the treatment of infarctions and aneurysms. The point of insertion is often the thigh, so the catheters and guidewires must be 1 m or more in length and less than 1 mm in diameter for treatment of the brain. Because wide incisions of tissue are not necessary to approach lesions using catheters and guidewires, this surgery can produce equivalent results with less pain and better functioning compared to open surgery. However, manipulation of these devices is limited to pushing, pulling, and twisting at the proximal portion outside the human body, and the procedure is very difficult due to the small diameter and tortuosity of blood vessels. For example, the inner diameter of the distal part of an internal carotid artery is about 4 mm [
1,
2]. Many cerebral aneurysms form at the internal carotid artery, which has a highly curved part called the carotid siphon [
3]. In this curved part, the guidewire contacts the blood vessel wall, and friction from this contact makes control of the guidewire difficult. The curvature of many parts of the carotid artery is less than 0.5 1/mm [
3,
4]. Furthermore, the surgeon’s sensory perception (visual and tactile) is severely reduced during manipulation in such a surgery because these tools are long and flexible with few degrees of freedom. One method to improve the manipulability of these medical devices is measurement of the tactile force (e.g., excessive contact force) between the device tip and the vessel wall. Therefore, various catheter-type sensors have been developed to measure the contact force [
5‐
10].
Unlike these researches, in this study, we developed a catheter-type tactile sensor with another function such as “palpation in vivo” [
11,
12]. The mechanical properties of tissues changes due to disease, and tactile sensors can detect this change. In clinical practice, doctors palpate various parts of the human body, such as breast [
13] and liver [
13,
14]. Nevertheless, manual palpation is subjective and the outcome depends on the experience of the doctor. Therefore, a minimally invasive method that allows quantitative measurement of the mechanical properties is desirable. In vivo measurements are advantageous because the mechanical properties of living tissue change once the tissue is removed from the human body. If an untouchable part can be measured in vivo using a miniaturized tactile sensor quantitatively, as if traced by a finger, it would be possible to obtain new knowledge about living tissue and to establish more accurate diagnosis during minimally invasive surgery. For example, it would be useful to measure the minute surface roughness of the carotid arterial wall to detect an early stage of atherosclerosis, where the luminal surface of the arterial wall becomes rough as a result of endothelial damage [
15,
16]. Consequently, it could become possible to prevent the occurrence of strokes and heart attacks. On the other hand, intravascular-imaging modalities such as intravascular ultrasound (IVUS) and intravascular optical coherence tomography (IVOCT) are usually used during treatment for evaluating the blood vessel walls [
17‐
19]. However, the resolution of IVUS (100–150 μm) is tenfold lower than that of OCT (10–20 μm). To obtain OCT images from the vessels, the blood needs to be removed from the field of view because near-infrared light is attenuated by the presence of red blood cells. On the other hand, our sensor may be capable of characterizing the surface structure of the vessel wall in greater detail without the necessity of the blood removal.
Various catheter simulators using a computer or blood vessel biomodels have been developed to improve safety in endovascular treatments [
20]. Using the tactile sensor, accurate determination of the physical parameters for the numerical simulation and the evaluation of the biomodels could become possible.
Tactile sensors composed of organic ferroelectrics
Typical catheter-type tactile sensors measure force by the piezoresistance effect [
5,
7,
11], capacitance [
7], pressure-sensitive rubber [
12] or the optical measurement [
6,
9,
10]. In comparison with other methods, organic ferroelectrics, such as poly(vinylidene fluoride) (PVDF) [
8,
14,
21‐
26], which is a copolymer with trifluoroethylene [P(VDF/TrFE)] [
27,
28] and VDF oligomer [
29,
30], exhibit piezoelectric responses and are useful for tactile sensors. They are promising materials for catheter-type tactile sensors due to the following characteristics:
1.High piezoelectric voltage sensitivity.
2.Flexibility, thinness and low weight: One advantage of organic ferroelectrics is their flexibility. For example, the elastic moduli of PVDF and lead zirconate titanate (PZT) are 2.5 GPa and 83.0 GPa, respectively, the densities of PVDF and PZT are 1.8 and 7.5, respectively [
31]. The thickness of the PVDF film used in this study is 40 μm. Because blood vessels are tortuous, an intravascular sensor needs to be flexible. A stiff sensor reduces the overall flexibility of the accompanying medical device, and this rigidity can damage living tissue. Moreover, when used for palpation, the sensor should be flexible because contact with tissue is necessary for palpation.
3.Responsiveness over a wide frequency range.
4.Durability and inertness to chemical agents: PVDF has been successfully used in a number of commercially available products, such as pipes and valves for the chemical industry.
6.The stress rate, not the stress, can be measured (see Eq. (
1) for more details).
The piezoelectric coefficient inherent to each material determines the relationship between the mechanical input and the electrical output. Each direction within a film has a different constant, and the output charge of the sample is due to the combination of piezoelectric constants along all directions. Therefore, the output current of a ferroelectric material (
I) is expressed as follows:
$$I = A\left( {d_{31} \frac{{\text{d}\sigma_{1} }}{{\text{d}t}} + d_{32} \frac{{\text{d}\sigma_{2} }}{{\text{d}t}} + d_{33} \frac{{\text{d}\sigma_{3} }}{{\text{d}t}}} \right)$$
(1)
where
A is the area of overlap of the two electrodes,
d31,
d32, and
d33 are the piezoelectric coefficients for the material,
σ1 is the applied tensile stress in the drawn direction for PVDF,
σ2 is the applied tensile stress in the transverse direction, and
σ3 is the normal stress to the plane of the film. Note that, for a given applied force, the output current from the film in the lateral direction is much higher than that in the thickness direction [
22]. This is because the extreme thinness of the film results in much higher stresses applied to the film and because of the similarity of the absolute values of
d33 and
d31 for PVDF.
Measurement of the stress rate is suitable for palpation because it is necessary to measure the difference between healthy and diseased tissues. For example, when measuring the surface roughness of a target object, the detection of subtle differences in height by moving a finger over the surface, much like the way a needle moves over the surface of a record, is a form of active sensing that uses the frequency response characteristics of tactile receptors, as is palpation by a hand. Organic ferroelectrics are suitable for active sensing because the output current is proportional to the stress rate, as shown in Eq. (
1). For example, PVDF tactile sensors have already been developed to evaluate human skin and prostate abnormalities [
14,
24]. Moreover, using a PVDF film, we also developed a thin plate type tactile sensor to detect micro-step shapes and rough shapes for the quality-check process of manufacturing [
32]. For the detection of the early stage of atherosclerosis described in “
Tactile sensor for medical applications” section, sub-micron resolution is required because endothelial cells are 10–20 μm thick [
15]. However, the surface roughness caused by atherosclerosis in the early stage cannot be measured by conventional B-mode imaging using ultrasonography [
15]. On the other hand, as our thin plate type tactile sensor can evaluate the micro-step heights (more than 10 microns) and surface roughness (
Ra: 1.6 to 6.3 microns) [
32], the application of PVDF tactile sensors to this diagnosis can be expected although the sensor should be inserted invasively and detect the lesion without the help of any image modalities. On the other hand, since there is greater spatial resolution with OCT compared with IVUS, OCT make it possible to assess thin neointimal coverage of drug-eluting stent, and to identify fibrous cap erosion and intracoronary thrombus [
17‐
19]. Our sensor with high resolution could be applied to these diagnosis without the necessity of the blood removal.
It is possible to measure the stress itself by measuring the charge using the other preamplifier [
26]. But, this technique has several disadvantages. Pyroelectricity, which is concomitant with the piezoelectric effect, is sometimes regarded as a disadvantage of ferroelectric materials because pyroelectricity can cause undesired artifacts in the detection of mechanical signals. When the output current is measured, the signal caused by the stress change can be separated from the signal caused by the temperature change because the stress changes much faster than the temperature [
26]. It is also possible to measure the acceleration by the addition of an inertial mass [
33]. However, the addition of an inertial mass is not suitable for miniaturization of the sensor.
Based on the characteristics of organic ferroelectrics, we fabricated a prototype catheter-type tactile sensor composed of a PVDF film, whose outer diameter is 2 mm. First, we evaluated the relationship between the tip deformation and the output from the prototype sensor by a simple weight-drop test not affected by electrical noise. Then, assuming the diagnosis of the atherosclerosis and other lesions, we measured the output of the prototype sensor as it was inserted into a blood vessel model with unevenness. Furthermore, because the inner diameter (ID) may become thinner due to stenosis, we also used a blood vessel model with protrusions. We investigated the possibility of determining the protrusion position and inter-convexity interval in regions of surface roughness by output filtering and frequency analysis. Note that, not limited to our group, there is a lack of studies evaluating the outputs from the catheter-type tactile sensor which contacts against the lesion-like shapes.