Andrew C. Ahn

Objective. Acupuncture points are reportedly distinguishable by their electrical properties. However, confounders arising from skin-to-electrode contact used in traditional electrodermal methods have contributed to controversies over this claim. The Scanning Kelvin Probe is a state-of-the-art device that measures electrical potential without actually touching the skin and is thus capable of overcoming these confounding effects. In this study, we evaluated the electrical potential profiles of acupoints LI-4 and PC-6 and their adjacent controls. We hypothesize that acupuncture point sites are associated with increased variability in potential compared to adjacent control sites.Methods. Twelve healthy individuals were recruited for this study. Acupuncture points LI-4 and PC-6 and their adjacent controls were assessed. A 2 mm probe tip was placed over the predetermined skin site and adjusted to a tip-to-sample distance of 1.0 mm under tip oscillation settings of 62.4 Hz frequency. A surface potential scan spanning a 1.0 cm × 1.0 cm area was obtained.Results. At both the PC-6 and LI-4 sites, no significant differences in mean potential were observed compared to their respective controls (Wilcoxon rank-sum test, and 0.79, resp.). However, the LI-4 site was associated with significant increase in variability compared to its control as denoted by standard deviation and range ( and 0.0005, resp.). At the PC-6 site, no statistical differences in variability were observed.Conclusion. Acupuncture points may be associated with increased variability in electrical potential.

The Kelvin probe measures surface electrical potential without making physical contact with the specimen. It relies on capacitive coupling between an oscillating metal tip that is normal to a specimen's surface. Kelvin probes have been increasingly used to study surface and electrical properties of metals and semiconductors and are capable of detecting material surface potentials with submillivolt resolution at a micrometer spatial scale. Its capability for measuring electrical potential without being confounded by electrode-specimen contact makes extending its use towards biological materials particularly appealing. However, the theoretical basis for applying the Kelvin probe to dielectric or partially conductive materials such as biological tissue has not been evaluated and remains unclear. This study develops the theoretical basis underlying Kelvin probe measurements in five theoretical materials: highly conductive, conductive dielectric with rapid charge relaxation, conductive dielectric with slow charge relaxation, perfect dielectric, and tissue with a bulk serial resistance. These theoretically derived equations are then computationally analyzed using parameters from both theoretical specimens and actual biomaterials—including wet skin, dry skin, cerebrospinal fluid, and tendon. Based on these analyses, a Kelvin probe performs in two distinct ways depending on the charge relaxation rates of the sample: The specimen is treated either as a perfect dielectric or as highly conductive material. Because of their rapid relaxation rate and increased permittivity biomaterials behave similarly to highly conductive materials, such as metal, when evaluated by the Kelvin probe. These results indicate that the Kelvin probe can be readily applied to studying the surface potential of biological tissue.

As the intermediate layer between the muscle and skin, the subcutaneous tissue frequently experiences shear and lateral stresses whenever the body is in motion. However, quantifying such stresses in vivo is difficult. The lack of such measures is partly responsible for our poor understanding of the biomechanical behaviors of subcutaneous tissue. In this study, we employ both ultrasound imaging and a novel spatial anisotropy measure – incorporating Moran’s I spatial autocorrelation calculations – to investigate the structuromechanical features of subcutaneous tissues within the extremities of 16 healthy volunteers. This approach is based on the understanding that spatial anisotropy can be an effective surrogate for the summative, tensile forces experienced by biological tissue. We found that spatial anisotropy in the arm, thigh and calf was attributed to the echogenic bands spanning the width of the ultrasound images. In both univariable and multivariable analyses, the calf was significantly associated with greater anisotropy compared with the thigh and arm. Spatial anisotropy was inversely related to subcutaneous thickness, and was significantly increased with longitudinally oriented probe images compared with transversely orientated images. Maximum peaks in spatial anisotropy were frequently observed when the longitudinally oriented ultrasound probe was swept across the extremity, suggesting that longitudinal channels with greater tension exist in the subcutaneous layer. These results suggest that subcutaneous biomechanical tension is mediated by collagenous/echogenic bands, greater in the calf compared with the thigh and arm, increased in thinner individuals, and maximal along longitudinal trajectories parallel to the underlying muscle. Spatial anisotropy analysis of ultrasound images has yielded meaningful patterns and may be an effective means to understand the biomechanical strain patterns within the subcutaneous tissue of the extremities.

The scientific basis for acupuncture meridians is unknown. Past studies have suggested that acupuncture meridians are physiologically characterized by low electrical impedance and anatomically associated with connective tissue planes. We are interested in seeing whether acupuncture meridians are associated with lower electrical impedance and whether ultrasound-derived measures – specifically echogenic collagenous bands - can account for these impedance differences. In 28 healthy subjects, we assessed electrical impedance of skin and underlying subcutaneous connective tissue using a four needle-electrode approach. The impedances were obtained at 10 kHz and 100 kHz frequencies and at three body sites - upper arm (Large Intestine meridian), thigh (Liver), and lower leg (Bladder). Meridian locations were determined by acupuncturists. Ultrasound images were obtained to characterize the anatomical features at each measured site. We found significantly reduced electrical impedance at the Large Intestine meridian compared to adjacent control for both frequencies. No significant decrease in impedance was found at the Liver or Bladder meridian. Greater subcutaneous echogenic densities were significantly associated with reduced impedances in both within-site (meridian vs. adjacent control) and between-site (arm vs. thigh vs. lower leg) analyses. This relationship remained significant in multivariable analyses which also accounted for gender, needle penetration depth, subcutaneous layer thickness, and other ultrasound-derived measures. Collagenous bands, represented by increased ultrasound echogenicity, are significantly associated with lower electrical impedance and may account for reduced impedances previously reported at acupuncture meridians. This finding may provide important insights into the nature of acupuncture meridians and the relevance of collagen in bioelectrical measurements.