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Ingeniería e Investigación

Print version ISSN 0120-5609

Ing. Investig. vol.31 no.2 Bogotá May/Aug. 2011

 

Electrodermal activity - a review

María Elena Moncada1, Jorge de la Cruz2

1 Electrician Engineer, Universidad de Antioquia. PhD in Engineering , Universidad del Valle. Research Professor, Instituto Tecnológico Metropolitano, Colombia. mariamoncada@itm.edu.co

2 Electronic Engineering Student,, Universidad del Valle. jodelacruz@hotmail.com


ABSTRACT

the electricity in living tissue was widely studied around the 19th century. Such study was suspended for many years but has then been started again during recent decades. New research into bioelectricity is creating alternatives in the health field; one of them is an electrodermal response associated with the wound healing, cell stimulation and psychopathology diagnostic. This article presents some of the first responses and models concerning electrodermal activity. Theoretical, clinical and review papers were studied and classified to show the amplitude and variety of bioelectrical responses. Electrodermal activity is only one of many applications having an abundant amount of evidence regarding diagnosis and treatment starting from bioelectrical signals. Electrical tissue response requires more experimental, theoretical and clinical research in many fields involving an organism' s behaviour to ascertain, propose and create new treatment alternatives for different pathologies.

Keywords: bioelectricity, wound healing, skin, electric potential.


Received: February 25th 2010

Accepted: July 6th 2011

Introduction

The skin is one of our body' s most important organs. If more than 40% of the skin is lacking, then this is often incompatible with life. The skin has a selective protective function preventing the entry of harmful substances and its presence forms part of the body' s immune system. It has metabolism regulatory functions as it prevents the exit of substances, regulates the body’s temperature and converts sunshine into vitamin D.

Electric signal treatment has been studied since the 19th century. Researchers have demonstrated the presence of bioelectric endogenous systems (Klotet et al., 1996; Charman, 1990). References about the effects of electrical stimulation concerning ulcers generated by pressure, vascular insufficiency, trauma, diabetes, surgery abound in the literature (Sandoval et al., 2007; Poltawski, 2008 a, b; 2009). Electrical stimulation of the skin has been used as a bactericide (Nelson et al., 1999; Kincaid et al., 1989) to increase blood flow (Mohr et al., 1987; Goldman et al., 2001) and to promote healing (Im et al., 1990; Brown et al., 1987, Chi-Sing, 1996). Electrodermal activity (EDA) has also been studied and widely used in psychology as a stress level indicator (Clements and Turpin, 2000), including neurosis (Norris, 2007). It has high sensitivity when being measured; its variations are related to a person' s cognitive emotional state (Hugdahl, 1995).

The existence of electrical signals in biological tissue is a fact; however, studies to date have been unrelated, thereby preventing overall conclusions being reached. Developments in diagnosis, measurement and therapy have been achieved through the evidence reported to date. However, there is still much to explore and the need for new experimental, clinical and mathematical studies leading to knowledge about the body' s bioelectrical aspects has become apparent. Studies in this field and some clinical, experimental and computational work presenting the electrodermal system as a tissue component have been started in Colombia (Moncada et al., 2007; 2008 a, b; Sandoval et al., 2007) and it is expected that support alternatives to current medicine will be created in the future.

Electrodermal measurement

The electrodermal response (EDR) is a phenomenon which is very connected with sweat gland response nowadays (Fowles, 1986). Two main EDR measurements have traditionally involved resistance or conductance (Féré, 1888)] (exosomatic) and voltage detection (Tarchanoff, 1889) (endosomatic). These measurements can also be tonic (depth-L) or phasic (variable response-R). Table 1 shows the abbreviations used for types of measurement.

Measurement is usually done with electrodes on the palm; when an electrode is reversible, polarisation and slanted potentials become minimised. The recorded signals are characterised because they present slow recovery around 40s, phase amplitude for conductivity around 2μS, and 10-20 mV potency (depending on the electrode area (Venables et al., 1980).

Fowles (1974) developed a global EDR model (Fig. 1) which is a useful qualitatively. The top of the model represents the skin surface and the bottom the interface between hypodermis and dermis. R1 and R2 represent, respectively, resistance to current flow in epidermis and dermis, E1 and R4 represent access to the dermis and E2 and R3 access to the epidermis. Transduction potentials E1 and E2 emerge as a result of the inequality of ion concentration through the ducts. R5 is surface resistance and E3 is the potential.

Physiological mechanisms

Physiological electrical stimulation mechanisms would include increased circulation, decreased oedema, increased epithelial cell migration, neutrophils and macrophages, mast inhibition, DNA synthesis stimulation and increased growth factors. Increased fibroblast production, collagenase levels, bacterial inhibition, debridement and the restoration of healing bioelectric potential has also been suggested. Electrical stimulation has also been used as a bactericide and for increasing blood flow and promoting healing.

Rowley et al., (Rowley, 1974) observed a proliferation of bacterial decrease having negative polarity extra high voltage (EHV) applied to infected wounds. Barranco and et al., (1974) inhibited staphylococci growth using current from 0.4 to 400 mA. Kincaid and Lavoie (1989) manage to destroy three microorganisms present in human wounds in an in vitro study with 150, 200, 250 and 300 V negative polarity. Analysis revealed that more microorganisms were inhibited with greater voltage intensity and longer application time.

EHV stimulation also accelerates tissue debridement and thrombosis; negative polarity current allows fast tissue reperfusion and hematomas dissolution (Nelson, 1999). Eberhardt et al., (1986) did a study about people suffering forearm dermis wounds which had been treated with 100 Hz, 1 ms and 3 to 35 mA rectangular pulse. They observed that granulocytes increased by 63.5% in people who received maximum stimulation compared to 44.7% of cells which did not receive stimulation. Mohr et al., (1987) used EHV for rats' hind limb blood flow, using three frequencies (2.20, 80 and 120 pps) and both polarities. A significant increase in blood flow speed was found for each pulse at both polarities. The highest increases were at 20 pps and with negative pulse. Goldman et al., (2001) applied 80 to 330 V at 100 pps voltages to diabetic patients suffering inframaleolar ischemic wounds with increased microcirculation and promotion of their wound healing.

Electrical stimulation for promoting wound healing

Brown et al., (1987) researched high voltage pulsed monophasic electrical stimulation regarding wound healing in rabbits. They found that positive polarity was contraindicated during the first four days but later accelerated healing better than negative polarity. Chi-Sing et al., (1996) found positive effects in rats' oedema and burn healing when using direct current (4 and 40 mA). Kloth and Feedar (1988) studied patients suffering ulcers having 1 month to 2.5 years' evolution using EHV (100-175 V and 105 Hz), finding that stimulated patients' wounds closed in about 7.3 weeks, giving 45% per week healing rate compared to 11.6% per week increase for control group ulcers.

Griffin et al., (1991) evaluated ulcers in a clinical trial using high voltage current (200 V, 100 pps). The closing percentage for ulcers in the stimulated group compared to the control group was greater after the 5th day. Hounghton et al., (2003) evaluated chronic ulcers formed by diabetes and arterial or venous insufficiency (150 V, 100 ms, 100 Hz, negative polarity) where they found 50% reduction in initial wound area, twice more than in the placebo group. Baker et al., (1997) evaluated healing from electrical stimulation in diabetic patients, finding 60% increase in the healing of the group receiving biphasic asymmetrical stimulation.

Gentzkow et al., (1993) carried out a study on patients with diabetes-related ulcers, cardiovascular or renal illness, impaired sensory-motor function and varying degrees of chronicity; 60.7% of ulcers improved after two weeks treatment, 80.4% after four weeks, 82% after seven weeks and 23% healed completely in 8.4 weeks. In another of their studies, the ulcers healed more than twice as fast as those of the control group, total healing for the stimulated group being 9 weeks on average and 11.8 weeks for the control group (Gentzkow et al, 1991). Franek et al., (2000) found a significant reduction in the femoral venous ulcer size and pressure ulcer when using EHV.

Electrodermal activity models

The skin’s potential is located between the external surface of skin and the internal body' s environment which can be considered as skin potential response (SPR) to a stimulus or skin potential level (SPL). SPR can be monophasic, biphasic or occasionally triphasic and is mainly attributed to sweat gland activity (Fowles, 1974). SPL is usually negative on the surface (Christie, 1981) and involves more epidermal mechanisms than SPR. Edelberg has suggested that skin electrical measurements can reflect sweat gland activity level, the local blood vessels' state and the state of one or more living cell layers.

Skin hydration model

Martin and Venables (1966) found that there was no skin resistance response (SRR) in the absence of sweat activity. Edelberg (1968) built a hydration model considering two sources of potential: sweat gland generator and epidermis membrane generator (Fig. 2). In Figure 2, S is the sweat gland battery and E the epidermal membrane battery, Rs and Re are a combination of internal resistance and batteries, P is transcutaneous potential and Rv is measured resistance. An i current circulateds with S being more negative than E. Fig. 2b shows Rv being constant and infinite (Edelberg, 1977).

The skin battery

It has been established in amphibian regeneration that amputation potential was derived from the skin as a current generator (Borgens, 1982). Electrical potential through the epidermis (in the 50mV range) is generated by the movement of positive sodium ions; potential existence between two skin sites lower than 1 mV has also been demonstrated (Barker, 1982; Foulds, 1983). Skin battery potential variation depends on sweat gland concentration, speed and density but apparently is not related to dermatome distribution. Burr et al., in the 1930s and Becker et al., in the 1960s conceptualised skin potential as a DC control system. Becker (1962) demonstrated the existence of an equipotential pattern of lines for complex electric field in amphibians and humans.

Wound potential

Wound current is essentially the potential difference and subsequent current flow between injured and normal tissue due to damaged membranes, altered potential or charged particle movement (Thakor et al., 1978). There is a voltage of over 200 mV / mm at the edge of an injury and a 1 to 1.5 mA current (Charman, 1990); this causes fibroblast and keratinocyte cell migration and proliferation (Nishimura et al., 1996) thereby increasing scar thickness. Injury current is initially positive in non-regenerative species while it frequently changes to negative after an initial positive phase in regenerative species (Fig. 3) until returning again to pre-wound levels (Becker, 1967; 1974). According to Becker, a DC control system is responsible for detecting injury and controlling repair (Nishimura et al, 1996). O' Leary and Goldring (1964) suggested that injury potential is developed in nerves, muscle or skin which are wounded and that any cell can be vulnerable to membrane depolarisation caused by trauma.

Wound potential in muscle-skeletal tissues

Barker et al., (1982) and Jaffe and Vanable (1984) have done work on mammals. They showed the existence of a current in wounded skin and measured zero potential at the site of the wound. Illingworth and Barker (1980) obtained finger amputation- associated 22μAcm-2 current density in children; the results showed that the wound in the skin activated an epidermal battery having 17-42mV open circuit voltage and bone injury activated a 15-56mV endosteal battery. The periosteum cell layer did not present electrogenic properties and the bone battery was associated with the endosteal layer. It was later concluded that muscle wound was the dominant factor in generating the detected voltage; the larger currents were 10-32 μA on soft tissue surface.

Applications in psychology

EDA is one of the main psychophysiological indexes used to correlate psychological processes and is related to emotion, arousal and attention. Its measurement is highly sensitive (Wieland et al, 1970) and its variations related to changes in cognitive or emotional status in an individual (Hugdahl, 1995). EDA has been used as a stress state indicator (Clements and Turpin, 2000) and as clinical index in studying various stressrelated psychophysiological disorders (Hugdahl, 1995) and neuroticism (Norris, 2007).

Some neurosis studies have shown higher depressive symptomatology in patients after experiencing stressful events, such as unemployment (Creed et al., 2001), caring for a spouse (Gallant et al., 2003) and breast cancer surgery (Millar et al., 2005). Other studies have reported higher skin conductance values in neurotic people, as well as reduced values in SCR to repetitive sensory stimulation (Akdag et al., 2003) and slow SPR habituation to anguish, anxiety and "excitability" (Horan et al, 2005; Mardaga et al., 2006).

Conclusion

The existence of electrical signals in biological tissue was investigated many years ago and was suspended at least one hundred years ago. Nowadays, the application and reading current and electrical potential has begun to be accepted as alternatives to pharmacological treatment; this is the case of skin injuries, mainly ulcers (Tomonori et al., 2004; Sakarai et al., 2008) and bone pathologies such as non-union or pseudarthrosis (Aaron et al., 2004; Anglen, 2003). Older diagnostic techniques which are already accepted and which have been implemented would include ECG and analysis of psychological disorders by EDR (Norris, 2007, Horan et al, 2005). This review has presented theoretical, clinical and experimental evidence about work concerning the study of an electrodermal response, revealing bioelectricity to be a very young area where more research is needed to deepen knowledge about the human body and lead to generating new and better treatment alternatives.

Acknowledgments

We would like to thank Tim Watson from the School of Health & Emergency Professions, University of Hertfordshire, UK, for the valuable information provided.


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