Recent research has shown that space explorers engaged in Extra Vehicular Activities (EVAs) may be exposed, under certain conditions, to undesired electrical currents. This work focuses on determining whether these undesired induced electrical currents could be responsible for involuntary neuromuscular activity in the subjects, possibly caused by either large diameter peripheral nerve activation or reflex activity from cutaneous afferent stimulation. An efficient multiresolution variant of the admittance method along with a millimeter-resolution model of a male human body were used to calculate induced electric fields, resistance between contact electrodes used to simulate the potential exposure condition, and currents induced in the human body model. Results show that, under realistic exposure conditions using a 15V source, current density magnitudes and total current injected are well above previously reported startle reaction thresholds. This indicates that, under the considered conditions, the subjects could experience involuntary motor response

Cela, Carlos J.

Hamilton, Douglas

Lazzi, Gianluca

Lee, Raphael C.

Loizos, Kyle

NASA Technical Reports Server

1On the Modeling of Electrical Effects Experienced
by Space Explorers During Extra Vehicular
Activities: Intracorporal currents, Resistances, and
Electric Fields.
Carlos J. Cela, Member, IEEE, Kyle Loizos, Student Member, IEEE, Douglas Hamilton, Member?, IEEE,
Raphael C. Lee, Fellow, IEEE, and Gianluca Lazzi, Fellow, IEEE
Abstract—Recent research has shown that space explorers
engaged in Extra Vehicular Activities (EVAs) may be exposed,
under certain conditions, to undesired electrical currents. This
work focuses on determining whether these undesired induced
electrical currents could be responsible for involuntary neu-
romuscular activity in the subjects, possibly caused by either
large diameter peripheral nerve activation or reflex activity
from cutaneous afferent stimulation. An efficient multiresolution
variant of the admittance method along with a millimeter-
resolution model of a male human body were used to calculate
induced electric fields, resistance between contact electrodes
used to simulate the potential exposure condition, and currents
induced in the human body model. Results show that, under
realistic exposure conditions using a 15V source, current density
magnitudes and total current injected are well above previously
reported startle reaction thresholds. This indicates that, under the
considered conditions, the subjects could experience involuntary
motor response.
I. INTRODUCTION
UNDER certain circumstances, Extravehicular activity(EVA) adjacent to the International Space Station (ISS)
can be associated with risk of electrical shock to the as-
tronaut. During EVA the astronaut wears a specialized suit
with a cooling undergarment connected to a heat exchanger
apparatus. Despite the cooling mechanism, the astronaut often
becomes soaked in perspiration while working on the exterior
of the ISS. The sweat soaked undergarment is in direct contact
with metal coupling rings in the Extravehicular Mobility Unit
(EMU, Figure 1) and the astronaut’s body.
When the astronaut exits the ISS, a steel safety cable
is attached from the EMU’s waist ring to the ISS. This
electrically clamps the waist ring potential to that of the ISS.
The ISS is immersed in Earths ionosphere. Therefore, when
the astronaut exits the ISS door, he enters conducting plasma.
The rotation of the Earths magnetic field through the con-
ducting plasma induces currents, along with the corresponding
electrical potential gradients. As the astronaut moves away
from the ISS door, the voltage of the EMU relative to the ISS
changes. The circuit that enables unwanted electrical shock is
Carlos J. Cela, Kyle Loizos, and Gianluca Lazzi are with the Department of
Electrical and Computer Engineering, University of Utah, Salt Lake City, UT,
USA. Douglas Hamilton is with NASA, Johnson Space Center, TX, USA.
Raphael C. Lee is with the Medical Center of the University of Chicago,
Chicago, IL, USA.
Fig. 1. Extravehicular Mobility Unit (EMU). Green bands correspond
to places where metal is in contact with both the astronaut’s skin and
ionosphere’s plasma.
formed by the ISS, the steel cable connecting the waist ring
of the EMU, the astronaut, other rings in different locations
in the EMU (Figure 1), and the plasma of the ionosphere.
To estimate the risk of involuntary muscular activation, the
electric field strength induced in the body, the dynamics of the
change in field strength, and the threshold fields and currents
for nerve or muscle activation need to be considered. Because
large sensory and motor neurons are much more sensitive
to electric field stimulation than with direct stimulation of
skeletal muscle, considering only nerve excitation is adequate
for determining maximum risk. The purpose of this study
is to determine the possibility of electrical shock induced
involuntary neuromuscular activity resulting from EVA.
We investigated two different current paths through the
body. The objective was to predict anatomical regions in which
neurons of different diameters would be excited by electrical
shocks consistent with those anticipated during EVA.
A first numerical model, which calculated the distribution
of electric fields (E-fields) in the body, was built and solved
using a multi-resolution variant of the admittance method
[1][2]. The current paths were determined by the location
https://ntrs.nasa.gov/search.jsp?R=20110008768 2019-08-30T15:08:28+00:00Z
2of electrical contact with metal coupling rings in the EMU,
each having 100 cm2 area. The first current path was from
the sweat-soaked left anterior chest to the right waist. A
second path was from the left wrist to the right waist. The
electrical stimulus considered in the simulation was a 4ms,
15V , electrical pulse, assuming sub-millisecond rise time. For
the purposes of defining tissue conductivity, an operational
frequency of 10kHz was assumed. The tissue conductivities
considered are listed on Table I. To address the probability
of involuntary nerve excitation, the electric field distribution
within major peripheral nerves was determined, as well as
current densities and total current injected in the vicinity of
the metal contacts. The possibility of reflex activity from
cutaneous afferent stimulation is weighted in the context of
known experimental studies [3], and on a second phase of this
work, the occurrence of nerve excitation on terminal endings
of peripheral nerve will be estimated with the help of the
Spatially Extended Nonlinear Node (SENN) model [4][5].
The calculated body impedance was consistent with mea-
sured values under condition of wet skin. The peak current
injected by a 15 V contact was 18mA for the chest-to-hip
current path and 15mA for the wrist-to-hip current path;
both of these values are well within the order of the current
threshold for startle response [3]. Based on these results, a
conclusion of no electrical hazard is not supportable.
II. METHOD
A discrete model was derived from the Visible Human Male
dataset [6]. The resolution of the model was 1 mm and its size
586x340x1878 voxels. Tissue conductivities were taken at 10
kHz from [7] and reported in Table I.
A software program was written to insert two circular
cutaneous electrode pads in the numerical model. At 1mm
resolution, a radius of 56 voxels was used for each electrode
pad to approximate a total contact surface of 100 cm2. The
current return electrode was positioned on the right side of the
lumbar region. Two different stimulating electrode placement
cases were studied, the first one on the chest, and the second
on the wrist as shown on Figure 2. The skin was removed
from the model under the electrodes to obtain perfect contact,
but the skin impedance was included as a 300Ω per contact
series resistance later in the numerical simulation. This series
resistance value fits within the range considered by [7] and [8]
for similar conditions.
A DC current injection of 1A was considered for the
stimulation, with its source at the chest (or wrist) electrode,
and its sink at the hip electrode. The hip electrode was
considered the electrical ground as well.
The admittance method was used to numerically solve the
resulting electrical state. Because of the large model size,
in excess of 335 million voxels, a multiresolution meshing
algorithm [1][2] was used to reduce the order of the model
by joining adjacent voxels inside large homogeneous portions
of the body, while preserving higher level of detail at material
boundaries. Each voxel was then represented as a network of
lumped electrical elements [1] and all partial networks were
joined into a large electrical network representing the entire
Fig. 2. Numerical model, based on the Visible Human Male 1mm dataset
(frontal view). Nerves, heart and brain are shown in blue color for anatomical
reference. Electrodes are represented in red color. Two cases were considered
for electrode placement: chest to hip path (left) and wrist to hip path (right).
model. A linear system was then formed and solved using
a preconditioned biconjugate gradient method; the result of
this operation was the electrical potential at every point in the
model.
Since 1A was the injected current, the resulting calculated
potential between the electrodes corresponds to the impedance
between electrode pads. All the voltages were normalized
so that the voltage between stimulating electrode pads was
15V : the E field magnitude was then calculated from the
scalar electric potential field and voxel dimensions. A software
program was written and used to filter out all the E field
magnitude values outside the nervous system and heart, and
to convert the simulation result to a format compatible with
ParaView [9] for visualization.
III. RESULTS
A. Body Resistance and Total Current Injected
The simulations were run with a 1A DC current from
electrode to electrode to obtain body resistance, and subse-
quently scaled to the equivalent of 15 V between electrodes.
In our model, the skin under the electrodes was removed to
obtain perfect contact with the underneath tissue, but later
added as a 300 Ω series bulk resistance per electrode. A
second simulation to verify the results obtained with the skin
as described above was also performed. For this later case
the model was reconstructed and a layer of skin under the
electrodes was introduced; results obtained with the added skin
3TABLE I
TISSUE CONDUCTIVITIES AT 10 kHz [7]
Tissue σ [Ω ·m]
Bladder 2.130E-01
Blood 7.000E-01
Blood Vessel 3.131E-01
Body Fluid 1.500E+00
Bone Cancellous 8.262E-02
Bone Cortical 2.043E-02
Bone Marrow 2.735E-03
Brain Grey Matter 1.149E-01
Brain White Matter 6.948E-02
Cartilage 1.759E-01
Cerebellum 1.349E-01
Cerebro Spinal Fluid 2.000E+00
Colon 2.399E-01
Cornea 4.425E-01
Eye Sclera 5.103E-01
Fat 2.383E-02
Gall Bladder 9.001E-01
Gall Bladder Bile 1.400E+00
Gland 5.297E-01
Heart 1.542E-01
Kidney 1.377E-01
Lens 3.354E-01
Liver 5.350E-02
Lung Deflated 2.429E-01
Lung Inflated 9.317E-02
Lymph 5.297E-01
Metal 1.000E+04
Mucous Membrane 2.932E-03
Muscle 3.408E-01
Nail 2.043E-02
Nerve 4.240E-02
Pancreas 5.297E-01
Retina 5.103E-01
Skin Dry (body) 2.041E-04
Small Intestine 5.597E-01
Spleen 1.108E-01
Stomach 5.297E-01
Tendon 3.864E-01
Testis 4.298E-01
Tooth 2.043E-02
Vitreous Humor 1.500E+00
layer were in good agreement with those considering the skin
as a series resistance.
In the chest-to-hip configuration, the largest resulting volt-
age in the model for 1A injection was 202V . From this, the
resistance between electrodes without considering contact ca-
pacitances or skin is 202Ω. Scaling for 15V requires a scaling
factor of 15V /202V = 0.074; this results in a total injected
current between electrodes of 74mA when 15V are applied,
without considering the effect of the skin or contact capaci-
tances. Considering the effect of the skin as a bulk resistance
of 300Ω per electrode contact in series, the total resistance for
the chest-to-hip configuration is 300Ω+202Ω+300Ω = 802Ω.
Thus, the total current injected for a 15V pulse in the chest-
to-hip configuration and considering the effect of the skin is
74mA× 202Ω/802Ω = 18mA.
Similarly, for the wrist-to-hip configuration a resistance be-
tween electrodes of 369Ω was calculated without considering
the effect of the skin and contact capacitances, yielding a total
injected current of 40mA for 15V . When the effect of skin
under electrodes was taken into account, a resistance of 969Ω
was estimated and a current of 15mA.
Fig. 3. Electric field in excitable tissue caused by a 15V electric shock for
chest-to-hip configuration.
B. Electric Fields and Induced Current Densities Near the
Electrodes
The electric field magnitudes were calculated from the
electric potentials and mesh geometry. We were interested in
the electric field present in excitable tissue, namely peripheral
nerves, heart, and brain. Figure 4 and Figure 4 show the elec-
tric fields on excitable tissue for both configurations modeled.
The current density magnitudes at each point in the model
were calculated from the electric fields and dielectric proper-
ties of the tissues. We were particularly interested in quanti-
fying current densities close to the electrodes, where they are
presumed to be larger, and in proximity of cutaneous afferent
neurons. The current density magnitudes at a transverse plane
crossing the center of each stimulating electrode are plotted in
Figure 5 and Figure 6 for the chest-to-hip case and on Figure 7
for the wrist-to-hip case.
IV. DISCUSSION
We presented a study to help determine the possible effects
of undesired induced electric currents and the possibility of
inducing involuntary neuromuscular activity when the space
explorer is outside the ISS. Because of the relatively coarse
resolution of our model compared with finer nerve structures,
only large peripheral nerves are represented. We considered
the possibility of accidental activation of larger diameter
peripheral nerves, cardiac fibrillation, and cutaneous afferent
neuron activation. For the purpose of this discussion, we will
focus on the latter effect.
4Fig. 4. Electric field in excitable tissue caused by a 15V electric shock for
wrist-to-hip configuration.
Fig. 5. Chest-to-hip configuration. Transverse slice through center of
electrodes, frontal aspect of body towards bottom of plot. Current density
magnitudes in the proximity of chest electrode for 15V shock.
Direct effect of stimulation of sensory nerves in the skin
was not computable, since single axons are not resolvable with
the mesh resolution of the considered model. However, values
for current density magnitudes and total current injected were
calculated for areas close to the stimulating electrodes, and
can be used to compare to previously reported experimental
data. Current values injected for a 15 V shock are in the range
of 15mA to 18mA, and well above the startle reaction current
thresholds reported by [3]. Thus, we conclude that the risk of
involuntary motor response is likely with a 15V electric shock,
under the assumptions of the model.
We should also note that cutaneous nerves are primary
sensory, and stimulation of these nerves due the the electric
shocks is likely to produce spinal withdrawal reflexes, which
Fig. 6. Chest-to-hip configuration. Transverse slice slice through center of
electrodes, frontal aspect of body towards bottom of plot. Current density
magnitudes in the proximity of hip electrode for 15V shock.
Fig. 7. Wrist-to-hip configuration. Transverse slice slice through center of
hip electrode, frontal aspect of body towards bottom of plot. Current density
magnitudes in the proximity of hip (left) and wrist (right) electrodes for 15V
shock
may cause for a generalized involuntary motor response in the
entire body.
REFERENCES
[1] C. J. Cela. A Multiresolution Admittance Method for Large-Scale Bio-
electromagnetic Interactions. PhD thesis, North Carolina State University,
2010.
[2] C. J. Cela, Raphael C. Lee, and G. Lazzi. Modeling cellular lysis in
skeletal muscle due to electric shock. IEEE Trans. Biomed. Eng., PP(99),
2011.
[3] IEC/TR. Effects of current on human beings and livestock - part 5:
Touch voltage threshold values for physiological effects. TR 60479-5,
International Electrotechnical Commission, Geneva, Switzerland, 2007.
[4] J. Patrick Reilly, Vanda T. Freeman, and Willard D. Larkin. Sensory
effects of transient electrical stimulation - evaluation with a neuroelectric
model. IEEE Trans. Biomed. Eng., BME-32(12), 1985.
[5] J. Patrick Reilly and Alan M. Diamant. Spatial relationships in electros-
timulation: Application to electromagnetic field standards. IEEE Trans.
Biomed. Eng., 50(6), 2003.
[6] V. Spitzer, M. J. Ackerman, A. L. Scherzinger, and D. Whitlock. The
visible human male: A technical report. J. Am. Med. Inform. Assoc., 3(2),
April 1996.
[7] S. Gabriel, R. W. Lau, and C. Gabriel. The dielectric properties of
biological tissues: Iii. parametric models for the dielectric spectrum of
tissues. Phys. Med. Biol., 41:2271–2293, 1996.
[8] IEC/TR. Effects of current on human beings and livestock – part 1:
General aspects. TR 60479-1, International Electrotechnical Commission,
Geneva, Switzerland, 2007.
[9] A. Henderson. ParaView Guide, A Parallel Visualization Application.
Kitware Inc., 2007.


On the Modeling of Electrical Effects Experienced by Space Explorers During Extra Vehicular Activities: Intracorporal Currents, Resistances, and Electric Fields

On the Modeling of Electrical Effects Experienced by Space Explorers During Extra Vehicular Activities: Intracorporal Currents, Resistances, and Electric Fields

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