Near-infrared photonic energy penetration: can infrared phototherapy effectively reach the human brain?

Figures & data

Figure 1 Hypothesized mechanism of action of near-infrared light (NIR) photobiomodulation.

Notes: NIR (600–980 nm) penetrates tissue to variable depth depending on wavelength, coherence, time, and the tissue involved. A portion of the photonic energy reaches the mitochondria and is absorbed by cytochrome c oxidase. In addition to inducing increased adenosine triphosphate (ATP) production, NIR appears to initiate increased production of reactive oxygen species (ROS), reactive nitrogen species (RNS), and possibly (?) nitric oxide (NO). Downstream events include increased early response genes – c-fos, c-jun – and activation of nuclear factor kappa-B (NF-κB), which in turn induces increased transcription of gene products leading to neurogenesis, synaptogenesis, and increased production of growth factors and inflammatory mediators.
Abbreviation: ↑, increase.

Table 1 Data on infrared light penetration of ex vivo skin samples

Instrument	Watts	Watts at 0 mm	Watts at 5 mm	Skin thickness (mm)	Watts penetrating	Temp change (°C)	Pulse (Hz)	Watts at 5 mm	Skin thickness (mm)	Watts penetrating	Temp change (°C)
Custom LED	0.05	0.050±0.001	0.020±0.000	1.9	0.0000±0.0000	−0.08±0.00	NA	NA	NA	NA	NA
810 nm					0.00%
In Light LED	0.2	0.020±0.002	0.010±0.000	1.9	0.0000±0.0000	−0.05±0.01	NA	NA	NA	NA	NA
650/880 nm					0.00%
K-Laser/Eltech 6D LED	6	3.890±0.004	0.012±0.003	1.9	0.0027±0.0010	−0.19±0.79	NA	NA	NA	NA	NA
670/970 nm					21.50%a,b
LiteCure LT 1000	10	5.I60±0.153	4.790±0.058	1.9	0.4120±0.0360	0.228±0.306	10	2.20±0.050	2.0	0.304±0.126	0.928±1.600
810/980 nm					8.60%a–c					13.80%
Diowave	15	9.210±0.040	8.460±0.006	2.0	2.8720±0.6670	0.389±0.322	10	4.00±0.020	2.0	1.372±0.241	−0.055±0.457
810 nm					33.90%a–e					34.30%
Diowave	15	9.590±0.085	9.080±0.093	2.0	1.2620±0.3080	1.33±0.362	10	4.47±0.040	2.0	2.734±0.279	0.889±0.794
980 nm					13.90%a–c,f					61.10%

Notes: The manufacturer’s specified watt output, the actual output registered when the infrared light-emitting device was directly in contact with the meter detector, and the watts which traversed 5 mm of air are reported. The watts recorded after infrared light from the device penetrated the stated thickness of skin along with temperature (temp) change at the surface of the skin closest to the infrared emitter also are provided. Data on the effects of pulsing at 10 Hz are provided where relevant Numbers in bold are percentage change in photonic energy with penetration of interposed tissue versus an equal distance of air. Significance is indicated with superscripted letters – significance level at P<0.0001 as determined by one-tailed t-test with corrections for multiple comparisons.

a Custom versus other;

b ln Light versus other;

c K-Laser versus other;

d LiteCure versus other;

e Diowave 810 nm versus other;

f Diowave 980 nm versus other (P<0.01).

Abbreviations: NA, not applicable; LED, light-emitting diode.

Figure 2 Ex vivo human skin studies illustrated.

Notes: (A) The pad of LEDs is held 2 mm from the surface of the light meter detector. The arrow indicates a row of near-infrared light (NIR) LEDs with a wavelength of 880 nm. The meter reads 0.01 W. (B) Human skin 1.9 mm thick is interposed between the NIR LED and the light meter detector. Thin plastic wrap covers the detector. (C) The NIR LED is covered with thin plastic wrap and placed directly against the sample of human skin. Photonic energy could not be detected passing through 1.9 mm of human skin.

Table 2 Data on infrared light penetration of ex vivo human skin samples

Instrument	Watts	Watts at 0 mm	Watts at 2 mm	Skin thickness (mm)	Watts penetrating	Temp change (°C)	Pulse (Hz)	Watts at 2 mm	Skin thickness (mm)	Watts penetrating	Temp change (°C)
Custom LED	0.05	0.050±0.001	0.020±0.000	1.9	0.000±0.000	−0.080±0.000	NA	NA	NA	NA	NA
810 nm					0.00%c,d
In Light	0.20	0.020±0.004	0.010±0.002	1.9	0.000±0.000	0.000±0.000	NA	NA	NA	NA	NA
650/880 nm					0.00%c,d
LiteCure LT 1000	10.00	9.140±0.042	8.680±0.049	1.9	0.994±0.067	0.870±0.412	10	4.590±0.023	1.9	0.372±0.013	0.550±0.170
810/980 nm					11.5%a,b,d					8.1%a,b,d
Diowave	15.00	13.050±0.440	12.030+0.011	1.9	2.008±0.092	1.810±0.447	10	6.198±0.019	1.9	0.790±0.027	1.00±0.50
810 nm					16.7%a–c					12.7%a–c

Notes: The manufacturer's specified watt output, the actual output registered when the infrared light-emitting device was directly in contact with the meter detector, and the watts which traversed 2 mm of air are reported. The watts recorded after infrared light from the device penetrated the stated thickness of skin along with temperature (temp) change at the surface of the skin closest to the infrared emitter also are provided. Data on the effects of pulsing at 10 Hz are provided where relevant. Numbers in bold are percentage change in photonic energy with penetration of interposed tissue versus an equal distance of air. Significance is indicated with superscripted letters – significance level at P<0.0001 as determined by one-tailed t-test with corrections for multiple comparisons.

a Custom versus other;

b In Light versus other;

c LiteCure versus other;

d Diowave 810 nm versus other (P<0.01).

Abbreviations: NA, not applicable; LED, light-emitting diode.

Figure 3 Ex vivo brain tissue studies illustrated.

Notes: (A) The photonic energy penetrating a fixed distance (3 cm) of air was determined. (B) A section of ex vivo lamb head was prepared which included skull, tissue, and brain. (C) The section was interposed in the space between the infrared light emitter and the light meter detector, both of which were fixed in place. The amount of infrared light energy penetrating the fixed distance (3 cm) through tissue was determined. (D) The temperature change was determined using a digital thermometer before and immediately after infrared light exposure.

Table 3 Data on infrared light penetration of ex vivo lamb skull, tissue, and brain

Instrument	Watts	Watts at 0 mm	Watts at 30 mm	Watts penetrating brain at 30 mm	Temp change at skin (°C)	Temp change at brain (°C)	Pulse (Hz)	Watts at 30 mm	Watts penetrating brain at 30 mm	Temp change at skin (°C)	Temp change at brain (°C)
Custom LED	0.05	0.050±0.001	0.000±0.000	0.0000±0.0000	0.000±0.000	−0.040±0.000	NA	NA	NA	NA	NA
810 nm				0.00%c–e
In Light LED	0.20	0.020±0.002	0.000±0.000	0.0000±0.0000	0.010±0.000	−0.050±0.010	NA	NA	NA	NA	NA
650/880 nm				0.00%c–e
K-Laser/Eltech 6D LED	6.00	3.890±0.004	0.009±0.001	0.0020±0.0000	0.295±0.877	0.113±0.670	NA	NA	NA	NA	NA
670/970 nm				22.2%a,b,e,f
LiteCureLT1000	10.00	5.160±0.153	2.000±0.030	0.0070±0.0050	3.331±1.410	0.110±0.106	10	1.580±0.010	0.0230±0.0050	2.84±3.900	0.340±0.478
810/980 nm				0.35%a,b					1.46%a–c,e,f
Diowave	15.00	9.210±0.040	4.000±0.055	0.1160±0.0200	0.220±0.304	0.944±0.179	10	2.017±0.029	0.0478±0.0043	1.533±1.200	1.000±1.360
810 nm				2.90%a–c					2.37%a–d,f
Diowave	15.00	9.590±0.085	6.220±0.127	0.0764±0.0130	1.330+1.390	2.890±2.770	10	3.137+0.011	0.0492±0.0070	1.550+1.140	1.000±0.465
980 nm				1.23%a–c					1.57%a–e

Notes: The manufacturer's specified watt output the actual output registered when the infrared light-emitting device was directly in contact with the meter detector, and the watts which traversed 30 mm of air are reported. The watts recorded after infrared light from the device penetrated 30 mm of skull, tissue, and brain, as well as temperature (temp) change at the exterior of the skull (skin surface) and at the interior surface of the brain are provided. Data on the effects of pulsing the infrared light at 10 Hz also are provided where relevant. Numbers in bold are percentage change in photonic energy with penetration of interposed tissue versus an equal distance of air. Significance is indicated with superscripted letters – significance level at P<0.0001 as determined by one-tailed t-test with corrections for multiple comparisons.

a Custom versus other;

b ln Light versus other;

c K-Laser versus other;

d LiteCure versus other;

e Diowave 810 versus other;

f Diowave 980 versus other (P<0.01).

Abbreviations: NA, not applicable; LED, light-emitting diode.

Table 4 Data on infrared light penetration of in vivo human tissue

Tissue	Distance to meter (mm)	Watts		Temp change (°C)	Pulse (Hz)	Watts		Temp change (°C)
		Air penetration	Tissue penetration			Air penetration	Tissue penetration
Palm of hand 1	25	3.8±0.00	0.023±0.009	+2	10	2.020±0.017	0.018±0.002	+1.0
			0.60%a				0.89%a
Palm of hand 2	30	3.9±0.01	0.023±0.007	+2	10	2.010±0.006	0.017±0.001	+1.0
			0.59%a				0.85%a
Sub-cut flesh	20	NA	NA	NA	10	1.583±0.015	0.005±0.003	+0.2
							0.32%a
Cheek	9	NA	NA	NA	10	3.800±0.010	0.013±0.003	+1.0
							0.47%a
Hand web	7	NA	NA	NA	10	4.100±0.167	0.018±0.002	+0.2
							0.44%a
Ear with cartilage	5	NA	NA	NA	10	4.400±0.030	0.129±0.007	+0.5
							2.93%a

Notes: For each human tissue, the distance measured, the Watts traversing the distance of air, the penetrating that distance in human tissue, and the temperature (temp) change at the skin surface immediately after infrared light exposure are provided. Data on the effects of pulsing the infrared light at 10 Hz also are provided where relevant. Numbers in bold are percentage change in photonic energy with penetration of interposed tissue versus an equal distance of air.

a Significance level at P<0.0001 as determined by one-tailed t-test with corrections for multiple comparisons.

Abbreviations: NA, not applicable; LED, light-emitting diode.