Panagiotis S Ioannou

Evripides Kyriakides

Olivier Schneegans

John Giapintzakis

evidence of Biorealistic Synaptic Behavior in Diffusive Li-based Two-terminal Resistive Switching Devices

Following the recent advances in artificial synaptic devices and the renewed interest regarding artificial intelligence and neuromorphic computing, a new two-terminal resistive switching device, based on mobile Li + ions is hereby explored. Emulation of neural functionalities in a biorealistic manner has been recently implemented through the use of synaptic devices with diffusive dynamics. Mimicking of the spontaneous synaptic weight relaxation of neuron cells, which is regulated by the concentration kinetics of positively charged ions like Ca 2+ , is facilitated through the conductance relaxation of such diffusive devices. Adopting a battery-like architecture, using LiCoO 2 as a resistive switching cathode layer, Sio x as an electrolyte and tio 2 as an anode, Au/LiCoO 2 /SiO x /TiO 2 /p ++ -Si two-terminal devices have been fabricated. Analog conductance modulation, via voltage-driven regulation of Li + ion concentration in the cathode and anode layers, along with current rectification and nanobattery effects are reported. Furthermore, evidence is provided for biorealistic synaptic behavior, manifested as paired pulse facilitation based on the summation of excitatory post-synaptic currents and spike-timingdependent plasticity, which are governed by the Li + ion concentration and its relaxation dynamics.

CMOS simulation of neural networks enables the implementation of artificial intelligence through machine learning [START_REF] Furber | The spinnaker project[END_REF][START_REF] Markram | The blue brain project[END_REF] . However, it suffers from great hardware and computational cost due to the von Neumann efficiency bottleneck [START_REF] Backus | Can programming be liberated from the von Neumann style?: a functional style and its algebra of programs[END_REF] . Novel, bioinspired computing paradigms are expected to overcome these limitations by following the blueprint of the biological brain. Contrary to von-Neumann-type systems, the astounding performance of the biological brain emanates from the highly parallel, low-energy processing and storing of information at a single node. This is achieved through the propagation of electrical and electrochemical signals between neurons, through electrical and chemical synapses, respectively, enhancing or diminishing the synaptic strength (weight) according to their preceding activity. While electrical synapses traditionally allow the bidirectional propagation of signals (action potentials), chemical synapses, which are regulated by the influx of ions such as Ca 2+ (due to an action potential), strictly allow the unidirectional propagation of neurotransmitters from the pre-synaptic neuron to the synaptic cleft [START_REF] Dharani | The Biology of Thought: A Neuronal Mechanism in the Generation of Thought-a New Molecular Model[END_REF] . Activity-dependent plasticity of vastly interconnected neural networks enables functionalities such as long-and short-term memory, associative learning, etc., which constitute the building blocks of cognitive processes [START_REF] Morris | Elements of a neurobiological theory of the hippocampus: the role of activity-dependent synaptic plasticity in memory[END_REF] .

Although still in its embryonic state, bioinspired computing appears more promising than ever before. Advancements in neural activity monitoring [START_REF] Musk | An integrated brain-machine interface platform with thousands of channels[END_REF] , which will effectively lead to a higher understanding of brain functionality, along with hardware implementation and system-level simulations of neuromorphic circuits with binary and analog resistive switching devices [START_REF] Ignatov | A memristive spiking neuron with firing rate coding[END_REF][START_REF] Covi | Analog memristive synapse in spiking networks implementing unsupervised learning[END_REF][START_REF] Sheridan | Sparse coding with memristor networks[END_REF][START_REF] Ielmini | Brain-inspired computing with resistive switching memory (RRAM): Devices, synapses and neural networks[END_REF] , constitute the foundation for the development of bioinspired computing paradigms.

While multibit information storage as synaptic weights, through the conductance modulation of such devices, enables the realization of in-memory computing and non-von Neumann architectures [START_REF] Borghetti | Memristive'switches enable 'stateful'logic operations via material implication[END_REF][START_REF] Rosezin | Crossbar logic using bipolar and complementary resistive switches[END_REF][START_REF] Kvatinsky | Memristor-based material implication (IMPLY) logic: Design principles and methodologies[END_REF] , synaptic plasticity enables the direct emulation of biorealistic neural activity [START_REF] Yang | Synaptic plasticity and memory functions achieved in a WO3-x-based nanoionics device by using the principle of atomic switch operation[END_REF][START_REF] Li | Activity-dependent synaptic plasticity of a chalcogenide electronic synapse for neuromorphic systems[END_REF][START_REF] Du | Biorealistic implementation of synaptic functions with oxide memristors through internal ionic dynamics[END_REF] . Biorealistic synaptic plasticity can be emulated by devices with transient switching behavior, such as diffusive (or second-order) memristors, which present a conductance dependence on the timing of stimulating pulses, enabling an internal timing mechanism through their inherent diffusive dynamics [START_REF] Kim | Experimental demonstration of a second-order memristor and its ability to biorealistically implement synaptic plasticity[END_REF] . Mimicking the spontaneous synaptic weight relaxation of neuron cells, through the conductance relaxation mechanism of such diffusive devices, enables the implementation of short-term memory effects based on biorealistic adaptive synaptic plasticity principles.

Towards the goal of implementing such artificial synapses, resistive switching devices based on the nanoinonic diffusion of Li + ions appear highly promising, both in three-terminal transistor configuration [START_REF] Upadhyay | Emerging memory devices for neuromorphic computing[END_REF][START_REF] Fuller | Li-ion synaptic transistor for low power analog computing[END_REF][START_REF] Yang | All-Solid-State Synaptic Transistor with Ultralow Conductance for Neuromorphic Computing[END_REF][START_REF] Zhu | Ion gated synaptic transistors based on 2D van der Waals crystals with tunable diffusive dynamics[END_REF][START_REF] Sharbati | Electrochemically Tunable Graphene Synapses for Neuromorphic Computing[END_REF] , as well as in two-terminal vertical configuration. The latter case (top electrode/Li-based material/electrolyte/bottom electrode) can be facilitated by a combination involving Li x CoO 2 as cathode material, SiO x as solid electrolyte and (doped) Si as bottom electrode [START_REF] Mai | Memristive and neuromorphic behavior in a Li × CoO 2 nanobattery[END_REF][START_REF] Hu | Modulation of resistive switching in Pt/LiCoO 2/SiO 2/Si stacks[END_REF] . As Li + ions can migrate reversibly from Li x CoO 2 towards Si 25 , the Li x CoO 2 conductance (which depends on the lithium content [START_REF] Ménétrier | The insulator-metal transition upon lithium deintercalation from LiCoO2: electronic properties and 7Li NMR study[END_REF][START_REF] Milewska | The nature of the nonmetal-metal transition in LixCoO2 oxide[END_REF] ) can be tuned between several conductance states over a range of 3-4 orders of magnitude.

However, using Si as the bottom electrode and anode, although facilitating CMOS integration, causes problems such as switching instabilities, and potentially leads to limited endurance due to a repeated high-volume expansion/contraction of Si during Li + ion migration. Si anodes can exhibit up to 280% volume expansion at full lithiation [START_REF] Obrovac | Structural changes in silicon anodes during lithium insertion/extraction[END_REF] which results in the degradation of the Si electrode. Optimization of the LiCoO 2 -based two-terminal architecture, through material selection and nano-engineering of the electrolyte and anode layer, has been shown to improve the switching characteristics of these devices [START_REF] Choi | Structural Engineering of Li-Based Electronic Synapse for High Reliability[END_REF] .

Taking the aforementioned limitations into consideration, a new two-terminal architecture is herein explored, based on the interjection of a thin TiO 2 interface layer (~ 30 nm thick) between the SiO x electrolyte [START_REF] Ariel | Electrochemically controlled transport of lithium through ultrathin SiO 2[END_REF] and the Si bottom electrode. TiO 2 has been chosen as a result of a review of recent advances in lithium-based batteries [START_REF] Madian | Current advances in TiO2-based nanostructure electrodes for high performance lithium ion batteries[END_REF] , showing that Anatase and Rutile polymorphs are extremely stable under long cycling -even at high rates -with little volume expansion (~4%) upon lithium insertion/extraction [START_REF] Wagemaker | Multiple Li positions inside oxygen octahedra in lithiated TiO2 anatase[END_REF] .

Thus, this study reports the fabrication and characterization of Li-based two-terminal resistive switching devices in the form of a Au/Li x CoO 2 /SiO 2 /TiO 2 /p ++ -Si stack, presenting for the first time evidence of the simultaneous manifestation of a number of memristive phenomena in the same device, like the rectification and nanobattery effects, as well as analog conductance modulation along with a range of synaptic functionalities such as spike-timing-and frequency-dependent-plasticity, which are emulated in a biorealistic manner through the device's inherent diffusive dynamics.

Results and Discussion

Device fabrication and structural characterization. Au/Li x CoO 2 /SiO x /TiO 2 /p ++ -Si devices were fabricated using a bottom-up approach, as shown in Fig. 1 (right). Pulsed laser deposition (PLD) was used for the deposition of Li x CoO 2 on sputter-coated SiO x /Ti/p ++ -Si substrates. The deposition of nearly stoichiometric Li x CoO 2 at conditions of elevated temperature (600 °C) and O 2 -rich atmosphere transforms the Ti layer to crystalline TiO 2

33

, which effectively serves as the device anode. Figure 1 (left) shows a typical grazing incidence X-ray diffraction pattern of the Li x CoO 2 /SiO x /TiO 2 /p ++ -Si stack, indicating the presence of the mixed Anatase-Rutile TiO 2 phases, as well as the nearly stoichiometric Li x CoO 2 layer.

DC electrical characterization.

For the electrical characterization of the devices, gold top electrodes were used to cap the abovementioned multilayer stacks. In order to examine the cyclic behavior of the devices, voltage sweeps, commencing with negative bias, were applied to the devices, as shown in Fig. 2. Negative biasing, which corresponds to the application of negative voltage on the bottom Si electrode, forces Li + ions to deintercalate from the Li x CoO 2 cathode and migrate towards the TiO 2 anode layer, through the SiO x electrolyte. Li + ion deintercalation prompts the oxidation of Co 3+ to Co 4+ , contributing an electron to the external circuit, thus generating a hole in the Li x CoO 2 valence band, which results in an insulator-metal transition (IMT) [START_REF] Milewska | The nature of the nonmetal-metal transition in LixCoO2 oxide[END_REF] . Subsequently, Li + ion insertion in the TiO 2 anode, utilizing an electron provided by the external circuit, leads to the reduction of Ti 4+ to Ti 3+ and the formation of lithium titanate (Li x TiO 2 ) complexes [START_REF] Van De Krol | Spatial extent of lithium intercalation in anatase TiO 2 . The[END_REF] . Conversely, a subsequent positive bias will force the inserted Li + ions to leave the TiO 2 anode and re-intercalate back to the Li 1-x CoO 2 cathode. Thus, voltage-controlled Li + ion migration leads to the reversible resistance modulation of the devices, through the IMT of the LiCoO 2 cathode.

As can be observed in Fig. 2, initiating the DC cycling (sweep rate: 0.1 V/s) by applying a negative voltage with respect to the bottom Si electrode causes the current to increase significantly above approximately -3 V, signaling the initiation of Li + ion diffusion towards the negatively charged bottom electrode. Current increase occurs smoothly and in a non-abrupt manner compared to previous approaches (e.g. Au/Li x CoO 2 /SiO 2 /Si stack), which presented an abrupt switch at approximately -4 V [START_REF] Moradpour | Resistive switching phenomena in LixCoO2 thin films[END_REF] . Current continues to increase as the applied voltage proceeds up to -5 V, while current hysteresis is observed upon reversal of the negative bias, towards -1.5 V. This is indicative of an enhancement of the device conductance due to the migration of Li + ions from the Li x CoO 2 cathode towards the TiO 2 anode and the onset of IMT in the Li x CoO 2 .

Notably, with the bias still negative and progressing from -1.2 V towards 0 V, a small positive current (max: ~1.5 × 10 -7 A) arises, resulting in a non-zero crossing in the I-V characteristic (Fig. 2, inset (a)). Non-zero crossing behavior, attributed to a nanobattery effect, emanates from the generation of an electromotive force (EMF) inside the switching device, due to charge redistribution and concentration gradients of mobile ions [START_REF] Valov | Nanobatteries in redox-based resistive switches require extension of memristor theory[END_REF] . Positive current measured during negative bias is indicative of backpropagation of Li + ions, from the TiO 2 anode and the SiO x electrolyte towards the cathode because of the aforementioned EMF [START_REF] Sharbati | Electrochemically Tunable Graphene Synapses for Neuromorphic Computing[END_REF] . The non-zero crossing I-V characteristic, because of the Li + ion backpropagation (self-injection) [START_REF] Choi | Structural Engineering of Li-Based Electronic Synapse for High Reliability[END_REF] , denotes the relaxation of the conductance enhancement, highlighting the diffusive character of the devices.

Positive bias DC cycling (0 V  5 V), relative to the bottom Si electrode, leads to a rectification effect resembling a diode-like behavior (Fig. 2, 1 st quadrant), resulting in highly asymmetric I-V characteristics. Figure 2, inset (b), indicates that the current initially decays from ~1 × 10 -7 A (at 0 V) before reaching a value of ~4 × 10 -8 A (above 2 V). Subsequent bias decrease from 5 V to 0 V, leads to a further decrease of the measured current (~1.5 × 10 -8 A). These observations suggest that stimulated conductance depression, beyond the spontaneous relaxation, can be also achieved with the application of positive bias.

Consecutive DC sweeps (65 cycles), as depicted in Fig. 2, present similar characteristics. A maximum current (~-6.5 × 10 -4 A) is measured in the first sweep (at -5 V) and subsequently fades to a constant current value (~-4.5 × 10 -4 A) after the first 20 consecutive sweeps. The reduction in the maximum current can be attributed to partial Li + ion trapping inside the SiO x electrolyte during the voltage-driven Li + migration 37 . After the partial Li + ion loss, the device I-V characteristics gradually stabilize and continue to exhibit its distinctive traits, i.e. hysteresis, non-zero crossing nanobattery effect, rectification at positive bias and reset to a lower conductance.

It is worth noting that the devices present relatively high currents, mainly due to the large area of the top electrode (300 × 300 μm 2 ). Indeed, a dependence of the maximum current on the top Au electrode area was observed (Supplementary Information, Fig. S1), which in combination with the non-abrupt switching, indicates a homogeneous rather than a filamentary switching mechanism. Such a dependence on the top electrode area can be promising for the reduction of current levels, as well as more power efficient switching with device scaling [START_REF] Nguyen | Direct Evidence of Lithium Ion Migration in Resistive Switching of Lithium Cobalt Oxide Nanobatteries[END_REF] . Leakage currents during potentiation/depression and readout can result in cross-talk effects between the switching elements in a crossbar array, requiring the integration of selector diode devices between write/read lines. Inherent rectification of the investigated two-terminal device can effectively suppress leakage currents potentially rendering selector devices obsolete [START_REF] Wu | Flexible three-dimensional artificial synapse networks with correlated learning and trainable memory capability[END_REF] .

Emerging evidence suggest that coexisting chemical and heterotypic electrical synapses, between premotor "command" interneurons and downstream motor neurons, interact to cooperatively modulate the synaptic strength. Furthermore, unidirectional signal transmission from heterotypic electrical synapses, achieved through current rectification, has been shown to amplify the chemical transmission (strictly unidirectional) at functionally mixed electrical-chemical synapses [START_REF] Liu | Antidromic-rectifying gap junctions amplify chemical transmission at functionally mixed electrical-chemical synapses[END_REF] . The inherent unidirectional signal propagation of the rectifying Au/ Li x CoO 2 /SiO x /TiO 2 /p ++ -Si devices presented, in combination with the transient conductance enhancement, could potentially enable the emulation of chemical and rectifying electrical (heterotypic) synaptic behavior.

Analog conductance modulation.

Progressive enhancement and depression of the device conductance was observed, as shown in Fig. 3(a,b). Sequential negative cycling, progressively potentiated the devices to continuously higher conductance states.

This was observed when the Au/Li x CoO 2 /SiO x /TiO 2 /p ++ -Si devices were cycled with solely negative biases between 0 V and -4.2 V. When six successive negative sweeps [(0 V  -4.2 V) x6] were applied on the device, I-V curves with increasing hysteresis and maximum currents were induced, as shown in Fig. 3(a). This behavior, indicative of a cumulative analog conductance enhancement, was reversed when a positive sweep was interjected between each of the subsequent six negative sweeps [(4 V  -4.2 V) x6], as shown in Fig. 3(b).

Although consecutive negative sweeps clearly result in a progressively enhanced conductance, partial overlapping of I-V cycles indicates that the device has a diffusive, i.e. conductance relaxation, attribute (Fig. 3(a)). This is also suggested by the non-zero crossing I-V characteristics discussed previously. Additionally, the depression sweeps are not directly complementary to the corresponding potentiation sweeps (Fig. 3(b)), mainly due to the characteristic asymmetry and the diffusive attribute of the device. This behavior, of continuous potentiating or depressing during consecutive voltage stimulation, at negative or positive bias, respectively, is attributed to the motion of Li + ions in the presence of an electric field. At negative bias greater than ~-2 V, Li + ions migrate towards the bottom Si electrode and are inserted in the TiO 2 anode, resulting in a mixed ionic-electronic current, while the observed hysteresis in the I-V curves is related to the conductance increase because of the induced IMT in the (now Li-deficient) Li x CoO 2 layer. Measured current at negative bias has an ionic contribution coming from the moving Li + ions in the device and an electronic contribution coming from the concurrent redox reactions (in the anode and cathode, respectively) and the electron transport through the device due to the induced IMT. Consecutive negative cycling (Fig. 3(a)) leads to an increasingly higher degree of Li + depletion, hence the progressive enhancement of the device conductance with recurrent negative cycles. On the other hand, the interjection of positive cycles (Fig. 3(b)) leads to the stimulated extraction of Li + ions from the TiO 2 anode and their re-intercalation in the previously depleted Li x CoO 2 cathode, reversing the analog conductance enhancement. This reversal is attributed to the stimulated and progressive inversion of the IMT in the Li x CoO 2 , which increases the cathode resistance again.

Pulsed characterization.

Voltage pulse stimulation experiments have been carried out in order to explore the potential integration of the investigated devices in spiking neural networks (SNN). Applying voltage pulse trains (-/+4.2 V, with 60 ms width (w) and 120 ms spacing (s)) to the bottom p ++ -Si electrode enabled the potentiation/depression of the device's conductance. The presence of an EMF between -1.2 V and 1 V after a potentiating cycle, along with the rectification effect observed at positive bias (Fig. 2, inset (b)), has led to the employment of read voltage pulses of -2 V to monitor the current modulation after each potentiating or depressing pulse (Fig. 4). The pulse characterization begins with 400 read pulses (-2 V, 2 ms width, 180 ms spacing) that monitor the initial conductance of the device (green region in Fig. 4a). This is followed by a potentiating sequence of write/ read pulse trains consisting of two parts: (i) 200 potentiating write pulses (-4.2 V, 60 ms width, 120 ms spacing -red regions in Fig. 4a), with each potentiating write pulse followed by a read pulse (-2 V, 2 ms width) and (ii) 400 read pulses (green regions in Fig. 4a). The potentiating sequence is repeated five times and then followed by a similar-in-scheme depressing sequence, where potentiating write pulses are replaced with depressing write pulses (+4.2 V -blue regions in Fig. 4a), which is also repeated five times.

As can be observed in Fig. 4, the device has an initial low average conductance, of ~80nS. The current drastically increases upon the application of the potentiation pulse trains, growing exponentially. Upon the removal of potentiating pulses, the device conductance relaxes, following an exponential decay. Exponential current-decay can be ascribed to the backpropagation of Li + ions from the TiO 2 anode and SiO x electrolyte and their intercalation back to the Li x CoO 2 cathode, thereby gradually reducing the device conductance.

Relaxation of the conductance in a predictable exponential decay, due to the internal Li + -diffusion dynamics of the device, render these devices an inherent timing mechanism. The device's diffusive dynamics are described by the conductance relaxation time constant (τ), which lies in the range of several seconds (Supplementary Information, Fig. S3), providing a large enough window for calculations with millisecond pulses. It is noted that with proper material selection and engineering of the solid electrolyte properties, it is possible to drastically modify the relaxation dynamics [START_REF] Choi | Structural Engineering of Li-Based Electronic Synapse for High Reliability[END_REF] , thus enabling the optimization of the programming characteristics depending on the memory or synaptic behavior needed to be emulated.

Similar to the first potentiation train, application and removal of a second potentiation train stimulates the enhancement of the device conductance, resulting in an even higher maximum current. Moreover, every subsequent incoming potentiation pulse train, temporally placed in the relaxation tail of the previous potentiation, causes a cumulative enhancement on the device conductance, resulting in a saw-like conductance oscillation. Additionally, the application of the first depression pulse train, after the last potentiation train and spontaneous conductance relaxation, results in the stimulated relaxation of the device conductance. The read train following the first depression train reveals a new state of enhanced conductance (~90nS), compared to the initial state of the device. The subsequent depression trains further reduce the conductance in a descending stepladder manner, returning the device closer to its initial conductance state. This behavior indicates that the device can be potentiated/depressed at various different conductance states, through the manipulation of the Li + ion concentration in the cathode and anode of the device. It is worth noting that these devices exhibited enhanced durability (up to 150,000 read/write operations without failure) compared to previous device configurations, where the Si substrate was used as the anode layer.

Activity-dependent plasticity. Considering the top and bottom electrodes (Au and Si, respectively) as pre-and post-synaptic neurons and the actively switching volume between them as the synapse, the conductance modulation of the two-terminal device under investigation can be considered as a stimulated synaptic weight variation. In this respect, the relaxation current after each potentiation, observed only for post-synaptic potentials (negative bias applied on the bottom Si electrode), that exponentially decays with a time constant of several seconds and actively contributes to the cumulative enhancement of the device conductance, can be contemplated as an excitatory post-synaptic current (EPSC).

The diffusive dynamics observed in the device, and expressed as an EPSC, can be ascribed to the nanobattery effect and specifically to the diffusion of Li + ions from the TiO 2 anode and SiO x electrolyte back to the parent Li x CoO 2 cathode, due to the generation of an EMF. This effect is analogous to the spontaneous weight relaxation, caused by the gradually decaying Ca 2+ residual concentration in the pre-synaptic region, of biological neuron cells. Summation of EPSCs, through the cumulative increase of the Li + ion concentration in the post-synaptic region (further to the residual Li + ions previously migrated), biorealistically mimics the origin of the paired pulse facilitation (PPF) mechanism observed in biological neurons. PPF of action potentials, because of the summation of EPSCs, is greatly involved in the activity-dependent plasticity of biological neurons [START_REF] Katz | The role of calcium in neuromuscular facilitation[END_REF][START_REF] Cuttle | Facilitation of the presynaptic calcium current at an auditory synapse in rat brainstem[END_REF] .

Cumulative increase of the measured current during consecutive potentiating pulse trains can be described by a PPF behavior, where a successive pulse stimulates the device to an even higher conductance value, through the summation of EPSCs. PPF in the investigated two-terminal devices has a temporal dependence and its effect can be enhanced or suppressed when the pulse width of the consecutive write pulses/trains is reduced or increased respectively (Supplementary Information, Fig. S2).

The combined effect of the PPF mechanism and the spontaneous EPSC decay, can be illustrated when the stimulation frequency is modulated. When 100 write (-4.2 V, 60 ms) pulses were applied on the device, the conductance variation (measured immediately after with read pulses of -2 V amplitude and 2 ms width) is enhanced with increasing stimulation frequency. The stimulation frequency was varied through the increase of spacing between write pulses (s = 20 ms, 50 ms, 80 ms, 120 ms). As shown in Fig. 5, increasing the stimulation frequency, results in a more pronounced impact on the device conductance. The stimulation frequencies appearing in the inset of Fig. 5 correspond to the real time rate of the applied write pulses.

The frequency dependence of the conductance enhancement is a manifestation of the equilibrium achieved between the Li + ion accumulation in the TiO 2 anode due to the potentiating pulses and the relaxation of the conductance due to the diffusion of Li + ions from the TiO 2 anode and SiO x electrolyte to the Li x CoO 2 cathode. Such an internal balancing between stimulated accumulation and spontaneous decay of Li + ion concentration, provides a physical timing mechanism, rendering to these devices an inherent activity-dependent plasticity attribute.

Frequency-dependent synaptic plasticity, controlled by the Li + ion concentration in the pre-synaptic region is analogous to the frequency-dependent plasticity of biological neurons, governed by the regulation of pre-synaptic Ca 2+ concentration. Additionally, the ability of low frequency synaptic weight variation closely resembles the ability of chemical synapses to follow low frequency stimulations [START_REF] Jackson | Action potential broadening and frequency-dependent facilitation of calcium signals in pituitary nerve terminals[END_REF] .

Figure 5.

Frequency dependent plasticity effect. Dependence of the conductance variation over time on the stimulation frequency of the Au/Li x CoO 2 /SiO x /TiO 2 /p + +Si device (top Au electrode: 300 × 300 μm 2 ). It is evident that the conductance variation increases with increasing stimulation frequency. Additionally, the conductance variation of a 10 MΩ resistor, stimulated with the same protocol, is depicted in order to elucidate that the observed conductance oscillations throughout the voltage pulse characterization are externally induced.

In order to elucidate the origin of the conductance oscillation, evident in the pulse characterization measurements in this study, we applied the stimulation protocol described above on a resistor. A 10MΩ resisitor was selected because it roughly matches the series resisistance of the Au/Li x CoO 2 /SiO x /TiO 2 /p + +Si device. As can be seen in Fig. 5, application of the stimulation pulse train with alternating read (-2V) and write (-4V) pulses, results in a sinusoidal oscillation. As such, we can deduce that the oscillations of the measured current on the passive resistor element and the resistive switching devices during voltage pulse characterization are externally induced. The oscilations could possibly originate from the measurement configuration.

Another manifestation of the impact of temporal association of pulses on the conductance modulation can be observed when a pair of potentiating and depressing pulses (spikes) are applied on the device (Fig. 6). Considering the investigated two-terminal device as a synapse, a pulse of -4.2 V applied to the bottom electrode was considered as a postsynaptic stimulation. On the other hand, a pulse of +4.2 V applied on the bottom electrode was considered as a presynaptic stimulation. While modifying the temporal spacing between postsynaptic and presynaptic spikes, the conductance response was monitored immediately after with read pulses (-2 V, 2 ms). A positive temporal difference (Δt > 0) is defined as the case where postsynaptic stimulation precedes presynaptic stimulation, while a negative temporal difference (Δt < 0) is defined as the reverse case (Fig. 6, insets).

By varying Δt, the spike-timing-dependent effect on the device conductance was monitored. A spike-timing-dependent-plasticity (STDP) behavior is observed between the device's conductance variation (ΔG/ G 0 , where ΔG = G f -G 0 ) and the temporal association (Δt) between presynaptic and postsynaptic spikes (data corresponding to the red curve, Fig. 6). It is shown that with reducing Δt in either direction an emphasized effect on the potentiation or depression of the relative conductance results.

For negative temporal difference in the range of -60 ms to 0 ms, the cumulative effect of the pair of presynaptic/postsynaptic spikes lead to a conductance depression and the devices presented a negative conductance variation. While such a conductance reduction, below the stack's initial conductance, would not be expected, it can possibly be explained by the Li + ion loss effect discussed previously. As can be seen in Fig. 2, the first I-V cycles show a gradual decrease and an eventual stabilization of the maximum current, which was ascribed to Li + ion accumulation inside the SiO x electrolyte until a certain equilibrium concentration is reached. Therefore, the devices were stabilized prior to any other electrical characterization, using consecutive voltage sweeps. For temporal differences of -60 ms to 0 ms the leading positive voltage pulse possibly forces a fraction of the Li + ions trapped inside the SiO x electrolyte, to re-intercalate in the Li x CoO 2 cathode reducing marginally the device's conductance below its stabilized initial conductance.

In the range of -80 ms to -400 ms the device presented a positive conductance variation of roughly 0.5 (irrespective to the temporal difference of spikes), meaning that the synapse is relatively potentiated to G f = 1.5G 0 , (Fig. 6, red curve). As can be seen, by the data corresponding to the black curve, for Δt > 0 the effect of a single postsynaptic spike on the conductance variation is very similar to that of a pair of presynaptic and postsynaptic spikes in the temporal range -80 ms < Δt < -400 ms. Thus, it is believed that for larger absolute temporal differences (for Δt < 0) the effect of the preceding presynaptic spike decays, and the device returns to its equilibrium state due to its internal diffusive dynamics, leaving only the effect of the succeeding postsynaptic spike to be reflected on the conductance variation. Insets illustrate the presynaptic and postsynaptic spike sequence. Dependence of the conductance variation when a single postsynaptic spike (Δt > 0) or a single presynaptic spike (Δt < 0) is applied on the device is also depicted (data corresponding to the black curve).

Furthermore, for positive temporal differences the device presented a more pronounced enhancement of the conductance, compared to negative Δt, and this effect is enhanced with decreasing Δt, (Fig. 6, red curve Δt > 0). Increasing the temporal spacing between the preceding postsynaptic and succeeding presynaptic spikes, has a diminishing effect on the overall conductance enhancement due to the inherent diffusive dynamics involved in the relaxation of the conductance enhancement induced by the preceding postsynaptic spike.

More importantly, Fig. 6 indicates that, in addition to the dependence of the conductance state on Δt, there is a clear difference between applying a single spike and applying a pair of opposite spikes. While a single postsynaptic spike (black curve, for Δt > 0) slightly increases the device's conductance, the introduction of a succeeding presynaptic spike leads to a more substantial enhancement through the cumulative effect of the spike pair similar to the synergistic effects of presynaptic and postsynaptic stimulations in biological synapses. Although the origin of this interesting synergistic effect is not clear at the moment, the mechanisms involved in the diffusive resistive switching Au/Li x CoO 2 /SiO x /TiO 2 /p ++ -Si two-terminal devices are actively investigated.

STDP behavior has been directly correlated to long-term potentiation of biological neurons [START_REF] Markram | Regulation of synaptic efficacy by coincidence of postsynaptic APs and EPSPs[END_REF][START_REF] Bi | Synaptic modifications in cultured hippocampal neurons: dependence on spike timing, synaptic strength, and postsynaptic cell type[END_REF] and is actively involved in associative learning processes [START_REF] Hebb | The organization of behavior[END_REF] . Forms of STDP learning algorithms are widely adopted in CMOS-based SNN computing paradigms, enabling the regulation of synaptic weights based on the temporal difference between pre-synaptic and post-synaptic spikes [START_REF] Diehl | Unsupervised learning of digit recognition using spike-timing-dependent plasticity[END_REF][START_REF] Kheradpisheh | STDP-based spiking deep convolutional neural networks for object recognition[END_REF] , Moreover, STDP has been observed in several memristive systems with either conventional or diffusive behavior [START_REF] Srinivasan | Magnetic tunnel junction based long-term short-term stochastic synapse for a spiking neural network with on-chip STDP learning[END_REF][START_REF] Wang | Memristors with diffusive dynamics as synaptic emulators for neuromorphic computing[END_REF] . Implementation of diffusive-memristor-based STDP learning algorithms in SNN models has demonstrated an improved accuracy over conventional-memristor-based STDP. This is attributed to the ability of diffusive memristors to retain frequently transmitted information while forgetting less significant information, through their inherent weight decay dynamics [START_REF] Panda | Learning to forget with adaptive synaptic plasticity in spiking neural networks[END_REF] . Frequency-dependent plasticity and STDP response render the investigated two-terminal diffusive devices useful for direct emulation of activity-dependent bioinspired functionalities like adaptive synaptic plasticity, and the realization of unsupervised learning in SNNs.

Conclusion

In an effort to reduce switching instabilities and limited durability observed in battery-like Au/LiCoO 2 /SiO 2 / p ++ -Si two-terminal devices, a thin TiO 2 layer was introduced between the Si anode and the SiO x electrolyte.

The Au/Li x CoO 2 /SiO 2 /TiO 2 /p ++ -Si two-terminal devices exhibited analog conductance modulation, based on the voltage-driven regulation of Li + ion concentration in the cathode, reminiscent of the synaptic weight modulation according to the regulation of pre-synaptic Ca 2+ concentration in biological neurons. Non-symmetric diode-like I-V characteristics observed in these devices allow the unidirectional signal propagation, similar to chemical or electrical (rectifying) biological synapses.

Furthermore, biorealistic synaptic functionalities, such as paired pulse facilitation -through the summation of excitatory post-synaptic currents -as well as frequency-and spike-timing-dependent plasticity, were facilitated by the inherent diffusive dynamics of the investigated two-terminal devices.

Materials and Methods

Au/Li x CoO 2 /SiO x /TiO 2 /p ++ -Si devices were fabricated using a bottom-up approach, starting with the deposition of a Ti thin film on a p ++ -Si [(111), heavily doped: Boron, Resistivity: 0.001 -0.003 ohm cm wafer] via DC sputtering. Subsequently, approximately 10 nm of SiO x were deposited by reactive RF sputtering from a Si target. The SiO x /Ti/Si coated substrates were transferred to a pulsed laser deposition (PLD) chamber where the substrate temperature was raised to 600 °C and O 2 background gas was backfilled. Ablation of a stoichiometric LiCoO 2 target, using a pulsed UV Kr:F excimer laser (COMPexPro 201, λ = 248 nm and τ = 25 ns) operated at 1 Hz, with fluence of ~1.3 J/cm 2 resulted in the deposition of approximately 45 nm of Li x CoO 2 (x~0.99) (Li x CoO 2 ) on the SiO x /Ti/Si substrates. During the deposition of Li x CoO 2 , the Ti metal thin film transformed to crystalline TiO 2 (~30 nm), which served as the device's new anode. The crystal structure and thickness of the individual films were investigated using Grazing-Incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR) (Rigaku SmartLab) and stylus profilometry (Bruker DektakXT), respectively.

The devices were completed with the deposition of Au electrodes (~100 nm thick) of various areas through a Ni shadow mask via DC sputtering, on top of the Li x CoO 2 /SiO x /TiO 2 /p ++ -Si stacks.

Electrical measurements were conducted at room temperature, in air, using a two-probe setup, sourcing voltage and measuring current. 20 μm-radius Be-Cu tips, controlled using micro-manipulators, were used to contact the top (Au) and bottom (Si) electrodes. A Keithley 6487 Picoammeter/Voltage Source, controlled by homemade LabVIEW routines (Virtual Instruments) via a GPIB bus, was employed to generate the excitation signals (voltage) and measure the device response (current). Current-voltage (I-V) characteristics of all cells were measured using voltage sweeps with maximum current compliance (25 mA) to enable observation of device behavior without artifacts. For pulse measurements, the programming signal (write pulses) consisted of customizable voltage spikes with respect to amplitude, length, and spacing. To avoid modification of the resistive state of the device, the cell response was measured using low-voltage spikes (read pulses) and simultaneous measurement of the current. Special attention was paid to the elimination of all delays in the sourcing and measurement of signals, other than those inherent in the software/hardware combination.

Figure 1 .

 1 Figure 1. Grazing incidence X-ray diffraction pattern of the Li x CoO 2 /SiO x /TiO 2 /p ++ -Si device. The diffraction pattern indicates the presence of the mixed Anatase-Rutile TiO 2 phases along with the nearly stoichiometric layered Li x CoO 2 phase (a). Schematic illustration of the Li x CoO 2 /SiO x /TiO 2 /p ++ -Si multilayer stack (b).

Figure 2 .

 2 Figure 2. Current-Voltage (I-V) characteristic curves of the Au/Li x CoO 2 /SiO x /TiO 2 /p ++ -Si device. I-V hysteresis due to the IMT of Li x CoO 2 is evident during negative biasing of the bottom Si electrode (top Au electrode: 300 × 300 μm 2 ). Non-zero crossing, shown in inset (a), is attributed to the nanobattery effect. Current rectification during positive biasing and reset to a lower conductance can be observed in inset (b). Schematic of the measurement setup is also provided (c).

Figure 3 .

 3 Figure 3. Analog conductance modulation. Consecutive sweeps with solely negative biases (a) results in the progressive enhancement of the Au/Li x CoO 2 /SiO x /TiO 2 /p ++ -Si conductance (top Au electrode: 300×300 μm 2 ). Cumulative analog conductance enhancement was partially reversed (b), when a positive sweep was interjected between the subsequent six negative sweeps (sweep rate: 0.1 V/s). Positive sweeps are omitted for clarity.

Figure 4 .

 4 Figure 4. Conductance enhancement, relaxation and depression during voltage pulse stimulation. Pulsed stimulation, of the Au/Li x CoO 2 /SiO x /TiO 2 /p ++ -Si device with potentiating and depressing voltage pulse trains (write: -/+4.2 V, pulse width: 60 ms) while monitoring the conductance modulation (read: -2 V: pulse width: 2 ms), (top Au electrode: 300 × 300 μm 2 ). Stimulated enhancement and subsequent conductance relaxation along with stimulated conductance depression can be observed (a). Schematics of the measurement setup (b) and the read/write pulse sequence applied on the device (c) are also illustrated.

Figure 6 .

 6 Figure 6. Spike-timing-dependent-plasticity (STDP) effect. When a pair of spikes (equal magnitude and opposite polarity, ±4.2 V, 300 ms) are sequentially applied on the two-terminal Au/Li x CoO 2 /SiO x /TiO 2 /p ++ -Si device (top Au electrode: 300 × 300 μm 2 ) a STDP behavior is recorded (data corresponding to the red curve).Insets illustrate the presynaptic and postsynaptic spike sequence. Dependence of the conductance variation when a single postsynaptic spike (Δt > 0) or a single presynaptic spike (Δt < 0) is applied on the device is also depicted (data corresponding to the black curve).

Scientific RepoRtS |(2020) 10:8711 | https://doi.org/10.1038/s41598-020-65237-0

Data availability

The datasets generated during and/or analysed during the current study are available in figshare repository, [http://doi.org/10.6084/m9.figshare.12251300].

Author contributions

P.S.I. fabricated the devices, performed electrical measurements and film characterizations; All authors discussed the results; P.S.I. wrote the main manuscript text and prepared the figures with the help of J.G. and E.K.; All authors reviewed the manuscript.

Competing interests

The authors declare no competing interests.


Britt-Isabelle Berg

Aure ´lien Laville

Delphine S Courvoisier

Philippe Rouch

Thomas Schouman

Experiences with a new biplanar low-dose Xray device for imaging the facial skeleton: A feasibility study

Background

This study aimed to evaluate the feasibility of a new biplanar low-dose X-ray device for facial skeletal imaging.

Methods

We evaluated 48 biplanar radiographs from 12 patients (posteroanterior/lateral), originally taken for a scoliosis examination with a biplanar low-dose X-ray device. For this study, the images were further evaluated for the perceptibility of 38 facial skeleton landmarks. To determine the reliability and reproducibility of perceptibility, two independent observers determined the landmarks twice, during a time interval of at least two weeks.

Results

Both interoperator and intraoperator reliability were excellent for all landmarks [intraclass correlation coefficient (ICC) > 0.92].

Conclusions

The biplanar low-dose X-ray device demonstrated good feasibility for precisely assessing the anatomical landmarks of the facial skeleton. Given its excellent precision, the biplanar low-dose X-ray device data sets should be forwarded from the treating orthopedic surgeon or neurosurgeon to the orthodontist or dentist for further assessment in their field.

Introduction

In 1992, the Nobel Prize in Physics was awarded to Georges Charpak "for his invention and development of particle detectors, in particular the multiwire proportional chamber" [START_REF] No | The Nobel Prize in Physics[END_REF]. This technology was used to develop a new biplanar X-ray imaging system. Ille ´s & Somoskeo ¨y published the detailed technology behind the system, stating that the method is highly sensitive, and could potentially detect even one photon. In addition, it is less affected by scattered radiation [START_REF] Ille ´s | The EOS™ imaging system and its uses in daily orthopaedic practice[END_REF]. Until now, the biplanar low-dose X-ray imaging system was used primarily in orthopedics to image the spine, hip, shoulder, joints, and extremities [START_REF] Gheno | Three-dimensional measurements of the lower extremity in children and adolescents using a low-dose biplanar X-ray device[END_REF][START_REF] Gaume ´tou | EOS analysis of lower extremity segmental torsion in children and young adults[END_REF]; furthermore, its role in the detection of bone lesions has been assessed [START_REF] Boutry | Low-dose biplanar skeletal survey versus digital skeletal survey in multiple myeloma[END_REF]. Since the biplanar X-ray imaging system produces head-to-foot images, while the patient maintains a standing position, it frequently captures the facial skeleton [START_REF] Dietrich | Comparison of radiation dose, workflow, patient comfort and financial break-even of standard digital radiography and a novel biplanar low-dose X-ray system for upright full-length lower limb and whole spine radiography[END_REF][START_REF] Jolivet | Fast 3D reconstruction of the rib cage from biplanar radiographs[END_REF][START_REF] Ille ´s | Breakthrough in three-dimensional scoliosis diagnosis: significance of horizontal plane view and vertebra vectors[END_REF], even when it is of no relevance for an initial patient-related assessment. Moreover, use of these images to assess dentofacial structures could enhance safety by avoiding additional exposure to radiation and reduce healthcare costs. The imaging system simultaneously acquires posteroanterior and lateral images. Both views are of great interest for dentists, orthodontists, maxillofacial surgeons, and pediatric surgeons, due to the use in cephalometric analyses. Cephalometric analyses based on landmarks allow orthodontists and maxillofacial surgeons to assess the severity and cause of dentofacial deformities, growth patterns of the facial skeleton, the ability to set a treatment plan, and eventually assess the efficacy of the treatment and its stability. Facial asymmetry can include a distortion of the entire face, if there appears to be a relationship between facial asymmetry and adolescent idiopathic scoliosis [START_REF] Hong | Correlation between facial asymmetry, shoulder imbalance, and adolescent idiopathic scoliosis[END_REF]. Panoramic radiographs and cephalograms, rather than standard X-rays, are routine diagnostic tools in these cases [START_REF] Mu ¨ssig | Indications for digital volume tomography in orthodontics[END_REF]. The analysis is based on a number of specific landmarks on frontal and lateral cephalograms. Thus, patients (mainly children or young adults) will be subject to repeated radiation exposure of the head over a couple of years for orthodontic or orthognathic treatment.

Keeping radiation exposure as low as possible is necessary, as radiation accumulates over an individual's lifetime. An association between dental X-rays and the risk of intracranial meningioma has been reported by Claus et al. [START_REF] Claus | Dental x-rays and risk of meningioma[END_REF]: they found that a history of exposure from dental X-rays during an earlier period (when radiation exposure was greater than now) appears to be associated with an increased risk of intracranial meningioma. A plain dental X-ray on average has a similar radiation dosage [START_REF]European guidelines on radiation protection in dental radiology[END_REF] as X-rays captured for cephalometric measurements. Therefore, it is sensible to use X-rays that are already available, even if they are not captured for the purpose of cephalometric analyses. Until now, no other research group assessed the feasibility of cephalometric analysis based on this biplanar imaging or any other biplanar low-dose X-ray system that captured both views of the face at the same time for precise depiction of the facial skeleton. One publication assessed cephalograms in posteroanterior and lateral views with a dry skull, but the X-rays were not captured at the same time [START_REF] Wu | Three-dimensional analysis of biplanar cephalograms[END_REF]. Therefore, we investigated whether this low-dose biplanar X-ray imaging system would be a useful tool for facial skeletal assessments. Our hypothesis was that biplanar X-ray imaging quality was acceptable for sufficient facial landmark placement, as used in orthodontic planning and orthognathic surgery. Our aim was determine if the biplanar low-dose X-ray imaging of the facial skeleton was sufficient for landmark assessment, even if artifacts occur (as in the images we assessed for patients who had their hands in front of the face). We decided to review the images of the same patients, once with hands in front of the face and once without hands in front of the face (hands on the shoulders).

Competing interests: I have read the journal's policy and the authors of this manuscript have the following competing interests: Prof. P. Rouch and Dr. A. Laville have to declare a conflict of interest with the EOS-Imaging company, as the laboratory has received EOS-Imaging funding associated to anterior studies. All other authors have no conflicts of interest to daclare. Herewith, we confirm that this does not alter our adherence to PLOS ONE policies on sharing data and materials.

Materials and methods

Volunteers

Ethical approval was received from the local ethical committee "Comite ´s de protection des personnes en Ile-de-France" (CPP IDF VI, No 90-06). Patients had provided written informed consent for the original study. We used a total of 48 biplanar radiographs from 12 patients, which were acquired with a biplanar low-dose X-ray imaging system, initially for spinal assessments with various upper extremity positions (patients provided informed consent for the original study). The images included 24 data sets (12 posteroanterior and 12 lateral) from patients in a standing position with hands in front of their face (Group A), and 24 data sets (12 posteroanterior and 12 lateral data sets) from the same patients without hands in front of their face (Group B). Depicted in Figs 1 and2 is the exact position of the patient in the biplanar lowdose X-ray device.

The biplanar low-dose X-ray imaging system

The device is the EOS1 system (EOS1, EOS Imaging SA, Paris, France). All patients were standing. The biplanar X-ray imaging system used two perpendicular X-ray beams that were collimated in two extremely thin, horizontal, fan-shaped beams with two detectors. Since this was a retrospective study, we used X-rays of a phantom head during data acquisition to illustrate the installation (see Fig 3).

Effective resolution was reduced to 193 μm × 185 μm for the frontal view and 179 μm × 185 μm for the lateral view [START_REF] Deschênes | Diagnostic imaging of spinal deformities: reducing patients radiation dose with a new slot-scanning X-ray imager[END_REF]. The reference settings were chosen as follows: Frontal: tube voltage 90 kV, current-time product 50 mAs, DAP 716 mGy cm 2 ; Lateral: tube voltage 105 kV, current-time product 320 mAs, DAP 1082 mGy cm 2 . The morphology was adult, the scanned length was 76 cm, scanning speed was 5 s, and acquisition time was 12.5 s.

Measurements

To evaluate the feasibility of the biplanar X-ray imaging system for pictorial depiction of the facial skeleton, two craniomaxillofacial surgeons (experienced in orthognathic surgery and dentomaxillofacial radiology) placed 38 landmarks on each image. The landmarks for assessment of the frontal and lateral images were placed according to the description provided in "Essentials of Orthognathic Surgery" [START_REF] Reyneke | Essentials of Orthognathic Surgery[END_REF]. For both views, landmark C2 was added to represent the odontoid process. Table 1 gives detailed descriptions of the 38 anatomical regions of the landmarks as seen in Fig 4.

Two independent raters were instructed to place the landmarks twice, i.e., on two separate occasions, at an interval of two weeks to assess consistency of the landmark placement at two time points, given consistency of the estimates of placement (interoperator reproducibility). The landmarks were digitally placed (computer mouse on a high-resolution monitor) with image-treatment software (Idefix, an imaging processing software based on C++, developed at the Laboratoire de Biome ´canique/Institut de Biome ´canique Humaine Georges Charpak, Arts et Me ´tiers ParisTech, Paris, France) to provide coordinates of each landmark in a two-dimensional reference system. The stereo-corresponding landmarks (visible in both views: N, ANS, Orl, Orr, Ml, Mr, Gol, Gor, Me, see Table 1) were selected in one view, as the level (y-axis) of the same landmark was indicated in the other view by an epipolar line. Values in the x-and yaxis were transferred to an Excel spreadsheet for further calculation.

Analysis

Interoperator and intraoperator reliability were evaluated for each landmark as recommended by the International Organization for Standardization [START_REF]1994 Accuracy (trueness and precision) of measurement methods and results-Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement method[END_REF]. One-way random effects, absolute agreement, and single rater (or single measurement) intraclass correlation coefficients [Ref. to (ICC (1,1), according to the Shrout and Fleiss convention [START_REF] Koo | A Guideline of Selecting and Reporting Intraclass Correlation Coefficients for Reliability Research[END_REF]], were calculated for each landmark and presented according to group membership (Group A vs. Group B). The ICC expressed the proportion of true variance and the proportion of total variability due to differences among the participants. ICC values > 0.91, 0.71-0.91, 0.51-0.70, or < 0.51 were considered to represent excellent, good, moderate, or poor agreement, respectively [START_REF] Ilharreborde | Angle measurement reproducibility using EOS three-dimensional reconstructions in adolescent idiopathic scoliosis treated by posterior instrumentation[END_REF]. The R v.3.0.1 software package (R Development Core Team, R Foundation for Statistical Computing, Vienna, Austria) was used to perform all statistical analyses. We used the moment distance to report the difference between the two observers in millimeters. Further information on this method can be found in the publication by S.D. Springate [START_REF] Springate | The effect of sample size and bias on the reliability of estimates of error: a comparative study of Dahlberg's formula[END_REF].

Results

The average age of the 12 volunteers was 46 (range, 17-71 years). Both interoperator and intraoperator reliability were excellent for all landmarks (ICC > 0.92). The ICCs of Group A were not significantly different from those of Group B (interoperator: t = 1.39; p = 0.17; d = 0.32; intraoperator rater 1: t = 1.13; p = 0.26; d = 0.26; intraoperator rater 2: t = 0.69; p = 0.49; d = 0.17). Detailed ICC values are in S1 and S2 Tables.

The greatest difference in millimeters between the two observers, for the x and y values (in Groups A and B) was found for the Orbital (Or) landmark in the lateral view; this is likely due to the fact that one value was aberrant. Most likely, the rater did not push the button on the computer mouse strongly enough, although the overall values were still excellent. The detailed millimeter differences for each landmark are shown in S3 Table.

Table 1. Landmarks according to descriptions provided in "Essentials of Orthognathic Surgery" [START_REF]1994 Accuracy (trueness and precision) of measurement methods and results-Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement method[END_REF]. For both views, landmark "C2" was added to represent the odontoid process.

Posterior view (SF)

Where the smaller wing of the sphenoid bone crosses the medial orbital ridge

Discussion

Our study was the first to investigate the feasibility of a biplanar low-dose X-ray imaging system for the facial skeleton. Image quality and excellent reproducibility/repeatability of the facial landmarks are promising. No significant differences are found between the two groups in terms of ICC values; therefore, the landmarks were precisely detectable even if patients had their hands in front of the face. This indicates the potential use of this system, not only in the field of orthopedics, but for imaging of craniofacial bony structures.

The patients in our retrospective study had been diagnosed with scoliosis. Scoliosis requires continuous follow-up with imaging. In cases of congenital scoliosis, for which surgery is indicated, annual follow-up imaging is also necessary until age 20 [START_REF] Faria | The EOS 2D/3D X-ray imaging system: a cost-effectiveness analysis quantifying the health benefits from reduced radiation exposure[END_REF]. Additional radiation doses can be prevented with extended spinal biplanar X-ray imaging. Previously, these images were not used to assess facial asymmetries. Our assessment showed that these images were of adequate quality for evaluation of the facial skeleton, despite evidence of a correlation between facial asymmetry and scoliosis. A previous publication reported that children with scoliosis were more likely to have malocclusions [START_REF] Hong | Correlation between facial asymmetry, shoulder imbalance, and adolescent idiopathic scoliosis[END_REF]. Another study compared the occlusal parameters of 103 children with idiopathic scoliosis in a random group and found more asymmetric features of malocclusion relative to the control group [START_REF] Damet | Occupational and patient exposure as well as image quality for full spine examinations with the EOS imaging system[END_REF]. Scoliosis patients are not the only group in which assessment of the captured faces might have demonstrated this. Proffit et al. reported that 57% to 59% of the US population is in need of some degree of orthodontic treatment [START_REF] Proffit | Prevalence of malocclusion and orthodontic treatment need in the United States: estimates from the NHANES III survey[END_REF]; this indicates that imaging for cephalometric assessment is necessary, which would expose a substantial part of the population to radiation. Given our results, biplanar X-ray imaging would be acceptable for an initial radiological assessment and follow-up when orthodontic/ maxillofacial treatment becomes necessary.

A comparison of our results with other studies using the same landmarks, but other X-ray devices, reveals that our interoperater and intraoperater reproducibility was comparable or better, even with an aberrant value; this could have occurred when the person performing the digital marking did not push the button on the computer mouse strongly enough. The rater might have thought that the mouse was correctly positioned and clicked in a certain way, but in fact, the "click" was delayed. The authors of a study on the reliability of locating landmarks of the porion (Po) and condyle (Co) compared two groups of patients, one with open mouth and one with teeth in centric occlusion. This showed that the "average reproducibility of all measurements for observers I and II was 93% and 81%" [START_REF] Adenwalla | Porion and condyle as cephalometric landmarks-an error study[END_REF]. Superimposing both data sets on special foils, the set with the mouth open and the set in the centric position, were assessed in millimeters [START_REF] Adenwalla | Porion and condyle as cephalometric landmarks-an error study[END_REF]. Manual setting of landmarks is still common, although landmarks are mostly digitally set. In a study conducted by Durão et al. [START_REF] Durão | Accuracy and reliability of 2D cephalometric analysis in orthodontics[END_REF], the accuracy of two-dimensional cephalometric analysis was evaluated. They used the same landmarks as in our study (though not all of them: N-Nasion; Me-Menton; ANS-Anterior Nasal Spine; Co-Condylion; Gn-Gnathion; A-Point A; B-Point B; Pog-Pogonion; Po-Porion; Or-Orbitale; Go-Gonion). They compared it to measurements of skulls, the gold standard. They found intraoperator consistency ICC above 0.9, except for one landmark in which the ICC was 0.79 [START_REF] Durão | Accuracy and reliability of 2D cephalometric analysis in orthodontics[END_REF]. Interoperator reliability with ICCs between 0.67-0.99 depend on the landmark. In their study, two observers performed the measurements. The second measurement was done after a one-month interval. Differences of < 1 mm were accepted as within one standard deviation (SD) and were considered clinically acceptable in this study [START_REF] Durão | Accuracy and reliability of 2D cephalometric analysis in orthodontics[END_REF]. Landmarks were digitally placed on the monitor [START_REF] Durão | Accuracy and reliability of 2D cephalometric analysis in orthodontics[END_REF]. Paixão et al. [START_REF] Paixão | Comparative study between manual and digital cephalometric tracing using Dolphin Imaging software with lateral radiographs[END_REF]. found no statistically significant differences (p > 0.05) when comparing cephalometric measurements of digital cephalometric tracing, with manual placement in lateral cephalometric radiographs. Comparing our results with conventional digital lateral cephalogram radiographs, reproducibility measurements in that study were slightly more precise. Comparing our results with those of Durão et al. [START_REF] Durão | Accuracy and reliability of 2D cephalometric analysis in orthodontics[END_REF], in some landmarks, our differences were > 1mm. Using the middle of the sella as an example ("S-point"), measurements published by Greiner et al. [START_REF] Greiner | Variance of landmarks in digital evaluations: comparison between CT-based and conventional digital lateral cephalometric radiographs[END_REF] yielded a mean "x"/"y" value quality range of 0.28 mm/0.21 mm. In our measurements, sella "S-point" values of 0.42 mm/ 0.42 mm were found with respect to inter-reproducibility (in Group B). An accuracy of less than 0.5 mm is precise and will be more than sufficient for the assessment used in clinical routine. The landmarks that exhibit greater differences than 1 mm are, in general, subject to interpretation in placement and are therefore more difficult to position, which might explain greater interoperator differences. The detailed verification of all landmarks of the face guarantees that the biplanar X-ray can be used for most interventions in the dentofacial area. Other studies described difficulties in identifying the "Go" and "Me" landmarks [START_REF] Chien | Comparison of reliability in anatomical landmark identification using two-dimensional digital cephalometrics and three-dimensional cone beam computed tomography in vivo[END_REF][START_REF] Polat-Ozsoy | Differences in cephalometric measurements: a comparison of digital versus hand-tracing methods[END_REF][START_REF] Santoro | Accuracy of digital and analogue cephalometric measurements assessed with the sandwich technique[END_REF], which proved to not be the case in our study with the biplanar X-ray imaging system tool. When comparing reproducibility of landmarks/anatomical points of our results with other studies, using the biplanar low-dose X-ray imaging system, e.g., reproducibility of vertebrae angle measurements, we reported ICC values of 0.97-1.00 [START_REF] Ilharreborde | Angle measurement reproducibility using EOS three-dimensional reconstructions in adolescent idiopathic scoliosis treated by posterior instrumentation[END_REF]. A study of scapular landmarks found maximum interoperator differences of 1.48-13.24 mm and average intraoperator differences of 0.25-2.53 mm [START_REF] Ohl | Shoulder bony landmarks location using the EOS low-dose stereoradiography system: a reproducibility study[END_REF]. Images in our study were not originally captured for detection of facial skeletal structures, but to clarify if hands in front of the face had an impact on spinal assessment. Positioning of the tongue, the Frankfurter Horizontal, a line to orient the head or the chin, were not considered. The patient was not perfectly placed in the device, yet it was possible to achieve very good image quality.

Even with hands in front of the face, these captured images could also be used by maxillofacial surgeons or orthodontists in deciding between conservative treatment and orthognathic surgery. The lateral view could be assessed to obtain the growth pattern, and the posteroanterior images could be used to assess facial asymmetries.

Since this study was retrospective, dosage measurements of the face were not performed. It should be left to future investigation whether (in regard to radiation dosage), the biplanar lowdose X-ray imaging system is superior to conventional cephalometric devices. However, the results of the present study, e.g., the high quality of landmarks assessment, promote the use of images captured with the biplanar low-dose X-ray imaging system for the same measurements, usually performed by conventional cephalometric devices, when available.

Conclusion

The biplanar low-dose X-ray device showed good feasibility for precisely assessing the anatomical landmarks of the facial skeleton. Given such excellent precision, the biplanar low-dose Xray device data sets should be forwarded from the treating orthopedic surgeon or neurosurgeon to the orthodontist or dentist for further assessment in their field.

  Fig 1 shows the patient with hands in front of the face (Group A), and Fig 2 shows the same patient with hands on the shoulders (Group B).

Fig 1 .

 1 Fig 1. The positioning of a patient from Group A (hands in front of the face) in the biplanar low-dose X-ray device.

Fig 5 (

 5 Group A, with hands) and Fig 6 (Group B, without hands) contain ICC values, averaged for x-and y-axis values, as well as for intraoperator and interoperator reliability for the perceptibility of the 38 facial skeleton landmarks on the 12 biplanar radiographs.

Fig 2 .Fig 3 .

 23 Fig 2. The positioning of a patient from Group B (hands on the shoulders) in the biplanar low-dose X-ray device.

(ANS)

  Anterior nasal spine: the center at the base of the nose (J) Jugular: the most superior and medial point of the zygomatic buttress (M) Mastoid: the most inferior point of the mastoid point (A) The contact area between the maxillary incisors (B) The contact area between the mandibular incisors (Go) Gonion: the most inferior posterior point at the angle of the mandible (Me) Menton: the most inferior posterior point at the anterior mandibular area (CH) The most inferior posterior point on the anterior inferior border of the mandible most anterior point of the frontal bone (N) Nasion: the most anterior point of the frontal nasal suture in the midsagittal plane (Or) Orbital: the lowest point on the inferior orbital rim (S) Sella: the center of the sella turcica on the lateral cephalogram, which is located by inspection (Ptm) Pterygomaxillary: the apex of the teardrop-shaped pterygomaxillary fissure (lowest point of the opening) (ANS) Anterior nasal spine: anterior tip of the nasal spine (PNS) Posterior nasal spine: the most posterior aspect of the palate bone (A)-Point or subspinal: the most posterior midline point in the concavity where the lower anterior adage of the anterior nasal spine meets the alveolar bone, overlying the mandible incisors (infradentale) and the pogonion (B)-Point or supra-mental: the most posterior midline point in the concavity of the mandible between the alveolar bone overlying the mandible incisors (infradentale) and the pogonion (Pog) Pogonion: the most anterior point of the chin (Go) Gonion: the point defined by using two lines, with one tangential to the inferior border of the ramus; this can be assessed by bisecting the angle formed by the two lines and extending the bisectors through the curvature of the mandible (Gn) Gnathion: the lowest, most anterior midline point on the symphysis of the mandible in the midline (Me) Menton: the most inferior point on the symphysis of the mandible in the midline (Po) Porion: the most superior point of the external auditory meatus; the machine porion is the uppermost point on the outline of the rods of the cephalometer (Co) Condylion: the most posterosuperior point on the head of the condyle (C2) Upper border of the odontoid process (Ba) Basion: the median point on the anterior margin of the foramen magnum

Fig 4 .

 4 Fig 4. Posteroanterior and lateral images of a patient without hands in front of the face, using the biplanar low-dose X-ray imaging system. The 38 landmarks are placed to illustrate the anatomical regions.

Fig 5 .

 5 Fig 5. ICC values for intraoperator reliability for perceptibility of the 38 facial skeleton landmarks on the 12 biplanar radiographs of patients with hands/without hands in front of the face. IB and TS are the raters' initials; x and y are the shortening of the x-and y-axis.

Fig 6 .

 6 Fig 6. ICC values for interoperator reliability for the perceptibility of 38 facial skeleton landmarks on the 12 biplanar radiographs of patients with hands/without hands in front of the face. IB and TS are the rater's initials; x and y are the shortening of the x-and y-axis.

Acknowledgments

We greatly thank Prof. Jean-Marc Vital, Hospital Tripode, CHU de Bordeaux, France for providing us with the biplanar low-dose X-ray imaging data sets.

For this study, no author has received any funding. During the time this retrospective study took place, the institute/laboratory in which Prof. P. Rouch works and Dr. A. Laville worked received funding from the EOS-Imaging company for other EOS studies. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

All relevant data are within the manuscript and its Supporting Information files.

Supporting information

S1 Table. ICC values for intraoperator and interoperator reliability