The Myth of Current Strength: Why 200 µA on the display does not guarantee 200 µA in the tissue

On many microcurrent devices, a figure appears quite naturally on the display: 200 microamperes, for example. Many users interpret this as meaning that exactly this current is now flowing through their patient’s tissue. This assumption is understandable, as one wishes to administer precise doses and work in a reproducible manner. Physically, however, it only holds up to closer scrutiny to a limited extent.

Current, voltage, resistance: three quantities, one relationship

Ohm’s Law describes this relationship: current equals voltage divided by resistance (I = U / R). This can be illustrated using the analogy of a water pipe. The current is the volume of water, the voltage is the pressure, and the resistance is the pipe. How much flows does not depend solely on how much one wishes for.

A calculation example: for 200 µA, a device requires a voltage of 10 V at 50 kΩ, 30 V at 150 kΩ and 60 V at 300 kΩ. However, for safety reasons, microcurrent devices operate with a limited output voltage. With the HD2000+, this is a maximum of 60 V per channel. If the resistance exceeds this, 200 µA simply cannot flow. The technical user manual for the HD2000+ clearly illustrates this relationship in a table: at 95 kΩ, a maximum of around 630 µA is possible; at 350 kΩ, the figure is still around 170 µA (Luxxamed GmbH, 2025).

The resistance chain

The resistance between two electrodes is not a single figure, but a chain. This includes the electrode itself – comprising its material, age and gel layer – the interface between the electrode and the skin, and the stratum corneum, which is a particularly poor conductor. Only then does the actual tissue follow, comprising highly conductive interstitial fluid, poorly conductive fascia and barely conductive bone.

This chain is also constantly changing: due to skin moisture, sweat, temperature and the ionisation of the upper skin layer. Swelling or inflammation also alter the electrical behaviour. The resistance at the start of a session is therefore not the same as it is after twenty minutes.

The relevance of this was also demonstrated in the laboratory. In in vitro studies at Fraunhofer FEP using the HD1000 device, an agar bridge had to be inserted between the electrode and the cell culture to partially simulate skin resistance (Schönfelder et al., 2017, unpublished project report).

What Cheng et al. (1982) actually measured

The study by Cheng et al. (1982) is regarded as one of the most frequently cited works on microcurrent therapy. In rat skin, ATP concentrations increased three- to five-fold with direct currents between 50 and 1,000 µA. Above 1,000 µA, the effect levelled off; at 5,000 µA, the ATP level was even slightly below that of the control group. A higher current does not therefore automatically mean a greater effect.

Less well known is the methodology section. The authors used a constant-current source and determined what proportion of the current actually flowed through the skin. They found the greatest resistance in the system at the interface between the electrode and the skin. With skin in a buffer bath, the current through the tissue stabilised at around 6 µA above a threshold of approximately 50 µA; the remainder flowed through the solution. The authors themselves note that only a small proportion of the applied current is responsible for the metabolic effects (Cheng et al., 1982). The frequently cited figures therefore describe the set current of the experimental setup, not the current flowing through the tissue.

The body is not a simple resistor

Biological tissue does not behave like an ohmic resistor. Cell membranes are thin insulating layers between conductive fluids and thus act like capacitors. Fasciae envelop and separate almost all structures of the body and also contribute to this capacitive behaviour.

This has two practical consequences. Firstly, the current varies even within a single pulse because the capacitive components become charged. Secondly, the capacitive reactance depends on the frequency: the higher the frequency, the lower it is. Frequency and current are therefore not independent control parameters.

Adjustment and maintenance

This also has regulatory implications. The Medical Devices Regulation effectively requires that products which deliver energy to the patient must be able to set and maintain the delivered amount with sufficient accuracy (Regulation [EU] 2017/745, Annex I, 21.1). Technical tests are typically carried out on defined test loads. However, a patient is not a test load.

Measure, adjust, measure again

This is where the principle of the control loop comes into play, as described by Norbert Wiener (1948) under the term ‘cybernetics’. The human body itself is an example of this: temperature, blood pressure and pH are continuously measured and regulated. Impedance-controlled microcurrent therapy follows the same principle. With the HD2000+, tissue resistance is measured throughout the entire treatment, and both voltage and current are continuously adjusted (LuXXaMed GmbH, 2025). Frequency-specific protocols can be combined with this approach.

Those who work with fixed settings may continue to do so. However, fresh electrodes, proper skin preparation and sufficient gel then become particularly important. It is crucial to be aware that the figure on the display is a target value and not a guarantee.

Sources

Cheng, N., Van Hoof, H., Bockx, E., Hoogmartens, M. J., Mulier, J. C., De Dijcker, F. J., Sansen, W. M., & De Loecker, W. (1982). The effects of electric currents on ATP generation, protein synthesis, and membrane transport in rat skin. Clinical Orthopaedics and Related Research, 171, 264–272.

European Parliament & Council of the European Union. (2017). Regulation (EU) 2017/745 on medical devices. Official Journal of the European Union, L 117.

LuXXaMed GmbH. (2025). HD2000+ Technical Instructions for Use, Software Version 3.X (Rev. 23). Kassel.

Schönfelder, U., Walker, ?, & Kenner, ? (2017). [Unveröffentlichter Projektbericht] Fraunhofer FEP.

Wiener, N. (1948). Cybernetics: Or control and communication in the animal and the machine. Technology Press; John Wiley & Sons.