1. The Strip-type Conductive Fabric Sensor (CFS)
The equation of bio-impedance is given as follows, from Nyboer (1970) and Swanson (1976):
In equation (1), Z is the bio-impedance of the cylindrical limbs, L is the distance among the electrodes, A is the area of sensing, and ρ is the characteristic resistivity of the muscles and blood vessels.
Equations (2) and (3) show the bio-impedance of the muscles (Zm) and the blood vessels (Zb) on the cylindrical limbs [6] and [7].
Furthermore, if L is fixed, the more the areas of sensing on the muscles (ΔAm) and the blood vessels (ΔAb) increase, the more the volume variations of the muscles (ΔVm) and the blood vessels (ΔVb) can be measured from equations (4) and (5).
When moving a knee joint, the total volume variations (ΔV) of the increases in the muscles (ΔVm) and blood vessels (ΔVb) are determined as follows:
This shows, in conjunction with equation (6), that the sensing areas of limbs should be expanded when measuring the volume variations of increases in muscles and blood vessels.
Therefore, the strip-type conductive fabric sensor (CFS) measured 25 cm by 1 cm based on an expansion of the sensing areas of the limbs, as shown in Figure 1. In addition, the external part of the sensor consisted of contractible non-conductive fabric, as the volume of each subject's muscles differed. The CFS has an adjustable range of 25 cm by 3 cm to 50 cm by 3 cm.
The proposed strip-type CFS, W-290-PCN, was produced by Ajin electrons(Busan, Korea). The CFS was made of polyester and Ni-Cu-Ni and was produced by electro less plating method in order to be stronger the plating adhesive property. The surface resistance of the CFS has a range from 0.005 Ω/sq to 50 Ω/sq.
2. System and Sensor Configuration
MP150, EBI100C, and Lead130 (BIOPAC Systems) devices were used to determine optimum CFS configuration to evaluate knee joint movements. MP150 was used as an A/D converter and a communicator with a computer, EBI100C was used as a device to flow a fixed electrical current into a human body and an amplifier to magnify the bio-impedance, and Lead130 was used as a lead to measure bio-impedance. The excitation frequency on the CFS must be in the range of 20 kHz to 100 kHz, and the electric current can flow into a human body up to 4
to detect the bio-impedance [4] and [5]. The devices can send out the excitation frequencies of 25 kHz, 50 kHz, and 100 kHz and a fixed electrical current of 100
in a human body. Therefore, these devices are suitable for measuring bio-impedance changes.
A tilt sensor (SCA61T-FA1H1G, VTI Technologies) with a range of ± 90° was used to measure the angle changes of a knee joint and verify the movements of the knee joint. The sensitivity of the tilt sensor resulting from angle changes was 35 mV/° or 2 V/g. The offset voltage of the tilt sensor was 2.5 V.
The upper side (from the hip joint to the knee joint) of the subjects' lower limbs was divided into two areas and the lower side (from the knee joint to the ankle joint) of the subjects' lower limbs was divided into three areas. The CFS was then attached to two positions denoted as 1 of 3 and 2 of 3 from the hip joint on the upper side and to three positions denoted as 1 of 4, 2 of 4, and 3 of 4 from the knee joint on the lower side in turns. These positions are based on optimum positions for measuring the bio-impedance of the lower limbs using the standard disposable electrodes [2] and [3].
Figure 2 shows the sites at which the CFS was attached. Each spot was matched to another spot, resulting in 6 pairs selected. In addition, a tilt sensor was attached to the muscles above the ankle joint to measure the angle changes of the knee joint and verify the movements of the knee joint.
3. Experimental Methods
The CFS was excited at 50 kHz and 100
which are the optimum frequency and the electrical current of the MP150 and EBI100C (BIOPAC Systems) devices. The bio-impedance and tilt signal were measured from the CFS through the knee, ankle, and hip joints flexions/extensions for every 1 pair for 60 seconds. At this time, the knee, ankle, and hip joints flexions/extensions were matched by the rising/falling of the trigger signals generated in a generator device. The experiments were conducted using six pairs in total. Subjects were composed of 15 males (age: 30.7 ± 5.3, weight: 69.8 ± 4.2 kg, and height: 173.5 ± 2.8 cm) who did not report any problems with their knee joints. The study conformed to the Declaration of Helsinki, and approved by the local research ethics committee. All subjects gave written informed consent.