Large Animal Model for Development of Functional
Restoration Paradigms Using Epidural and Intraspinal
Stimulation
Jan T. Hachmann1,2, Ju Ho Jeong1,3, Peter J. Grahn2, Grant W. Mallory1, Loribeth Q. Evertz2,
Allan J. Bieber2,4, Darlene A. Lobel5, Kevin E. Bennet1,6, Kendall H. Lee1,7, J. Luis Lujan1,7*
1 Department of Neurologic Surgery, Mayo Clinic, Rochester, Minnesota, United States of America, 2 Mayo Graduate School, Mayo Clinic, Rochester, Minnesota, UnitedStates of America, 3 Department of Neurosurgery, Kosin University College of Medicine, Busan, Korea, 4 Department of Neurology, Mayo Clinic, Rochester, Minnesota,
United States of America, 5 Department of Neurosurgery, Cleveland Clinic, Cleveland, Ohio, United States of America, 6 Division of Engineering, Mayo Clinic, Rochester,
Minnesota, United States of America, 7 Department of Physiology and Biomedical Engineering, Mayo Clinic, Rochester, Minnesota, United States of America

Abstract
Restoration of movement following spinal cord injury (SCI) has been achieved using electrical stimulation of peripheral
nerves and skeletal muscles. However, practical limitations such as the rapid onset of muscle fatigue hinder clinicalapplication of these technologies. Recently, direct stimulation of alpha motor neurons has shown promise for evoking
graded, controlled, and sustained muscle contractions in rodent and feline animal models while overcoming some of theselimitations. However, small animal models are not optimal for the development of clinical spinal stimulation techniques forfunctional restoration of movement. Furthermore, variance in surgical procedure, targeting, and electrode implantation
techniques can compromise therapeutic outcomes and impede comparison of results across studies. Herein, we present a
protocol and large animal model that allow standardized development, testing, and optimization of novel clinical strategies
for restoring motor function following spinal cord injury. We tested this protocol using both epidural and intraspinal
stimulation in a porcine model of spinal cord injury, but the protocol is suitable for the development of other novel
therapeutic strategies. This protocol will help characterize spinal circuits vital for selective activation of motor neuron pools.
In turn, this will expedite the development and validation of high-precision therapeutic targeting strategies and stimulation
technologies for optimal restoration of motor function in humans.
Citation: Hachmann JT, Jeong JH, Grahn PJ, Mallory GW, Evertz LQ, et al. (2013) Large Animal Model for Development of Functional Restoration Paradigms Using
Epidural and Intraspinal Stimulation. PLoS ONE 8(12): e81443. doi:10.1371/journal.pone.0081443
Editor: Anthony E. Kincaid, Creighton University, United States of America
Received August 19, 2013; Accepted October 22, 2013; Published December 5, 2013
Copyright: ß 2013 Hachmann et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by the National Institutes of Health (K08 NS 52232 and R01 NS 70872 awards to KHL), The Grainger Foundation, and a giftfrom Louise Chapman. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: lujan.luis@mayo.edu

agonists [11–13]. Unfortunately, while existing small animalmodels are well established for performing proof-of-principle andpreliminary studies, they are ultimately inadequate for determining clinical utility or for successful clinical translation of functional
restoration techniques [14,15]. Therefore, development of ISMS
technology and functional stimulation paradigms for humanapplication requires a model with appropriate dimensions andstructure for accurate comparisons as well as evaluation of safety
and efficacy. Functional hand movement has also been reported
via cervical spinal stimulation in high-transection models of nonhuman primates [16]. However, primate models are costly andrequire long-term animal training. As such, there is a clear needfor a large animal model that closely resembles the neuroanatomyof the human spinal cord and allows the development of novel,
effective, safe, and cost-effective therapeutic paradigms. The
neuroanatomy of the porcine spinal cord closely resembles thatof humans [20] and represents a low-cost alternative to nonhuman primate models for the development and application of
ISMS technology.
Herein, we present a standardized surgical model and spinal
stimulation protocol in the adult white farm swine. This model

Introduction
Limb movement is controlled by effector motor neurons andsynaptic relays within the ventral horn grey matter of the spinal
cord that receive inputs from higher brain centers. When signaltransmission to these cells is interrupted, as in the case of spinal
cord injury (SCI), permanent loss of sensorimotor and autonomic
function can occur below the level of injury. Despite this, thecellular structure and neuronal networks remain intact and arecapable of evoking and coordinating limb movements whenelectrically stimulated [1–4].
Recently, multiple studies of intraspinal stimulation have beensuccessful in safely evoking coordinated limb movement andweight-bearing while improving fatigue-resistance in small animalmodels of SCI [5–9]. In previous studies, we demonstrated thatwireless control of intraspinal microstimulation (ISMS) can beused to evoke graded and sustained contraction of hip, knee, andankle muscles over prolonged periods of time in anesthetized SCI
and spinally intact rats [10]. Additionally, fatigue resistant stepping
has been demonstrated using intraspinal and epidural stimulation
combined with systemic injection of excitatory neurotransmitter

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allows the development and evaluation of novel therapeuticinterventions and will ultimately help fill a critical gap in thetranslation of ISMS technology from small animal research toclinical human therapies. Ultimately, application of this model forthe development and optimization of novel neural interface
technology holds the potential to improve functional gains using
spinal stimulation therapies and enhance quality of life forindividuals with spinal cord injury.

the paraspinous musculature was retracted and detached from thespinous processes and laminae using a combination of blunt
preparation and monopolar electrocautery. Self-retaining retractors were inserted and progressively deepened with exposure toallow adequate visualization of the surgical field. Dissection inclose proximity to the surface of the spinous processes reduced
bleeding and facilitated subsequent bone removal. A small Cobb
periosteal elevator was used to retract and sweep soft tissue fromthe spine, exposing the laminar surface (Fig. 1D). Bleeding wascontrolled using absorbable hemostats (Surgicel, Ethicon Inc,
Somerville, NJ) and mono2/bipolar electrocautery.

Methods
Ethics Statement
All study procedures were approved by the Mayo Clinic
Institutional Animal Care and Use Committee and were inaccordance with the National Institutes of Health Guidelines for
Animal Research (Guide for the Care and Use of Laboratory
Animals).

Epidural Stimulation
A partial laminotomy was performed at the caudal end of thefifth lumbar vertebral level (L5) to allow insertion of epiduralelectrodes (Fig. 1E). The facet junction was identified and the bonethinned using a Horsley rongeur to expose the interior corticalmargin. The spinal canal was then opened using a small angled
curette by removing the remaining bone of the inferior L5 andsuperior L6 articular processes. The opening on the lower borderof the lamina was extended using a 3 mm Kerrison rongeur. Thisdecreased the risk of inadvertent injury to the dura mater during
electrode insertion. The ligamentum flavum was incised to allow
access to the dural sac. A cylindrical 8-contact epidural stimulating
electrode (St. Jude Medical, St. Paul, MN) was inserted craniallythrough the laminotomy site (Fig. 1E). Accurate electrode contact
placement was verified using intra-operative bi-planar fluoroscopy(Fig. 2A,B). We applied comprehensive bipolar stimulation at afrequency of 50 Hz with 200 ms pulse durations to all adjacentpairs of electrode contacts with amplitudes ranging from 100 to1000 mA.

Animals
Six female domestic adult pigs (GeneticPork, Rochester, MN)
aged 8–12 months and weighing between 25–35 kg were used forthis model. Animals were kept in separate cages with visual accessto each other in a controlled environment (constant temperature at
21uC and humidity at 45%) on a 12-hour light/dark cycle with ad
libitum access to water. Animals were fed once daily.

Animal Anesthesia
Animals were placed under general anesthesia with intramuscular telazol (5 mg/kg) and xylazine (2 mg/kg) for induction and
1.5–3% isoflurane for maintenance. Fentanyl (2–5 mg/kg/hr) was
administered during surgery for analgesia. Heart rate (,120 bpm),
blood pressure (,110/80 mmHg), and core body temperature(36–37uC) were continuously monitored to detect signs of pain or
discomfort. Sufficient hydration was ensured by continuous
intravenous (I.V.) saline infusion (75–150 ml/hr). Body temperature was maintained via heated blankets and warmed saline
infusions. The short-acting non-depolarizing muscle relaxant
vecuronium (0.057 mg/kg) was administered prior to spinal
exposure to eliminate back musculature contractions during
surgical manipulation.

Intraspinal Microstimulation (ISMS)
The spinal cord lumbar enlargement was exposed via complete
laminectomies from L2 to S1, avoiding iatrogenic injury to large
blood vessels, spinal cord tissue, nerve roots, and dura mater. First,
the spinous processes were removed and the cortical margin
exposed by thinning the laminar bone with a Leksell rongeur.
Second, the bone shell was removed in a caudal to rostral direction
from L2 to S1 using a 3 mm Kerrison rongeur while sparing thefacet joints. Special attention should be placed not to extend thelaminectomy beyond the lateral masses. Third, the ligamentum
flavum and epidural fat pad were removed to expose the dural sac.
Fourth, the dura mater was tented up with a sharp dura-hook or
5–0 silk suture and incised in the midline. The durotomy can beextended using a groove director and 15-blade or blunt microscissors while avoiding iatrogenic injury to the spinal cord. The
dural flaps were bilaterally retracted using 5–0 silk sutures toexpose the spinal cord parenchyma (Fig. 1F). Continuous saline
irrigation was maintained to prevent cord dehydration.
Polyimide insulated (,200 mm in diameter, A–M Systems,
Sequim, WA) tungsten microwires were implanted into the
porcine ventral spinal cord for stimulation (Fig. 1G). Insertion
was guided by the dorsal fissure, bilateral posterior spinal arteries,
and posterolateral dorsal root entry zones. Each microwire had a
250–400 mm exposed tip to create the stimulating electrode.
Electrodes were labeled with tape using light microscopy prior to
the procedure to mark targeted insertion depth and protect theelectrode from damage during insertion. The tungsten microelectrodes allow a wide range of possible trajectories and insertion
depths. Current-controlled bipolar spinal stimulation was applied
through these electrodes using a four-channel portable stimulator(Mayo Investigational Neuromodulation Control System, MINCS)[17]. The stimulator was wirelessly-controlled in real-time using a

Lumbar Exposure
Animals were positioned prone on an open-ended convex
polymethyl-methacrylate cradle to allow stable surgical exposure,electrode insertion, and kinematic analysis of stimulation-evoked
responses without requiring animal repositioning. Moderatekyphosis of the lumbar area facilitated the surgical procedure.
The coat covering the surgical field was clipped using an electric
shaver. Unlike the human lumbar spine, the porcine spine
generally consists of six vertebrae. The lumbar enlargement
resides between the third and fifth lumbar vertebrae (L3–L5) and
was initially confirmed using pre-operative computed tomography(CT). Subsequent procedures were conducted using vertebral
anatomical landmarks (i.e., the iliac crest and spinous processes)
and intra-operative fluoroscopy (Fig. 1A,C). Location of the
spinous processes allows orientation to midline. The spinous
processes are connected via supraspinous and interspinous
ligaments. A median skin incision from the second lumbar to thefirst sacral vertebral levels (L2–S1) was performed to allow accessto the paraspinal musculature. Skin was retracted and the
subcutaneous connective tissue was superficially incised tovisualize the lumbodorsal fascia. The paraspinal musculature
inserts at the facet joints and spinous processes via paravertebral
tendons. The paravertebral tendons were bilaterally dissected and
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Figure 1. Experimental setup. (A) Anatomical landmarks (i.e., sacrum, iliac crest, and spinous processes) for localization of the lumbar spine (L2–S1); (B) Lateral view of anatomical motion analysis markers (1-Lateral iliac crest, 2-Trochanter major, 3-Patella, 4-Lateral malleolus, 5-Fourth
metatarsal); (C) Pre-operative computed tomography: Lower thoracic vertebrae, lumbar vertebrae and sacrum; (D) Exposure of the lumbar spine; (E)Laminotomy at L5-left (circle) and epidural electrode (arrow); (F) Exposure of the spinal cord for intraspinal microstimulation; (G) Typical

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microelectrode implantation in the left hemicord (arrow). Insulating adhesive tape was used for protection of the electrode during forceps-insertionand color-coded to determine insertion depth (e.g., blue tape corresponded to an electrode depth of 6 mm). A reference ruler (centimeters) was used
to determine cord dimensions and estimate electrode location.
doi:10.1371/journal.pone.0081443.g001

Windows-based personal computer. We used a frequency of
50 Hz with 200 ms pulse duration and amplitudes of 50–300 mA.
Microwire electrodes were orthogonally inserted at a trajectorythat avoids the posterior spinal arteries to target lateral or medial
ventral horn motor neurons, respectively. Insertion depth wasvaried from 2 to 6 mm from the posterior surface of the cord. A
return tungsten microelectrode with a 10 mm exposed tip was
implanted into the contralateral abdominal muscle. Electrodemigration was prevented using a biocompatible cyanoacrylateadhesive applied to the dorsal surface of the spinal cord.

musculature. EMG data were wirelessly recorded and transmitted
to a Windows-based computer via Bluetooth technology for offline analysis using Matlab (Mathworks, Natick, MA). Changes injoint angle were calculated by subtraction of joint position offsets
before stimulation onset from the entire signal. To allow
comparison across different muscles and subjects, EMG signal
amplitude was determined by calculating the root mean squared(RMS) of the sampled signal and normalizing it to the maximum
EMG amplitude recorded.

Results

Spinal Cord Injury Model

We tested our porcine model using epidural and intraspinal
microstimulation techniques to evoke hind limb movement in six
pigs. Preparation of the lumbar enlargement (including muscle
dissection and bone removal from L2 to S1), radiographic
identification of the stimulation targets, and electrode placement
was achieved within 60 minutes of the initial incision. The timerequired for experimental testing depended upon the specificepidural or intraspinal stimulation protocol (i.e., stimulation
paradigms, targets, and parameters) and can be safely extended
under anesthesia. The maximum experimental time for the
protocol described herein was 480 minutes. Injury to major bloodvessels was the most common risk. However, bleeding was
minimal (,0.3 L) and easily controlled using electrocautery,hemostats, gelfoam (Pfizer, New York, NY), and bone wax.
Animal temperature was safely maintained within 36.5uC 60.5uC
throughout this time. One animal presented with borderline
hypothermia (35uC) within the first 30 minutes of the procedure.
Overall motion response to stimulation was decreased during thistime. Normal temperature was restored using a combination of
I.V. heated saline and heated blankets. There were no other majorcomplications.
Optimal target location of the epidural electrode was readily
achieved using lateral and posterior-anterior fluoroscopy(Fig. 2A,B). Ex-vivo imaging confirmed successful ISMS-targetingof the ventral horn gray matter (Fig. 2C). Epidural andintraspinal stimulation successfully evoked selective extension,
flexion, adduction, and abduction of the knee and hip joints.
Maximal hip extension was evoked via ISMS at the L5 level(Fig. 2H) with combined abduction/extension motion (Fig. 2E,F)
and normalized EMG showed maximal activation of the gluteus
medius, hamstrings, biceps femoris, and gastrocnemius muscles(Fig. 2G). Both epidural and intraspinal stimulation confirmedthat spinal motor neurons controlling distal portions of the hindlimb were located approximately 3 mm lateral from midline,
while neurons controlling proximal portions of the hind limb
were found approximately 1 mm from midline. Similarly,
extensor neurons were found ventrally approximately 6 mmfrom the posterior surface of the cord and flexor neurons werelocalized more dorsally approximately 4 mm from the cordsurface. Electromyography and kinematic analysis showed thatjoint angle change and EMG amplitude increased proportionally to stimulation amplitude. Furthermore, both types ofstimulation were successful in evoking sustained tetanic muscle
responses. The magnitude of these responses was not different
for epidural and intraspinal stimulation (p.0.05). However,current amplitude thresholds required for evoking limb movement were five times lower for intraspinal compared to epidural

In addition to exposure of the lumbar spine, a complete spinal
cord transection was performed at the fourth thoracic level (T4)following the surgical principles described previously. The target
segment was localized using the most prominent spinous process(T2) and the turning point of the curvature from kyphosis tolordosis, usually at T7 or T8. This was verified using intraoperative fluoroscopy.

Verification of Electrode Location
Placement of the epidural electrode was verified following
epidural stimulation using intra-operative bi-planar fluoroscopy(Fig. 2A–B). In accordance with the recommendations of theAmerican Veterinary Medical Association Guidelines on Euthanasia, animals were euthanized while still under anesthesia using
a lethal dose of sodium-pentobarbital (50–75 mg/kg, I.V.) uponcompletion of the experimental protocol. Following euthanasia,
the lumbar spinal cord (L2–L6) was surgically removed andimmersed in a 4% paraformaldehyde solution. The resectedspinal cord segment was imaged ex vivo using a 9.7 Tesla
Magnetic Resonance Imager (Bruker 300 NMR) with a T2weighted fast spin-echo (FSE) sequence. This allowed threedimensional reconstruction and visualization of the intraspinal
electrode tracts through the grey and white matter into theventral horn (Fig. 2C).

Analysis of Stimulation-evoked Motor Function
Kinematic and EMG analyses were performed during spinal
stimulation. All animals were suspended in a custom-made systemthat allowed for electrode repositioning and unrestricted hindlimb
movement across all degrees of freedom (Fig. 2D). Motion analysis
markers were placed on the fourth metatarsal, ankle joint, knee
joint, and iliac crest (Fig. 1B). Evoked joint movement wasrecorded using Basler ace GigE cameras (204861088 pixels, 100fps) for parasagittal and posterior high-speed motion analysis(Contemplas, Templo, Kempten, Germany). Intramuscular EMG
electrodes were implanted into the gluteus medius, quadriceps
femoris, gastrocnemius, hamstrings, and biceps femoris muscles.
Surface EMG sensors were placed over the gluteus medius,
quadriceps femoris, hamstrings and biceps femoris muscles.
Intramuscular and surface EMG data (common mode rejection
ratio .80 dB and gain of 300) were recorded at 4000 Hz across abandwidth of 20–450 Hz using a 16-bit Trigno system (Delsys,
Boston, MA). The skin underlying the surface electrodes was
shaved and cleaned to improve electrode–skin contact and reduce
impedance. The surface EMG sensors were attached to the skinusing a double-sided adhesive. For intramuscular EMG recordings, a reference electrode was inserted into the deep back
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Figure 2. Targeting analysis and functional outcomes. (A) Epidural electrode placement confirmed by lateral intra-operative fluoroscopy; (B)Posterior-anterior intra-operative fluoroscopy; (C) Ex-vivo axial MRI of spinal cord showing ISMS electrode tract (arrow); (D) Experimental setup for
kinematic analysis; (E–F) Typical hip extension evoked by ISMS at L5-segment; (G) Normalized intramuscular EMG amplitude as a function of spinal
cord segment evoked by ISMS at 300 mA (n = 3); (H) Kinematic analysis of joint angle change evoked by ISMS as a function of spinal cord segment(n = 3). Abbreviations: G.M. = Gluteus medius, H = Hamstrings, B.F. = Biceps femoris, G = Gastrocnemius, Q.F. = quadriceps femoris.
doi:10.1371/journal.pone.0081443.g002

stimulation. Initial muscle contractions in response to ISMS
were generally observed at 50 mA compared to 250–300 mA for
epidural stimulation. Motor response to epidural stimulation
depended majorly on electrode location and distance between
the contacts and spinal cord surface.

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Discussion and Future Directions
Traumatic injury to the spinal cord can result in permanent lossof motor, sensory, and autonomic function, significantly reducing
quality of life for affected individuals. Functional electricalstimulation (FES) has been used clinically to restore lost function
in paralyzed limbs by applying electrical currents either directly tothe muscles or to peripheral nerves [3–5]. However, these5

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techniques have had limited success due to early onset of muscle
fatigue, limited degrees of freedom, large neuroprosthetic device
size, and short battery life. Direct spinal stimulation techniques
have been associated with recruitment of fatigue-resistant motor
units in small-animal models [5,7,8,18,19]. Thus, the lower
stimulation amplitudes associated with spinal stimulation, compared to conventional stimulation techniques, have the potential toincrease battery life of implantable neuroprosthetic systems.
However, these spinal stimulation techniques for restoration of
motor function have only been attempted in small animal models,
which are ultimately inadequate for the successful translation offunctional restoration therapies into clinical application. As such,there is a significant need for a large animal model that not only
allows evaluation of therapeutic efficacy and safety, but also
optimization of targeting strategies and characterization of spinal
circuits vital for selective activation of motor neuron pools.
The porcine spinal cord offers distinct advantages over otheranimal models for the development of functional restoration
strategies following spinal cord injury. First, the spinal cord
anatomy and physiology in the pig closely resemble the humanspinal cord, thereby facilitating characterization of motor circuits.
Several studies have shown that topographical organization of
motor neurons and circuitry are maintained across species [20–22]. This allows the use of the porcine model to study therapeuticmechanisms and optimize the development of novel therapeuticparadigms for restoring function following spinal cord injury.
Overall, the superior neuroanatomical resemblance of the porcine
spine [23] and evolutionary conservation of topographical motor
neuron networks, make this large animal model primed forexpedited development of novel spinal therapies and evaluation of
in-vivo long-term efficacy and safety. Second, its large spinal cord
makes the porcine model ideal for optimizing targeting anddelivery of stimulating electrodes. The model presented hereinfacilitates comprehensive three-dimensional topographical mapping of lumbar spinal cord neurons, thereby allowing for enhanced
selectivity and control of motor function. Furthermore, this modelhas the potential for application in a variety of other research andclinical applications such as drug delivery systems for a host of
spinal therapies including chemotherapy, gene therapy, and stem
cell transplantation. It will also allow characterization of neuronal
activation profiles, e.g. through electrophysiological recording orelectrochemistry. Third, their low cost makes porcine models afeasible alternative to non-human primate models. However,having an appropriate animal model does not guarantee optimaltranslation of research developments into clinical therapeutic
application. Differences in neuroanatomy, targeting strategy, andsurgical skill can introduce significant variation in therapeutic
outcomes. As such, it is paramount to standardize surgical andexperimental protocols. This manuscript describes, step-by-step,
the development of a large-animal model for the study of spinal
cord physiology and restoration of function following neurologic
injury and disease. Despite presenting significant advantages overexisting models, the model described herein has inherent

limitations. First, this work describes an acute model of spinal
cord injury. Consequently, a stable model of chronic spinal cord
injury will be required in order to establish the long-term feasibilityof this type of therapeutic intervention. However, this model
presents a good platform for mapping spinal neuron populations
responsible for limb movement and for developing novel
neuroprosthetic technology. Second, stimulation spillover canactivate antagonist motor pools micrometers away from theelectrode site [24]. Therefore, an automated stereotactic systemneeds to be developed for precise and accurate stimulation oftarget neurons. In turn, this will ensure subject safety while
improving limb control and reliability of ISMS technology. Third,
mapping locomotor regions of the spinal cord via rigid microelectrodes requires multiple penetrations. This extends surgery timeand has the potential to damage spinal cord tissue [25]. For thisreason, future studies should use multi-electrode arrays that may
allow comprehensive and more precise activation of motor pools
with potential for less tissue disruption. Fourth, the efficacy of
microwire electrodes tends to diminish over time due to tissue
reactions in vivo. Thus, in order to minimize tissue damage to thespinal cord, future work should be focused on developing flexible,
implantable electrode arrays. Fifth, this model focuses on evoking
simple motor responses. To this end, future work will need to focus
on functional movements such as weight-bearing standing andambulation that require independent but coordinated activation ofmuscles across multiple joints.
Successful application of spinal stimulation therapy in humans
will require the use of versatile chronic neurostimulation
approaches for optimal restoration of motor function. The
standardized surgical procedure presented in this manuscript
reduces the risk of inadvertent iatrogenic injury to spinal cord andnerve roots while allowing for relatively easy adaptation to otherapplications. Furthermore, this model has the potential to facilitate
the development of novel clinical neural interfaces for chronic
spinal cord stimulation, as well as the development and validationof techniques for long-term evaluation of efficacy and safety. Inturn, the model and protocol presented herein represents animportant step toward developing novel technology, which may
ultimately lead to restoring independence and quality of life
following spinal cord injury.

Acknowledgments
The authors would like to thank Jill Anderson, Michael Marsh, and Bill
Flanary for their help during animal surgery and stimulation protocols.

Author Contributions
Conceived and designed the experiments: JTH JHJ PJG AJB DAL KEB
KHL JLL. Performed the experiments: JTH JHJ PJG GWM LQE AJBDAL JLL. Analyzed the data: JTH JHJ PJG AJB DAL JLL. Contributed
reagents/materials/analysis tools: KEB KHL. Wrote the paper: JTH JHJ
PJG GWM LQE AJB DAL KEB KHL JLL.

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December 2013 | Volume 8 | Issue 12 | e81443