Oliver Pabst

• Pabst, Oliver; Schülke, Christin; Balasingham, Ilangko; Krauss, Stefan Johannes Karl; Glover, Joel Clinton & Martinsen, Ørjan Grøttem (2025). Recording and modeling of pinched hysteresis loops, the fingerprint of a memristor, in neurons. Scientific Reports. 15. doi: 10.1038/s41598-025-21035-0. Full text in Research Archive Show summary

  A variant of the Hodgkin-Huxley model has been proposed in which time-varying Na+ and K+ ion conductances are replaced by memristors, based on the proposal that ion channels are memristors. This hypothesis predicts that current-voltage plots of neurons subjected to sinusoidal stimulation should exhibit a pinched hysteresis loop, the fingerprint of a memristor. We tested this using whole-cell patch clamp recordings from human neurons derived from neural progenitor cells and observed pinched hysteresis loops in 16% of all recorded neurons. Recordings with current clamp and voltage clamp with a holding potential of 0 mV yielded a higher success rate than voltage clamp recordings with a holding potential of around − 60 mV. In addition, more neurons exhibited pinched hysteresis loops when the applied voltage or current amplitude was high, and the frequency was low. The recording of pinched hysteresis loops in neurons gives evidence that these contain memristors and this is the first time to be shown experimentally. This is not just an indication that the memristor-based Hodgkin-Huxley model is valid; it also demonstrates that models of neurons based on memristors actually resemble nature. We then subjected simulated neurons expressing Hodgkin-Huxley voltage-gated Na+ and K+ channels to sinusoidal membrane potential oscillations and found that the presence of pinched hysteresis loops depended strongly on the density of voltage-gated K+ channels and on the membrane capacitance but not on the presence of voltage-gated Na+ channels. Because Na+ channels are expected to be largely inactivated in neurons at a holding potential of 0 mV, these findings together suggest that it is the presence of voltage-gated K+ channels that is critical for the observed memristive behavior.

• Tronstad, Christian; Amini, Maryam; Olesen, Eline; Qvigstad, Elisabeth; Pabst, Oliver & Martinsen, Tormod [Show all 11 contributors for this article] (2022). Diabetic foot assessment using skin impedance in a custom made sensor-sock. Journal of Electrical Bioimpedance. 13(1), p. 136–142. doi: 10.2478/joeb-2022-0019. Full text in Research Archive Show summary

  Abstract Diabetic peripheral neuropathy (DPN) may lead to several changes in the skin, and some of these may influence the skin impedance spectrum. In the present study we have developed a prototype solution for skin impedance spectroscopy at selected skin sites (big toe pulp, heel and toe ball) that was tested in a pilot study on five patients with DPN and five healthy controls. At the big toe, most of the controls had markedly lower impedance than the DPN group, especially in the range of 1-100 kHz. The separation between the groups seems to be weaker at the heel and weakest at the toeball. The results may indicate that monitoring of the skin impedance spectrum may be a method for detection of skin changes associated with DPN, encouraging further studies with the big toe sensor in particular.

• Kjølerbakken, Kai Morgan; Miloch, Wojciech Jacek; Martinsen, Ørjan Grøttem; Pabst, Oliver & Røed, Ketil (2022). Numerical Study of Non-Linear Effects for a Swept Bias Langmuir Probe. IEEE Transactions on Plasma Science. ISSN 0093-3813. 50(5), p. 1237–1245. doi: 10.1109/TPS.2022.3164220. Full text in Research Archive Show summary

  We present a numerical study disclosing non-linear effects and hysteresis loops for a swept bias Langmuir probe. A full kinetic particle in cell (PIC) model has been used to study the temporal sheath effects and the probe current. Langmuir "close to steady state" condition is required to characterize the plasma. However, during operations above frequencies normally used, capacitive and non-linear resistive effects are being unveiled. We demonstrate how ion and electron density and temperature change properties of the probe-plasma system. We also show that a swept Langmuir probe exhibits essential properties described as the "fingerprint of memristors" and how a Langmuir probe can be identified as a transversal memristor. Understanding non-linear processes might enable new ways to operate Langmuir probes with higher sampling rates and better accuracy.

• Tronstad, Christian; Pabst, Oliver; Amini, Maryam; Kleggetveit, Inge Petter; Elvebakk, Ole & Martinsen, Ørjan Grøttem [Show all 10 contributors for this article] (2022). Development of a prototype toe sensor for detection of diabetic peripheral small fiber neuropathy. Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC). doi: 10.1109/EMBC48229.2022.9871101. Full text in Research Archive
• Pabst, Oliver; Sørebø, Øystein; Andersen, Karoline Sjøen; Ousdal, Erlend Lemva; Bråthen, Sean William & Rehman, Badi Ur [Show all 15 contributors for this article] (2021). Storing information electrically in human skin. Journal of Electrical Bioimpedance. 12(1), p. 73–81. doi: 10.2478/joeb-2021-0010. Full text in Research Archive Show summary

  Abstract Human skin has been classified as a non-volatile memristor and it is shown that information can be stored within for at least three minutes. Here we investigate whether it is possible to store information up to 20 minutes. Furthermore, we investigate whether the information can be based on four different states, not just two (binary). We stored the information into the skin of the forehead of the test subjects under three different electrodes, which allows in principle for 64 different combinations (3 electrodes, 4 states) and one can think of numbers on the base of four. For this experiment, we decided on the numbers 123 4 and 302 4 (that correspond to numbers 27 and 50 in the decimal system). Writing of the different states was done by the application of DC voltage pulses that cause electro-osmosis in the sweat ducts (nonlinear electrical measurements). Based on our results, we were not able to distinguish between four different states. However, we can show that binary information storage in human skin is possible for up to 20 minutes.

• Pabst, Oliver & Martinsen, Ørjan Grøttem (2019). Simulation based comparison between a transversal and a tangential memristor model with a capacitance in parallel. PLOS ONE. 14(8). doi: 10.1371/journal.pone.0221533. Full text in Research Archive Show summary

  In non-linear measurements, the applied stimulus itself affects the electrical properties of the underlying tissue. If corresponding voltage-current plots exhibit pinched hysteresis loops with pinched point in the origin of coordinates, the tissue can be classified as a memristor. Several organic memristors like human skin, venus flytrap and slime mould memristors have been demonstrated. However, measurements on organic memristors are usually affected by parasitic elements like a capacitance which will influence the appearance of the recorded pinched hysteresis loops. Here we study the parallel connection of two different memristor types, one with tangential and the other with transversal pinched hysteresis loop, and a capacitance by simulations. The simulations are inspired by human skin; beside the sweat ducts that can be modelled as a transversal memristor, the surrounding tissue, the stratum corneum exhibits non-linear electrical properties, as well. Based on a systematic study we suggested that the stratum corneum may be modelled as a tangential memristor. We demonstrate here by simulations that hysteresis loops with two pinched points can be achieved if a tangential memristor model is connected in parallel to a capacitance. Similar results were obtained from the skin recordings of some subjects; examples are presented here. Furthermore, if both the tangential and the transversal memristor models contribute to the simulation, quite asymmetric pinched hysteresis loops are obtained which are similar to recordings of some other test subjects.

• Pabst, Oliver; Andersen, Steinar; Bhatti, Soban Ali; Brevik, Jørgen; Fallaas, Simen Anthony & Fjeldstad, Mads [Show all 18 contributors for this article] (2019). Questioning the aloe vera plant and apple memristors. Journal of Electrical Bioimpedance. p. 83–89. doi: 10.2478/joeb-2019-0012. Full text in Research Archive Show summary

  Non-linear electrical properties of a (biological) tissue can be revealed by non-linear electrical measurements, which means that the applied stimulus itself affects the measurement. If resulting voltage–current plots exhibit pinched hysteresis loops, the underlying tissue may be classified as a memristor, a state dependent resistor. The aloe vera plant and apples have been found to be memristors. However, polarization processes on the electrodes are also non-linear and may affect the measurement. Apples and aloe vera conduct electrical current very well and it is likely that the recordings are actually dominated by the polarization impedance of the electrodes. Here, we study the non-linear properties of aloe vera and apples with two different measurement electrode types. Furthermore, we measured also on the extracted liquids from one aloe vera leaf and one apple, leading to similar results. We concluded, unlike previous studies on these subjects, that the memristive properties originate from electrochemical reactions on the electrodes rather than the apples or aloe vera themselves.

• Pabst, Oliver; Martinsen, Ørjan Grøttem & Chua, Leon (2019). Information can be stored in the human skin memristor which has non-volatile memory. Scientific Reports. 9(1). doi: 10.1038/s41598-019-55749-9. Full text in Research Archive Show summary

  Much is already understood about the anatomical and physiological mechanisms behind the linear, electrical properties of biological tissues. Studying the non-linear electrical properties, however, opens up for the influence from other processes that are driven by the electric field or movement of charges. An electrical measurement that is affected by the applied electrical stimulus is non-linear and reveals the non-linear electrical properties of the underlying (biological) tissue; if it is done with an alternating current (AC) stimulus, the corresponding voltage current plot may exhibit a pinched hysteresis loop which is the fingerprint of a memristor. It has been shown that human skin and other biological tissues are memristors. Here we performed non-linear electrical measurements on human skin with applied direct current (DC) voltage pulses. By doing so, we found that human skin exhibits non-volatile memory and that analogue information can actually be stored inside the skin at least for three minutes. As demonstrated before, human skin actually contains two different memristor types, one that originates from the sweat ducts and one that is based on thermal changes of the surrounding tissue, the stratum corneum; and information storage is possible in both. Finally, assuming that different physiological conditions of the skin can explain the variations in current responses that we observed among the subjects, it follows that non-linear recordings with DC pulses may find use in sensor applications.

• Pabst, Oliver; Anwar, Abbas; Nieweglowski, Adam; Lindland, Christian Rolid; Rahman, Habibur & Siljedal, Håvard [Show all 18 contributors for this article] (2019). The non-linear electrical properties of silver/silver chloride electrodes in sodium chloride solution. Journal of Electrical Bioimpedance. 10(1), p. 113–123. doi: 10.2478/joeb-2019-0017. Full text in Research Archive Show summary

  An electrical measurement is non-linear when it is affected by the applied stimulus, i.e. when the measured phenomenon changes with amplitude. If pinched hysteresis loops can be observed in the voltage current representation, the underlying tissue can be classified as a memristor. Several biological memristors have been published, like human skin and apples. However, changes in the polarization impedance of electrodes may also cause pinched hysteresis loops. The question whether the reported biological memristors are real or whether the results just reflect changes in the polarization impedance arises. If the impedance of the measured object is close to or smaller than the polarization impedance of the used electrodes, the latter may dominate the measurement. In this study, we investigated the non-linear electrical properties of silver/silver chloride electrodes in a sodium chloride solution that has a similar concentration as human sweat and compared these to results from human skin. First of all, we found that silver/silver chloride electrodes in sodium chloride solution can be classified as memristors. However, the currents obtained from the sodium chloride solution are much higher than the currents recorded from human skin and there is a qualitative difference in the pinched hysteresis loops in both cases. We can conclude that the non-linear electrical measurements with silver/silver chloride on human skin are actually dominated by the skin and we can confirm that the human skin memristor really exists.

• Bjørhei, Andreas; Pedersen, Filip T.; Muhammad, Saeed; Tronstad, Christian; Kalvøy, Håvard & Wojniusz, Slawomir [Show all 8 contributors for this article] (2019). An investigation on bilateral asymmetry in electrodermal activity. Frontiers in Behavioral Neuroscience. 13(88). doi: 10.3389/fnbeh.2019.00088. Full text in Research Archive Show summary

  The Multiple Arousal Theory (Picard et al., 2016) was proposed to explain retrospective observations of bilateral differences in electrodermal activities occurring in threat-related high-stake situations. The theory proposes different cortical and subcortical structures to be involved in the processing of various facets of emotional states. Systematic investigations of this effect are still scarce. This study tested the prediction of bilateral electrodermal effects in a controlled laboratory environment where electrodermal activity (EDA) was recorded bilaterally during normal activity and two stress-tasks in 25 healthy volunteers. A visual search stress task with a performance-related staircase algorithm was used, ensuring intersubjectively comparable stress levels across individuals. After completion of the task, a sense of ownership of an attractive price was created and loss aversion introduced to create a high-stake situation. Confirmation of the theory should satisfy the hypothesis of a bilateral difference in EDA between the dominant and non-dominant hand, which is larger during high-stake stressors than during low-stake stressors. The bilateral difference was quantified and compared statistically between the two stress-tasks, revealing no significant difference between them nor any significant difference between the stress tasks and the period of normal activity. Subgroup analysis of only the participants with maximum self-rating of their desire to win the price (n = 7) revealed neither any significant difference between the two tasks nor between the stress-tasks and the period of normal activity. Although the theory was not confirmed by this study, eight cases suggestive of bilateral difference within the recordings were identified and are presented. Because the study is limited in using one of several possible operationalizations of the phenomenon, it is not possible to draw a general conclusion on the theory. Nevertheless, the study might contribute to a better understanding and encourage systematic review and hypothesis development regarding this new theory. Possible explanations and suggestions for future pathways to systematically investigate the Multiple Arousal Theory are discussed.

• Pabst, Oliver; Martinsen, Ørjan Grøttem & Chua, Leon (2018). The non-linear electrical properties of human skin make it a generic memristor. Scientific Reports. 8(1), p. 1–9. doi: 10.1038/s41598-018-34059-6. Full text in Research Archive Show summary

  An electrical measurement is non-linear when the applied stimulus itself affects the electrical properties of the underlying tissue. Corresponding voltage-current plots may exhibit pinched hysteresis loops which is the fingerprint of a memristor (memory resistor). Even though non-linear electrical properties have been demonstrated for different biological tissues like apples, plants and human skin, non-linear measurements as such have not been defined, yet. We are studying the non-linear properties of human skin systematically and initiate non-linear measurements on biological tissues as a field of research in general by introducing applicable recording techniques and parameterization. We found under which voltage stimulus conditions a measurement on human skin is non-linear and show that very low voltage amplitudes are already sufficient. The non-linear properties of human skin originate from the sweat ducts, as well as, from the surrounding tissue, the stratum corneum and we were able to classify the overall skin memristor as a generic memristor. Pinched hysteresis loops vary largely among subjects; an indication for the potential use in biomedical sensor applications.

• Pabst, Oliver (2018). Rectifying memristor bridge circuit realized with human skin. Journal of Electrical Bioimpedance. 9(1), p. 184–192. doi: 10.2478/joeb-2018-0023. Full text in Research Archive Show summary

  It has been demonstrated before that human skin can be modeled as a memristor (memory resistor). Here we realize a memristor bridge by applying two voltages of opposite signs at two different skin sites. By this setup it is possible to use human skin as a frequency doubler and half-wave rectifier which is an application of the non-linear electrical properties of human skin. The corresponding electrical measurements are non-linear since these are affected by the applied stimulus itself.

• Pabst, Oliver & Martinsen, Ørjan Grøttem (2017). Interpretation of the pinched point position in human skin memristor measurements. International Federation for Medical and Biological Engineering Proceedings. ISSN 1680-0737. 65, p. 430–433. doi: 10.1007/978-981-10-5122-7_108. Full text in Research Archive Show summary

  It is shown that human skin acts like a memristor but systematic studies on skin memristance are not published, yet. If e.g., a low frequency voltage is applied to human skin and the current is measured, a hysteresis loop can be seen in the voltage-current plot. In skin measurements, the pinched point of the hysteresis loop can be shifted from the coordinate origin. In this paper, possible reasons for this observation shall be investigated and interpreted by the use of simulation. Two example measurements on human skin are presented, as well.

• Skjærvø, Øystein; Brandtzaeg, Ole Kristian; Lausund, Kristian Blindheim; Pabst, Oliver; Martinsen, Ørjan Grøttem & Lundanes, Elsa [Show all 7 contributors for this article] (2017). Exploring bioimpendance instrumentation for the characterization of open tubular liquid chromatography columns. Journal of Chromatography A. ISSN 0021-9673. 1534, p. 195–200. doi: 10.1016/j.chroma.2017.12.060. Full text in Research Archive
• Pabst, Oliver; Tronstad, Christian & Martinsen, Ørjan Grøttem (2017). Instrumentation, electrode choice and challenges in human skin memristor measurement. IEEE Engineering in Medicine and Biology Society. Conference Proceedings. ISSN 1557-170X. p. 1844–1848. doi: 10.1109/EMBC.2017.8037205. Full text in Research Archive
• Pabst, Oliver; Tronstad, Christian; Grimnes, Sverre; Fowles, Don C. & Martinsen, Ørjan Grøttem (2016). Comparison between the AC and DC measurement of electrodermal activity. Psychophysiology. ISSN 0048-5772. 54(3), p. 374–385. doi: 10.1111/psyp.12803. Full text in Research Archive
• Martinsen, Ørjan Grøttem; Pabst, Oliver; Tronstad, Christian & Grimnes, Sverre (2015). Sources of error in AC measurement of skin conductance. Journal of Electrical Bioimpedance. 6(1), p. 49–53. doi: 10.5617/jeb.2640. Full text in Research Archive

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• Pabst, Oliver; Tronstad, Christian; Martinsen, Ørjan Grøttem & Grimnes, Sverre (2016). Comparison between AC and DC exosomatic EDA measuring methods. Proceedings of the 16th International Conference on Electrical Bioimpedance. p. 65–65. Full text in Research Archive
• Pabst, Oliver & Martinsen, Ørjan Grøttem (2016). Investigating the memristive properties of skin. Proceedings of the 16th International Conference on Electrical Bioimpedance. p. 33–33. Full text in Research Archive
• Pabst, Oliver; Martinsen, Ørjan Grøttem; Tronstad, Christian & Wilson, Steven Ray Haakon (2018). Electrical properties of human skin: From linear recordings of exogenous electrodermal activity to non-linear memristor measurements. Universitetet i Oslo. Full text in Research Archive Show summary

  This thesis is all about applying an electrical voltage to human skin and measuring the corresponding electrical current. A recording (on any tissue) is non-linear when the electrical properties are affected by the applied signal itself; The measurement is linear otherwise. Non-linear measurements on human skin initiate a very new field of research. Dependent on the polarity of the applied voltage, the sweat inside the ducts is moved towards the skin surface or towards deeper skin layers. Here we confirmed that the sweat ducts are memristors (memory resistor) and found that information can actually be stored inside. If you are a forgetful person, this might be a way to store your bank pin number in the future. A second memristor can be found in the stratum corneum (the tissue in the outer layer of the skin that surrounds the sweat ducts). However, the voltage-current characteristics recorded from human skin vary largely among subjects with potential applications in Diagnostics. Further, we found that human skin can act as a frequency doubler and halve-wave rectifier. An example of linear measurements will be given as an introduction. Changes in sweating due to sympathetic nervous activity are subject of (linear) electrodermal activity (EDA) recordings. Here, the test subjects were asked to do continuous subtractions and deep breaths, to stimulate their sweat activity. By doing so, it was possible to find out that EDA recordings using the alternating current (AC) method are comparable with recordings using the direct current (DC) method.

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