Principles and working mechanisms of water-filtered infrared-A (wIRA) in relation to wound healing

Abstract

The experience of the pleasant heat of the sun in moderate climatic zones arises from the filtering of the heat radiation of the sun by water vapor in the atmosphere of the earth. The filter effect of water decreases those parts of infrared radiation (most parts of infrared-B and -C and the absorption bands of water within infrared-A), which would cause - by reacting with water molecules in the skin - only an undesired thermal load to the surface of the skin. Technically water-filtered infrared-A (wIRA) is produced in special radiators, whose full spectrum of radiation of a halogen bulb is passed through a cuvette, containing water, which absorbs or decreases the described undesired wavelengths of the infrared radiation. Within infrared the remaining wIRA (within 780-1400 nm) mainly consists of radiation with good penetration properties into tissue and therefore allows - compared to unfiltered heat radiation - a multiple energy transfer into tissue without irritating the skin, similar to the sun's heat radiation in moderate climatic zones. Typical wIRA radiators emit no ultraviolet (UV) radiation and nearly no infrared-B and -C radiation and the amount of infrared-A radiation in relation to the amount of visible light (380-780 nm) is emphasized. Water-filtered infrared-A as a special form of heat radiation with a high tissue penetration and with a low thermal load to the skin surface acts both by thermal (related to heat energy transfer) and thermic (temperature depending, with a relevant change of temperature) as well as by non-thermal (without a relevant transfer of heat energy) and non-thermic (not depending on temperature, without a relevant change of temperature) effects. wIRA produces a therapeutically usable field of heat in the tissue and increases tissue temperature, tissue oxygen partial pressure, and tissue perfusion. These three factors are vital for a sufficient tissue supply with energy and oxygen. As wound healing and infection defense (e.g. granulocyte function including their antibacterial oxygen radical formation) depend decisively on a sufficient supply with energy and oxygen, one explanation for the good clinical effect of wIRA on wounds and wound infections can be the improvement of both the energy supply per time (increase of metabolic rate) and the oxygen supply. In addition wIRA has non-thermal and non-thermic effects, which are based on putting direct stimuli on cells and cellular structures.wIRA can considerably alleviate the pain (with remarkably less need for analgesics) and diminish an elevated wound exudation and inflammation and can show positive immunomodulatory effects. wIRA can advance wound healing or improve an impaired wound healing both in acute and in chronic wounds including infected wounds. Even the normal wound healing process can be improved.wIRA is contact-free, easily applied, without discomfort to the patient, with absent consumption of material and with a good effect in the depth. The irradiation of the typically uncovered wound is carried out with a wIRA radiator.

Keywords: absent expenditure of material; acute wounds; chronic venous stasis ulcers of the lower legs; contact-free method; energy supply; immunomodulatory effects; infection defense; inflammation; infrared-A radiation; oxygen supply; problem wounds; prospective, randomized, controlled, double-blind studies; quality of life; reduction of pain; thermal and non-thermal effects; thermic and non-thermic effects; tissue blood flow; tissue oxygen partial pressure; tissue temperature; water-filtered infrared-A (wIRA); wound exudation; wound healing; wound infections.

Figure 1. Spectral solar irradiance outside the atmosphere and at sea level

in both cases with the sun at the zenith and for a mean Earth-sun separation. Shaded areas indicate absorption at sea level due to the atmospheric constituents shown (from [97], adapted from [98]). For comparison of Figures 1, 3 and 4: 1000 W m^-2 µm^-1 = 100 mW cm^-2 µm^-1 = 1 mW cm^-2 (10 nm)^-1

Figure 2. Cross-section of a water-filtered infrared-A radiator

(Hydrosun, Müllheim, Germany) The whole incoherent broad-band radiation of a 3000 Kelvin halogen bulb is passed through a cuvette, containing water, which absorbs or decreases the undesired wavelengths within infrared (most parts of infrared-B and -C and the absorption bands of water within infrared-A). The water is hermetically sealed within the cuvette. A fan provides air cooling of the cuvette to prevent the water from boiling.

Figure 3. Spectral irradiance of a water-filtered infrared-A radiator

(Hydrosun® radiator 501 with 10 mm water cuvette and orange filter OG590) at approximately 210 mW/cm² (= 2.1 x 10³ W/m²) total irradiance (from [1]); (visible light (VIS): 380-780 nm; infrared-A (IR-A): 780-1400 nm; infrared-B (IR-B): 1400-3000 nm)

Figure 4. Comparison of the spectra of the sun at sea level and of a water-filtered infrared-A radiator

Spectral solar irradiance at sea level (with the sun at the zenith and for a mean Earth-sun separation) as in Fig. 1 (adapted from [97]) and spectral irradiance of a water-filtered infrared-A radiator (Hydrosun® radiator 501 with 10 mm water cuvette and orange filter OG590) at approximately 210 mW/cm² (= 2.1 x 10³ W/m²) total irradiance as in Fig. 3 (from [1]). The spectrum of the sun at sea level includes ultraviolet radiation (UV, <400 nm), visible light (VIS, 380-780 nm), and infrared radiation (IR, >780 nm). The spectrum of the water-filtered infrared-A radiator includes only visible light (VIS) and infrared radiation (IR); the visible part depends on the used color filter; the wIRA radiator does not emit ultraviolet radiation (UV). Both spectra show the decreased irradiance of the absorption bands of water.

Figure 5. Comparison of irradiation with water-filtered infrared-A and with conventional infrared

Thermographical comparison of skin surface temperatures in the lumbar region 12 minutes after beginning of irradiation with water-filtered infrared-A (left) and conventional infrared (right) with the same irradiance: the skin surface temperature is higher in case of irradiation with conventional infrared (presented in the thermography), while temperature in 1 cm depth of tissue is higher when irradiating with water-filtered infrared-A (from [45]). So water-filtered infrared-A presents a high tissue penetration combined with a low thermal load to the skin surface.

Figure 6. Example for an irradiation of a wound with a water-filtered infrared-A radiator

(published with kind approval of Prof. James Mercer, Tromsø/Norway) [27]