Understanding tVNS Waveforms: What the Research Actually Says About Symmetric, Alternating, Asymmetric and Monophasic Stimulation

<![CDATA[Modern transcutaneous vagus nerve stimulation devices can deliver several different pulse waveforms. The differences between them are not cosmetic — each waveform interacts differently with nerve tissue, skin electrochemistry, and the neural membrane's electrical properties. Here's what the published research says about each one.

Symmetric Biphasic: The Gold Standard

Figure 1: Symmetric Biphasic — equal and opposite phases, same duration and amplitude.

Symmetric biphasic is the dominant waveform in both clinical TENS and vagus nerve stimulation research. Each pulse consists of two equal and opposite phases: if the first phase pushes current in one direction for 200 microseconds at a given amplitude, the second phase pushes the same current in the opposite direction for the same duration and amplitude. The charge delivered in each direction is identical by construction, so the net charge at the electrode-tissue interface is always zero.

This matters because of what happens at the skin surface during electrical stimulation. During stimulation, negatively charged ions in the skin are attracted towards the anode electrode and their excessive accumulation causes an acid reaction in the skin area under the anode. Similarly, positively charged ions move to the cathode electrode and their excessive concentration causes an alkaline reaction under the cathode¹. The reversing current of the biphasic waveform neutralises the charge buildup that occurred during the first phase, significantly minimising the risk of adverse tissue reactions such as skin irritation, burns, or cellular damage².

On comfort, the evidence is clear. Bowman and Baker demonstrated that subjects preferred a symmetrical biphasic square waveform over an asymmetrical biphasic square waveform during neuromuscular electrical stimulation, and the symmetrical biphasic waveform appears to be the most comfortable for the patient³. In the larger quadriceps muscle group, a symmetric biphasic square wave was perceived as more comfortable than either a monophasic paired spike or any of three medium frequency waveforms³.

In terms of therapeutic efficacy, different waveforms appear to produce equivalent analgesic effects. Modulation of waveform (asymmetric versus symmetric) equally reduced hyperalgesia induced by joint inflammation in rats⁴.

Symmetric biphasic is the right default for most users: it’s the most studied, the most comfortable, and inherently charge-balanced.

Alternating Biphasic: Balanced Nerve Recruitment

Figure 2: Alternating Biphasic — leading polarity flips each cycle, balancing fibre recruitment and electrode wear.

Alternating biphasic uses the same symmetric pulse shape, but the leading polarity flips on each successive cycle. If cycle one starts with a cathodic (negative) phase, cycle two starts with an anodic (positive) phase, and so on.

This is not a cosmetic change. Cathodic and anodic phases activate nerve fibres through fundamentally different mechanisms. A cathodic pulse can directly depolarize the membrane immediately adjacent to the stimulating electrode. In contrast, an anodic pulse can hyperpolarize the adjacent membrane but depolarize the membrane at slightly distant sites through a return current⁵. The smaller current efflux to depolarize membrane by an anodic pulse makes it less efficient than a cathodic pulse in activating neurons⁵.

For vagus nerve stimulation specifically, polarity determines which direction action potentials preferentially travel along the nerve. Bipolar nerve stimulation causes depolarization and generation of action potentials near the cathode. When the elicited action potentials reach sufficiently hyperpolarized sections of axon near the anode, conduction of some of them is blocked. In this way, cathode caudad polarity favours the activation of efferent fibres, whereas cathode cephalad favours the activation of afferent fibres⁶.

By alternating the leading polarity each cycle, the stimulation alternates between preferentially recruiting different fibre populations and different directions of propagation along the nerve. The net effect over a session is more balanced, bidirectional nerve activation rather than consistently biasing afferent versus efferent pathways.

There is also a practical electrode-level benefit. Since each electrode alternates between serving as cathode and anode, the electrochemical ion accumulation that would otherwise build up under a fixed-polarity electrode is distributed evenly across both electrodes, further reducing the risk of skin irritation during longer sessions.

Asymmetric Biphasic: More Efficient Stimulation

Figure 3: Asymmetric Biphasic — full-amplitude active phase with time-chopped sub-threshold return at 25% duty cycle.

In a standard symmetric biphasic pulse, the return phase partially cancels the neural activation produced by the active phase. This is the fundamental efficiency problem that asymmetric biphasic waveforms address.

The evidence for this cancellation effect is well-established. Neural membranes integrate externally applied current, so the second phase of a biphasic stimulus reduces the effective strength of the leading stimulus phase⁷. The presence of the anodic phase of a pulse pair was found to abolish excitation in cases where the cathodic phase alone was near threshold. In addition, peak twitch force from the tibialis anterior muscle of cat was found to be reduced by the use of biphasic motor nerve stimulation in place of monophasic stimulation⁸.

An asymmetric biphasic waveform solves this by making the return phase sub-threshold: the same total charge is delivered in reverse (maintaining charge balance and electrode safety), but spread over a longer duration at a lower effective amplitude. The nerve membrane cannot integrate this weaker, slower return into a meaningful counter-activation. The active phase delivers its full therapeutic effect without interference.

In the cochlear implant literature, this class of waveform is called “pseudomonophasic” and has been extensively studied. Pseudomonophasic pulses consist of a brief phase of one polarity followed immediately by a longer and lower-amplitude phase of the opposite polarity⁹. The results are significant: pseudomonophasic pulse trains produced thresholds that were more than 10 dB lower than those obtained with symmetric biphasic pulses⁹. Modifying the pulse shape while maintaining charge balance has been shown to be beneficial in terms of reducing power consumption, increasing dynamic range, and limiting channel interactions⁹.

Beyond efficiency, asymmetric waveforms offer spatial selectivity. McIntyre and Grill modelled the effect of increasing the duration and proportionately decreasing the amplitude of the leading phase of charge-balanced waveforms on preferential activation of cell bodies or passing axons. Their studies concluded that charge-balanced asymmetric waveforms could selectively activate either cell bodies (cathodic first) or passing fibres (anodic first), depending on the leading polarity¹⁰.

In practical implementation, the charge-balancing return phase can be delivered as a continuous low-amplitude pulse or as a series of very brief full-amplitude sub-pulses spread across the same time window. Both approaches achieve the same charge balance. The sub-pulse approach takes advantage of the strength-duration relationship: each individual mini-pulse is far too brief to reach the nerve’s chronaxie threshold and therefore cannot trigger an action potential, regardless of its amplitude.

Monophasic: Maximum Stimulation Efficiency

Figure 4: Monophasic — active phase only, no return phase, maximum per-pulse efficiency.

Monophasic pulses deliver current in one direction only, with no return phase. This makes them the most efficient waveform for nerve activation — the effective charge equals the total pulse charge, with no cancellation from a reverse phase¹.

Monophasic waveforms are not a fringe or experimental option. They are used in clinical practice across multiple applications. All waveforms are capable of activating peripheral nerves; monophasic waveforms are most appropriate for wound healing; asymmetrical balanced biphasic waveforms may be more useful to stimulate small muscle groups¹¹. Some TENS devices deliver monophasic pulse waveforms without users complaining of adverse skin reactions¹². In devices using monophasic waveforms, the cathode electrode is placed proximal to the anode because the cathode activates the axonal membrane and generates the nerve impulse¹³.

The practical tradeoff is skin electrochemistry during prolonged use. Monophasic pulse trains produce direct current which can give rise to unwanted electrochemical effects at the electrode site leading to skin irritation and even damage¹⁴. Because there is no reversing phase to clear the ion buildup under each electrode, longer sessions carry a higher risk of localised skin reactions compared to charge-balanced waveforms. This is a duration-dependent concern rather than a fundamental safety issue — shorter sessions or well-maintained electrode contact can mitigate it.

Monophasic delivers the highest per-pulse stimulation efficiency of any waveform, with the practical caveat that users should be attentive to electrode contact quality and session duration.

Summary

Each waveform represents a different point on the tradeoff between stimulation efficiency, comfort, electrode safety, and nerve recruitment characteristics. Symmetric biphasic maximises comfort and electrode safety. Alternating biphasic adds balanced fibre recruitment across both polarities and both propagation directions. Asymmetric biphasic increases stimulation efficiency by preventing the return phase from cancelling the active phase’s neural effect. Monophasic maximises per-pulse efficiency at the cost of charge balance.

No single waveform is universally superior. The right choice depends on the user’s priorities, session parameters, and the specific therapeutic goals of stimulation.

References

Kaczmarek KA et al. “Transcutaneous electrical nerve stimulation using novel unbalanced biphasic waveform and novel electrode arrangement.” US Patent 10,112,040. 2018.
Biology Insights. “What Is a Biphasic Waveform in Electrical Stimulation?” 2025.
Bowman BR, Baker LL. “Effects of waveform parameters on comfort during transcutaneous neuromuscular electrical stimulation.” Annals of Biomedical Engineering, 1985.
Gopalkrishnan P, Sluka KA. “Differences in Waveform Characteristics Have No Effect on the Anti-Hyperalgesia Produced by Transcutaneous Electrical Nerve Stimulation (TENS) in Rats With Joint Inflammation.” The Journal of Pain, 2007;8(4):351-358.
Zhang Z et al. “Interactions between cathodic- and anodic-pulses during high-frequency stimulations with the monophasic-pulses alternating in polarity at axons — experiment and simulation studies.” Journal of Neural Engineering, 2023;20(5).
Patel YA, Butera RJ. “Anodal block permits directional vagus nerve stimulation.” Scientific Reports, 2020;10:9221.
Miller CA et al. “Auditory nerve responses to monophasic and biphasic electric stimuli.” Hearing Research, 2001;151(1-2):79-94.
Gorman PH, Mortimer JT. “Monophasic and biphasic stimulation evoke different responses.” 2003. (Published via ResearchGate.)
Macherey O et al. “Asymmetric Pulses in Cochlear Implants: Effects of Pulse Shape, Polarity, and Rate.” Journal of the Association for Research in Otolaryngology, 2006;7(3):253-266.
McIntyre CC, Grill WM. Modelling studies cited in: Stieger KC et al. “In vivo microstimulation with cathodic and anodic asymmetric waveforms modulates spatiotemporal calcium dynamics in cortical neuropil and pyramidal neurons.” bioRxiv, 2019.
Lane Community College. “Foundations of Electrical Stimulation.” Educational resource.
Johnson MI. “Resolving Long-Standing Uncertainty about the Clinical Efficacy of Transcutaneous Electrical Nerve Stimulation (TENS) to Relieve Pain: A Comprehensive Review of Factors Influencing Outcome.” Medicina, 2021;57(4):378.
Anesthesia Key. “Transcutaneous electrical nerve stimulation and acupuncture.” 2016.
Doucet BM et al. “Transcutaneous spinal cord stimulation and motor responses in individuals with spinal cord injury: A methodological review.” Journal of NeuroEngineering and Rehabilitation, 2021.

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