Hardware related issues
Electrocardiogram (ECG) is the most widely used method in traditional medicine, which measures electrical
heart activity from the body surface. During the hospitalization time standard wet gel electrodes are used.
However, during daily use, because of the sensitivity to motion artifacts [61, 64-66], data can be corrupted
during measurements. Furthermore, it requires special skin preparation before use, which could have side
effects like skin irritation or allergy. Moreover, gel electrodes are lifetime limited to several days of use.
Prolonged use produces problems like dehydration and bacterial growth. Dehydration leads to disrupt
electrodes impedance, by producing noise. Therefore, new existing solutions fitting in home care
environment would be required. Found, valuable state-of-the-art ECG electrodes fitting for the daily time
Dry foam electrodes
Main evident problems like walking, body movements, and breathing, are factors to
loose connection or make to change the impedance between skin and electrode. To overcome this problem
dry foam electrodes presented. Due to material properties new type of electrodes improves the contact
impedance between skin and electrode as well as reduces influence to motion artifacts . Moreover it is
designed for long term use. Flexibility and adaptation to the skin shape and hairs surrounding it is the key
feature of new type of electrode. Recommended shaving for standard gel electrodes can be avoided,
because impedance properties of hairy and hairless skin are similar as in standard gel electrodes.
Advantages can be concluded with figure bellow.
Proper foam type selection is the noticeable drawback in such electrodes. Even if only two best foam types
exists E103/HART and E103/XAC . Therefore, additional tests are required for each individual.
Micro-needle  and nano-spikes  electrodes.
Micro-needle as well as nano-spike electrodes are based
on similar topological concept, to improve the contact between skin and electrode. Improved contact
impedance as well as motion artifacts by needle penetration to the skin. Additionally, special skin preparation
also is not required like in traditional gel electrodes. Micro-needle electrodes are special surface comprised
of array of thousands of hollow micro-needless with diameter of 100um and length of 200um (Figure bellow).
Similarly nano-spike electrodes comprise similar structure as micro-needle, except that the length of the
nano-spikes (10um) is much shorter than micro-needles. As stated in  capillaries and nerves are around
200-300 um depth and dead skin layer called stratum corneum (SC) is around 10-20um depth. Thus, 10um
nano-spike electrodes length do not penetrates to depth and does not give any pain and skin damage .
Seems that better contact belong to micro-needle electrode than to nano-spike, however nano-spike
electrodes employ only dead skin, while micro-needles penetrates to depth and may produce a pain.
Capacitive coupling electrodes
Previously described electrodes belong to direct skin contact via resistance coupling. However, capacitive
coupling electrodes are able to measure electrical potential in the free space , by employing electrode
among clothes. First time electrical body potentials were detected by Richardson in 1968. Dry and capacitive
coupling electrodes addressed to comfort and long term use issues; however standard wet gel electrodes
are still irreplaceable in hospitals. As an example, wet gel electrodes are easily and fast to attach. However,
often mechanical solutions must be employed in capacitive as well as in dry electrodes. Main drawback in
capacitive (non-contact) electrodes is sensitivity to frictions and motion artifacts. Although, Quasar
developed a new state-of-the art capacitive coupling electrodes [67, 105], which are able to measure
electrical potentials in the free space (Fig 5.3). Measurements were performed through the cotton T-shirt
without any direct skin contact to the electrode. Electrodes showed very close correlation 99% as standard
wet gel electrodes. During still position standard wet gel electrodes were a little bit better, however during
movements Quasar IBE electrodes were significant better comparing to standard wet gel electrodes.
However, better performance seems due to factor that they were used elastic belt, which was tightly secured
Long settling time is another drawback in capacitive coupling electrodes, because friction and motion
artifacts can disrupt ECG signal for more than 10 seconds .
Concentric circular ring (Fig 5.4) electrode
Concentric circular ring (Fig 5.4) electrode  based on the hearts conduction system and dipole
model . Thus, body surface potentials are sensed as time function and thus produces electrocardiogram.
Electrical heart activity measured at two points belongs to previously described electrodes; however
concentric circular ring electrode is able to measure at the single body point. Therefore this makes it more
suitable for daily time use. However, tests were performed only at the near chest area . Matching
asymmetric impedance among pre-amplifier, electrode and skin should be considered, because area
between skin and electrode is not the same . Unmatched impedance produces weak and distort ECG
wave. Moreover, this type of is a little bit sensitive to motion artifacts than standard wet gel electrode.
Photoplethysmography, traditionally measures blood flow at the vascular system, and is useful method to
estimate vascular system function using pulse wave analysis . However in the hospitals ordinary finger
clip sensors are not feasible in daily life use. Main reasons that they are not comfortable to wear and are
obstructive. Additionally, indicated that traditional PPG sensors are much more sensitive to motion artifacts
than ECG . Therefore new kind of configurations and more robust solutions are required. Indirect-contact
photoplethysmography is new approach , which tries to solve previously described problems. Proposed
technology allows increase signal quality, also allows to measure through several cotton layer. However how
signal quality is improved in terms of movements not revealed. This study shows that different sensor
configurations depend on signal quality. Therefore more sensors more clear signal is.
Fig 5.5 concludes the configurations of proposed PPG indirect contact sensor, which is much more robust to
motion artifacts than traditional configuration found in hospitals.