Ismael Pellejero
email: ismael.pellejero@unavarra.es

Fernando Almazán

Marta Lafuente

Miguel A Urbiztondo

Martin Drobek
email: martin.drobek@univ-montp2.fr

Mikhael Bechelany
email: mikhael.bechelany@umontpellier.fr

Anne Julbe

Luis M Gandía

Functionalization of 3D printed ABS filters with MOF for toxic gas removal

Keywords: ZIF-8, 3D printing, ABS filter, Atomic Layer Deposition, VOCs abatement, gas treatment. Highlights

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Introduction

Additive manufacturing techniques based on 3D printing are nowadays having huge impact on the way the current fabrication methods are changing and adapting [START_REF] Laplume | Global value chains from a 3D printing perspective[END_REF][START_REF] Balletti | 3D printing: State of the art and future perspectives[END_REF]. Indeed, these technologies provide attractive fabrication strategies with major competitive advantages, as they are easy-handling and adapted to geometrical complexity and customised designs at low cost.

Among the different 3D printing methods the Fused Filament Fabrication (FFF), based on extruding thermoplastic materials, is the most extended one. A typical example of a key material for FFF is ABS (Acrylonitrile Butadiene Styrene) polymer featuring both remarkable chemical and physical stability coupled with easy extrusion. In order to widen the field of ABS applications, huge effort has been made in designing various ABS-composites with tuneable adsorption properties, wettability, good thermal stability and high mechanical strength. Such composites are typically prepared either by addition of the active materials (fillers) in the ABS matrix prior to its fabrication [START_REF] Shofner | Nanofiber-reinforced polymers prepared by fused deposition modeling[END_REF][START_REF] Jiang | Strengthening acrylonitrilebutadiene-styrene (ABS) with nano-sized and micron-sized calcium carbonate[END_REF][START_REF] Ma | Synergistic effect of carbon nanotube and clay for improving the flame retardancy of ABS resin[END_REF][START_REF] Schmitz | Hybrid composites of ABS with carbonaceous fillers for electromagnetic shielding applications[END_REF][START_REF] Dul | Filaments Production and Fused Deposition Modelling of ABS/Carbon Nanotubes Composites[END_REF][START_REF] Bible | 3D-Printed Acrylonitrile Butadiene Styrene-Metal Organic Framework Composite Materials and Their Gas Storage Properties, 3D Print[END_REF], or by post-synthesis surface modification/functionalization of the final ABS filter with the selected functional materials [START_REF] Kestila | Towards space-grade 3D-printed, ALD-coated small satellite propulsion components for fluidics[END_REF]. In this work, we propose the surface modification of 3D printed ABS filters with a thin layer of a Metal Organic Framework (MOF) material (here ZIF-8, Zeolitic Imidazolate Framework).

Due to their ultrahigh porosity, large surface areas, structural diversity with versatile micropore sizes, and rich functionalities, MOFs have been developed for multiple applications including gas storage and separation, catalysis and sensing [START_REF] Anson | ABS copolymer-activated carbon mixed matrix membranes for CO2/CH4 separation[END_REF][START_REF] Kreider | Toward 3D printed hydrogen storage materials made with ABS-MOF composites[END_REF][START_REF] Shekhah | MOF thin films: existing and future applications[END_REF]. Concerning their synthesis on polymer supports, number of research works have been focused on the development of MOFpolymer composite membranes and filters for either gas [START_REF] Zhang | Challenges and recent advances in MOF-polymer composite membranes for gas separation[END_REF] or oil/water emulsion separations [START_REF] Cai | Nanofibrous metal-organic framework composite membrane for selective efficient oil/water emulsion separation[END_REF]. Two kinds of membrane designs are typically considered in the literature: i) MOF Mixed-Matrix membranes (MMM) where MOF particles are used as fillers incorporated in a polymer matrix [START_REF] Basu | MOF-containing mixed-matrix membranes for CO2/CH4 and CO2/N-2 binary gas mixture separations[END_REF][START_REF] Sorribas | Mixed matrix membranes comprising silica-(ZIF-8) core-shell spheres with ordered meso-microporosity for natural-and bio-gas upgrading[END_REF] or ii) supported MOF layers which are grown or deposited on the surface of a prefabricated polymer material [START_REF] Shi | Renewable metalorganic-frameworks-coated 3D printing film for removal of malachite green[END_REF][START_REF] Wang | Three-dimensional Printed Acrylonitrile Butadiene Styrene Framework Coated with Cu-BTC Metal-organic Frameworks for the Removal of Methylene Blue[END_REF]. The first design is the most common for 3D printing applications. Typically the pre-synthesized MOF crystals are mixed with a thermoplastic material flowed by an extrusion of the composite (filament shape) in FFF 3D printer [START_REF] Kreider | Toward 3D printed hydrogen storage materials made with ABS-MOF composites[END_REF][START_REF] Evans | Chemically Active, Porous 3D-Printed Thermoplastic Composites[END_REF][START_REF] Li | 3D Printing of Mixed Matrix Films Based on Metal-Organic Frameworks and Thermoplastic Polyamide 12 by Selective Laser Sintering for Water Applications[END_REF], or in the case of stereolithography printing (SLA), MOF crystals are mixed with a photopolymer resin and incorporated to the 3D structure during the curing step [START_REF] Halevi | Hydrolytically Stable MOF in 3D-Printed Structures[END_REF]. The main drawback of both protocols bears on lowering both the mechanical stability of the composites and the accessibility of the active material flooded in the bulk of the polymer matrix. Hence, the latter design based on material surface modification provides significant advantage over bulk matrix modification. The surface modification approach, often based on MOF secondary growth, requires an optimized support seeding with MOF nanocrystals [START_REF] Ge | Porous Polyethersulfone-Supported Zeolitic Imidazolate Framework Membranes for Hydrogen Separation[END_REF][START_REF] Cacho-Bailo | ZIF-8 continuous membrane on porous polysulfone for hydrogen separation[END_REF][START_REF] Nagaraju | In situ growth of metal-organic frameworks on a porous ultrafiltration membrane for gas separation[END_REF]. A uniform seeding could be however very challenging on complex 3D structures and it is thus tricky to growth uniformly a MOF crystal layer on the whole support surface. To overcome this drawback, in this work we applied a synthesis protocol based on "reactive support seeding" involving the deposition of the metal precursor (ZnO) by Atomic Layer Deposition (ALD) [START_REF] Khaletskaya | Self-Directed Localization of ZIF-8 Thin Film Formation by Conversion of ZnO Nanolayers[END_REF][START_REF] Weber | Atomic Layer Deposition for Membranes: Basics, Challenges, and Opportunities[END_REF] and its solvothermal conversion to MOF (ZIF-8) [START_REF] Wang | Three-dimensional Printed Acrylonitrile Butadiene Styrene Framework Coated with Cu-BTC Metal-organic Frameworks for the Removal of Methylene Blue[END_REF]. Based on atom-by-atom reactive deposition mechanism, ALD enables the functionalization of ABS filters with a uniform ZnO layer, thus forming a uniform precursor distribution for homogeneously growing the MOF (ZIF-8) material on the filter. In fact, unlike other deposition techniques such as impregnation, sputtering or CVD, only ALD enables the synthesis of high quality ultrathin films of inorganic materials even on complex 3D structures. This unique ALD capability permits to achieve an excellent grade of metal oxide coverage [START_REF] Marichy | Atomic Layer Deposition of Nanostructured Materials for Energy and Environmental Applications[END_REF][START_REF] Tynell | Atomic layer deposition of ZnO: a review[END_REF][START_REF] Miikkulainen | Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends[END_REF] and was thus selected as a technique of choice for the functionalization of the studied ABS filters.

The metal oxide-to-MOF-conversion strategy has been already successfully applied for growing MOFs on flat silicon wafers [START_REF] Khaletskaya | Self-Directed Localization of ZIF-8 Thin Film Formation by Conversion of ZnO Nanolayers[END_REF], textile fibers (polypropylene (PP), polybutylene terephthalate (PBT), polyacrylonitrile (PAN) and cotton) [START_REF] Zhao | Highly Adsorptive, MOF-Functionalized Nonwoven Fiber Mats for Hazardous Gas Capture Enabled by Atomic Layer Deposition[END_REF][START_REF] Zhao | Conformal and highly adsorptive metal-organic framework thin films via layer-by-layer growth on ALD-coated fiber mats[END_REF][START_REF] Lee | UiO-66-NH2 Metal Organic Framework (MOF) Nucleation on TiO2, ZnO, and Al2O3 Atomic Layer Deposition-Treated Polymer Fibers: Role of Metal Oxide on MOF Growth and Catalytic Hydrolysis of Chemical Warfare Agent Simulants[END_REF][START_REF] Bechelany | Highly crystalline MOF-based materials grown on electrospun nanofibers[END_REF] or alumina membrane supports [START_REF] Drobek | An innovative approach for the preparation of confined ZIF-8 membranes by conversion of ZnO ALD layers[END_REF]. In the present work, the reactive seeding by ALD has been used for the first time for the synthesis of MOF functionalized 3D printed structures, targeting their application as active gas filters. To achieve the preparation of such composite materials, the applied synthesis protocol has to warrant MOF growth in aqueous medium (hydrothermal conversion), thus avoiding the use of organic solvents (often methanol) that could alter the polymer stability and the 3D structure of the filter.

In order to demonstrate the proof of concept for the application of such ABS/ZIF-8 filter systems in air purification, a series of adsorption experiments have been conducted with the composite materials using dimethyl methylphosphonate (DMMP) as a probe gas molecule. The DMMP is a well-known G-series nerve agents simulant, often used to imitate Sarin gas because of its similar chemical structure but much lower toxicity [START_REF] Zheng | Advances in the Chemical Sensors for the Detection of DMMP -A Simulant for Nerve Agent Sarin[END_REF]. Due to the exceptional adsorption and reaction capacities of MOFs, many research efforts have been already conducted focusing on their possible application in the capture and/or fast degradation/conversion of toxic agents [START_REF] Montoro | Capture of Nerve Agents and Mustard Gas Analogues by Hydrophobic Robust MOF-5 Type Metal-Organic Frameworks[END_REF][START_REF] Moon | Instantaneous Hydrolysis of Nerve-Agent Simulants with a Six-Connected Zirconium-Based Metal-Organic Framework[END_REF][START_REF] Liu | Catalytic degradation of chemical warfare agents and their simulants by metal-organic frameworks[END_REF][START_REF] Troya | Reaction Mechanism of Nerve-Agent Decomposition with Zr-Based Metal Organic Frameworks[END_REF]. Hence, the proposed combination of the 3D printed filter with a uniform ZIF-8 layer is expected to be advantageously integrated in such kind of toxic gas capture/elimination systems [START_REF] Ma | Metal-Organic Framework Films and Their Potential Applications in Environmental Pollution Control[END_REF].

Materials and Methods

Fabrication of 3D printed ABS filters.

Fig. 1: 3D printed ABS filter. A) CAD design, B) printed filter images.

Series of 3D ABS filters have been fabricated by FFF methodology using ABS (SMARTFIL® ABS Smart Materials 3D) and a desktop 3D printer ZortraxM200 3D printer (extrusion parameters: 235 ºC, 1.75 mm filament, 10 mm•s -1 ). Fig. 1 shows a typical 3D printed ABS filter with disc geometry (22 mm x 3.2 mm), designed by a computer-aided design software (AutoCAD®). Different mesh dimensions and spacing were tested for selecting the optimal grid design constituted of perpendicular lines (400 µm x 400 µm) and rectangular openings (400 µm x 500 µm). ZnO by Atomic Layer Deposition A home-made ALD set-up [START_REF] Viter | The influence of localized plasmons on the optical properties of Au/ZnO nanostructures[END_REF] was used for depositing ZnO layers on both 3D printed ABS filters and companion Si wafer samples (1 mm x 1 mm, polished; <100> oriented). ZnO layers were deposited at two different temperatures (60 °C and 100 °C) using sequential exposure of the metal precursor (diethyl zinc (DEZ), Zn(CH2CH3)2, Sigma Aldrich, 95% purity) and deionized water, separated by a purge with dry argon (Ar flow rate of 100 sccm). The deposition protocol was as follows: a) 0.5 s pulse of DEZ, 30 s exposure and 40 s purge with dry Ar; b) 2 s pulse of H2O, 30 s of exposure, and 40 s purge with dry Ar (i.e. total duration ~ 10 h). The selected pulse, exposure and purge times were chosen both to ensure completion of the ALD surface reactions and to prevent any mixing of the reactive species. The measured growth rate of ZnO layer on Si wafers was 0.2 nm per cycle. The thickness (typically in the range 50 nm -100 nm) and optical characteristics of the deposited layers were analysed by ellipsometry (Semilab GES5E) on flat Si samples.

ZIF-8 by ZnO conversion

The conversion of ZnO to ZIF-8 was carried out by hydrothermal treatment in a closed pressure vessel (Teflon-lined stainless steel autoclave -20 ml) containing an aqueous solution of 2-methyl imidazole (2-mIm) (Alfa Aesar, 97% purity). The influence of the main hydrothermal reaction parameters (temperature, reaction time and reactants concentration) was first conducted on flat ZnO-coated Si samples in order to determine the optimal conditions for growing polycrystalline ZIF-8 layers on ABS filters. The 2-mIm concentration was varied fixed at 0.2, 0.5 and 1wt.%, hence in stoichiometric excess compared to the deposited quantity of ZnO. The syntheses were conducted in the temperature range 25-100 °C in a conventional oven. Reaction times were varied from 4 to 24 hours depending on the synthesis temperature. After the hydrothermal treatment, samples were washed with deionized water and dried for 1 h at 60 °C.

Optimized ZIF-8 growth conditions were subsequently transferred to 3D printed ABS samples designed for VOCs sorption experiments.

Sample characterization

Full characterization of the prepared 3D filters was conducted by Field Emission Scanning Electron Microscopy (FESEM), Energy-dispersive X-ray spectroscopy (EDX), X-ray Diffraction (XRD), Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared spectroscopy (FTIR). The morphology and homogeneity of the supported ZnO and ZIF-8 on both Si wafers and ABS 3D printed filters were studied at different locations of the samples using field emission scanning electron microscopy (FESEM, Hitachi S-4800 and INSPECT-F50 with Energy-dispersive X-ray spectroscopy (EDX) for elemental analysis. Prior to FESEM observations, the filters were cut with a diamond saw and metalized with palladium by plasma sputtering. Crystalline structures were studied by X-ray diffraction (XRD, D-Max Rigaku, Ru300) using graphite filtered Cu-Kα radiation (X-ray power: 40 kV, 80 mA) and the scanning programme: 2 = 5-45°; step = 0.03°; time per step = 1s. FTIR spectra were measured on Jasco 4000 equipment with a high throughput monolithic diamond Attenuated Total Reflectance (ATR) accessory. All spectra were recorded by averaging 25 scans in the 4000-400 cm -1 range. The DSC spectra were measured using a DSC822e equipment from METTLER TOLEDO with a heating rate of 10 °C•min -1 up to 500 °C. Thermogravimetric analyses were performed in air flow (50 mL•min -1 ) with a heating rate of 5 °C•min -1 up to 600 °C (TGAQ5000, TA Instruments). N2 adsorption-desorption isotherms were obtained with a Micromeritics Gemini V 2380 analyser at 77 K, previously degassed at 100 °C overnight.

Adsorption studies with DMMP

Adsorption experiments with DMMP (dimethyl methylphosphonate) were carried out by loading the ABS/ZIF-8 filters into the adsorption column (quartz tube Øint = 22 mm) of a labmade set-up described in our previous work [START_REF] Almazan | Zeolite based microconcentrators for volatile organic compounds sensing at trace-level: fabrication and performance[END_REF]. Trace-level DMMP vapor atmospheres were generated by circulating 10 mL•min -1 dry N2 through a calibrated permeation tube (VALCO, permeation rate of 1629.96 ng/min ± 0.71% at 100 ºC) and further analyte dilution was obtained with additional dry N2 stream until target concentration C0 = 162 mg•m -3 (32 ppmV). A flow of 10 mL•min -1 of the DMMP stream (C0 = 32 ppmV) was circulated in the adsorption column at room temperature and the concentration of the downstream at any time (Ct) was monitored online by gas chromatography coupled with mass spectrometer (Shimadzu GCMS-QP2010). To fully characterize the total adsorption capacity and the associated kinetics, experiments were carried out until the outlet concentration (Ct) was equal to the feed concentration (C0). The adsorbent material was regenerated and reconditioned overnight at 80 ºC under dry N2 flow (10 mL•min -1 ).

Results and discussion

As already specified in the experimental part, the most important parameters influencing ZIF-8 growth have been investigated first on Si wafers before transferring the synthesis protocol to 3D printed ABS filters. A particular attention has been devoted to the examination of ALD deposition temperature, ZnO layer thickness, concentration of the organic linker, and hydrothermal treatment temperature and duration. All these parameters were selected to fit the chemical, thermal and mechanical stability of the ABS polymer [START_REF]REHAU. Acrylonitrile-butadiene-styrene[END_REF]. Indeed, compared to reference studies published in the literature [START_REF] Khaletskaya | Self-Directed Localization of ZIF-8 Thin Film Formation by Conversion of ZnO Nanolayers[END_REF][START_REF] Drobek | MOF-Based Membrane Encapsulated ZnO Nanowires for Enhanced Gas Sensor Selectivity[END_REF][START_REF] Weber | High-Performance Nanowire Hydrogen Sensors by Exploiting the Synergistic Effect of Pd Nanoparticles and Metal-Organic Framework Membranes[END_REF] using organic solvents for ZIF-8 layer synthesis (e.g. dimethylformamide and methanol) in this work we focused exclusively on the utilization of water as a solvent for the organic linker. The target was double: i) introducing a more environmentally friendly synthesis protocol avoiding application of organic solvents together with minimizing the amounts of reactants (e.g. intrinsically small quantities of organic precursors required for ALD of ZnO) and ii) preservation of the 3D printed filter structure which may be altered by organic solvents under specific reaction conditions.

ALD of ZnO on 3D printed ABS filters

The stability of ABS filters under ALD conditions has been also investigated in order to identify the possible temperature window for ALD deposition without compromising the pristine filter characteristics. In fact, the temperature and type of organic precursor used for ALD of metal oxides [START_REF] Kestila | Towards space-grade 3D-printed, ALD-coated small satellite propulsion components for fluidics[END_REF] might seriously affect the ABS structure by polymer swelling, deformation or surface damages. Standard ZnO growing procedures by ALD are usually carried out at temperatures ~100 °C or higher depending on the targeted crystalline characteristics and deposition rate.

Taking into account the glass transition temperature for ABS (~105 °C), only ALD temperatures below this value are recommended for this kind of support. Indeed, it has been shown that the simple exposure of ABS filters to temperatures ~ 100 °C does not affect their 3D structure.

However, under the ALD conditions at 100 °C the filters tend to deform, revealing that the combination of both high temperature and organometallic species (DEZ) can seriously affect the mechanical integrity of the polymeric material (thermochemical effect). This deformation phenomenon was fully avoided by carrying out the ALD of ZnO at ~60 °C, which corresponds to the highest applicable ALD temperature without any alteration of the ABS filter structure. Both optical and SEM observations (Fig. 2A and2B) confirmed the absence of any filter deformation at 60 °C, although some ZnO clustering was detected on the filter surface at this temperature (Fig. 2B). Nevertheless, a homogeneous coverage of the ABS sample with ZnO was achieved on all samples and confirmed by EDX spectroscopy (results not shown).

Fig. 2: SEM observation of an ABS filter coated with ZnO (ALD at 60 °C, 50 nm thick ZnO layer measured on Si wafer). A) and B) details of the ZnO-coated ABS surface.

The ALD temperature is a very important parameter impacting the crystallinity, grain size and grain boundaries of the deposited materials [START_REF] Miikkulainen | Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends[END_REF][START_REF] Iqbal | ALD grown nanostructured ZnO thin films: Effect of substrate temperature on thickness and energy band gap[END_REF]. In the present work, ZnO deposition temperature could thus affect the oxide reactivity during its hydrothermal conversion to ZIF-8.

Ellipsometry measurements were carried out on Si wafers with 50 nm thick ZnO layers. It has been verified that the thickness of ZnO layer measured on Si wafers corresponds well to the layer thickness deposited on ABS substrates. Indeed, as already observed in our previous studies [START_REF] Viter | Enhancement of Electronic and Optical Properties of ZnO/Al2O3 Nanolaminate Coated Electrospun Nanofibers[END_REF], the ALD rate of ZnO is influenced by the substrate chemistry only during the very first deposition cycles. This initial phenomenon has thus negligible impact on the final layer thickness after the as many as 250 ALD cycles applied to obtain ~ 50 nm thick ZnO layers. The absorption peaks (minimum amplitude Tan Ψ) of ZnO deposited at 60 °C and 100 °C were at 363 nm (3.41 eV) and 370 nm (3.35 eV), respectively, confirming no significant optical differences between the ZnO materials deposited at these two temperatures. Hence, similar crystal sizes and interconnectivity seems to be obtained, independently of the deposition temperature, suggesting similar reactivity for the corresponding layers. When ZnO conversion to ZIF-8 was carried out at lower temperatures (e.g. 25°C), only small and irregular ZIF-8 crystals were formed, leaving the ZnO layer practically intact. Similar results have been obtained even at longer reaction times ≥ 24 h, confirming low ZnO dissolution/conversion rate at temperatures below 60 °C. Indeed, at 60 °C a nearly full conversion of the 50 nm ZnO layer was achieved within 4 hours, forming ZIF-8 crystals (~1 µm in size) homogeneously spread on the Si wafer surface. In addition to the temperature for both the ALD of ZnO and the hydrothermal conversion of ZnO to ZIF-8, a series of other synthesis parameters such as ZnO layer thickness and organic linker concentration (2-mIm) has been also investigated. As expected (Fig. 5A), ZnO layers which are ~100 nm thick still partially remained after gentle hydrothermal conditions (4 h-60 °C). Full ZnO conversion required higher temperature (up to 100 °C for the same reaction time). Although none of these reaction conditions allowed obtaining a full and uniform coverage of the Si support with ZIF-8 crystals, improvement could be expected when using longer reaction times at 60 °C in order to slow the crystal growth kinetics. Indeed, the above results demonstrate the direct relation between ZnO layer dissolution and ZIF-8 nucleation/crystal growth. ZIF-8 crystal nucleation should start from ZnO surface by a dissolution mechanism, linker reaction with Zn 2+ and ZIF-species clustering in basic media. At 100°C, ZIF-8 crystals grow rapidly, consuming the surrounding ZnO and yielding non-uniform support coverage with large ZIF-8 crystals and spaces between them.

Synthesis of ZIF-8 by conversion of ZnO ALD layers on Si wafers

Fig. 3: FESEM images of ZIF-8 crystals obtained by hydrothermal conversion (4 h, 1wt% 2mIm in H2O): at 1) 25°C, 2) 60 °C and 3) 100 °C, of ALD ZnO layers (50 nm thick) deposited on Si wafers at A) 60 °C and B) 100 °C.

Fig. 4: XRD patterns of ZIF-8 obtained by hydrothermal conversion of ZnO (50nm layer on Si wafers) at: a) 100 °C-4 h, (ALD at 100 °C) and b) 60 °C -4h (ALD at 60 °C). The patterns of ZIF-8 crystal c) and ALD of ZnO

Fig. 5: FESEM images of ZIF-8 crystals obtained by hydrothermal conversion (4 h, 1 wt.% 2mIm in H2O) at: A) 60 °C and B) 100°C of ALD ZnO layers (100 nm thick) deposited on Si wafers at 60 °C.

Fig. 6: FESEM images of ZIF-8 crystals obtained by hydrothermal conversion (4 h-60 °C) with A) 0.2 wt.% 2-mIm; and B) 0.5 wt.% 2-mIm, of ALD ZnO layers (100 nm thick) deposited on Si wafers at 60°C.

Optimal reaction conditions yielding uniform support coverage with connected ZIF-8 crystals should correspond to a compromise between ZnO dissolution and ZIF crystal nucleation and growth rates. The dissolution rate of the ZnO layer is directly to the pH of the linker solution. Fig. 6 shows the effect of lower concentration of 2-mIm, down to 0.5wt.% (~ pH 10) and 0.2 wt.% (~ pH 9.5), using the same reaction time and temperature (60 °C, 4 h). In comparison with the Fig. 5A, obtained with a higher 2-mIm concentration of 1wt.% (pH 12), the lower pH values are not favourable for a complete dissolution of the ZnO layer and yield heterogeneous growth of ZIF-8 crystals.

Synthesis of ZIF-8 on ABS filters

As a result of the preliminary investigations on Si wafers, the reaction temperatures for both ALD deposition and solvothermal conversion were fixed at 60 °C for uniform functionalisation of the 3D ABS filters without altering its structure. The hydrothermal conversion of ZnO to ZIF-8 was carried out with 1 wt.% 2-mIm at different reaction times. It is important to note that the low synthesis temperature warrants the possible transfer of the synthesis protocol to other types of polymer materials typically used in FFF process and featuring lower glass transition point.

Fig. 7: FESEM images of ZIF-8 grown from ALD-ZnO on the outer surface of ABS filters at A) 60 °C -24 h and B) 60 °C -4 h.

As observed on the FESEM images (Fig. 7), the hydrothermal synthesis carried out at 60°C for either 24 h (Fig. 7A) or 4 h (Fig. 7B) led to well intergrown layers homogeneously covering the lines of the filter grid. However, the morphology of MOF crystals/layers grown on ABS support differs from those obtained on Si wafers. We already observed this phenomenon in a previous work focused on the growth of MOFs crystals on polymeric supports (Polyacrylonitrile-PAN) [START_REF] Bechelany | Highly crystalline MOF-based materials grown on electrospun nanofibers[END_REF] leading to well-intergrown continuous MOFs crystalline layers. Thus MOF crystals intergrowth seems to be better achieved on rough supports rather than on perfectly flat surfaces (Si wafers) featuring no geometrical constrains. This result highlights the asset of the reactive seeding protocol applying ALD to obtain a uniform and high-grade support coverage with MOF layers in comparison with direct MOF synthesis [START_REF] Shi | Renewable metalorganic-frameworks-coated 3D printing film for removal of malachite green[END_REF][START_REF] Wang | Three-dimensional Printed Acrylonitrile Butadiene Styrene Framework Coated with Cu-BTC Metal-organic Frameworks for the Removal of Methylene Blue[END_REF] in which full and homogenous coverage are difficult to achieve. To demonstrate the presence of a ZIF-8 layer also on the inner walls of the filter and evaluate its thickness, the MOF modified ABS supports were cut transversally for SEM observations as shown in Fig. 8. It can be clearly seen that, same as on the outer filter walls, homogenous and continuous ZIF-8 crystal layers are also present in the inner areas of the 3D printed filter (Fig. 8B 1-3). Performing the EDS mapping (Fig. 8A 3) of the filter cross-section (Zn La1 at 1.02 keV) the ZIF-8 layer thickness was evaluated to be ~ 2 µm. This value is well in the range of the theoretical layer thickness which could be expected by conversion of 50 nm thick ZnO layers (i.e. ~ 2.2 µm thick ZIF-8 layer). The lower values measured for ZIF-8 thickness suggest uncomplete ZnO conversion, as confirmed by XRD analysis (remaining ZnO phase).

Fig. 8: FESEM cross-section images in different part of an ABS/ZIF-8 filter. A) cross-section, B) slice in the middle of the filter.

It should be underlined that no filter damage was observed even after extended reaction time (24 h). Interestingly, the developed protocol resulted in more uniform layers with better intergrown ZIF-8 crystals than on silicon wafers, thus highlighting the influence of the support and ZnO characteristics on the properties of the derived ZIF-8 layer. In addition, the selected synthesis protocol was found to be very robust as equal amounts of ZIF-8 were grown on filters series having the same architecture (active surface). Typically 2.9 ± 0.6 mg of ZIF-8 were grown on filters with a weight of 520 ± 6 mg (3D architecture described in section 2.1). Due to practical reasons shorter reaction times (4 h) were favoured as the weight of ZIF-8 did not increase significantly after longer reaction times.

From N2 sorption measurements, the specific surface area of a typical ABS/ZIF-8 filter was evaluated to be 7-8 m 2 •g -1 , although it was below 1 m 2 •g -1 for a bare ABS filter. This increase in SBET could be clearly attributed to the contribution of ZIF-8 material. Considering the quantity of ZIF-8 grown on the surface of the filter (weight increase measurement), a SBET value of ~1300 m 2 •g -1 was estimated for the ZIF-8 material alone. In spite of the high error (+20%) associated to this estimated value, it perfectly fits the data reported in the literature for ZIF-8 [START_REF] Karagiaridi | Opening ZIF-8: A Catalytically Active Zeolitic Imidazolate Framework of Sodalite Topology with Unsubstituted Linkers[END_REF].

Fig. 9: A) XRD patterns and B) FTIR spectra of ABS filters before and after ZIF-8 synthesis. Graphs a) asprinted ABS filter, b) ABS filter + ALD ZnO at 60 °C, and c) ZIF-8 coated ABS filters (hydrothermal conversion at 60 °C, 4 h).

XRD analyses of the samples confirmed the presence of the main characteristic peaks of both ZIF-8 and ZnO which still remains partially preserved after the ZIF-8 synthesis (Fig. 9). The initial and remaining ZnO layer could serve as a protective shield hindering strong chemical attack of the ABS polymer during the hydrothermal synthesis. Still, when comparing the XRD patterns of a pristine ABS filter and its ZnO-coated analogue (Fig. 9A a) and b), respectively), a slight difference can be observed between 10° and 15°, 2, which could be attributed to a small change in the ABS structure during ALD. ABS filters were also subjected to FTIR-ATR analysis (Fig 9B). Bare ABS presents characteristic peaks at 3023 cm -1 and 2840-2915 cm -1 (C-H, and -CH3, -CH2 stretching) together with C-H bending in mono-substituted benzene rings at 755 and 690 cm -1 [START_REF] Puype | Evidence of waste electrical and electronic equipment (WEEE) relevant substances in polymeric food-contact articles sold on the European market[END_REF][START_REF] Thanh Truc | Selective separation of ABS/PC containing BFRs from ABSs mixture of WEEE by developing hydrophilicity with ZnO coating under microwave treatment[END_REF]. ABS/ZIF-8 shows bands at 1309 cm -1 and 1147 cm -1 corresponding to entire ring and aromatic C-N stretching, respectively, while the band at 995 cm -1 could be assigned to C-N bending vibration mode [START_REF] Bible | 3D-Printed Acrylonitrile Butadiene Styrene-Metal Organic Framework Composite Materials and Their Gas Storage Properties, 3D Print[END_REF][START_REF] Cravillon | Rapid Room-Temperature Synthesis and Characterization of Nanocrystals of a Prototypical Zeolitic Imidazolate Framework[END_REF] in the 2-mIm ligand. FTIR spectroscopy confirmed that a part of the ZnO remains unreacted (band related to stretching modes of ZnO around 500 cm -1 [START_REF] Anzlovar | Polyol-Mediated Synthesis of Zinc Oxide Nanorods and Nanocomposites with Poly(methyl methacrylate)[END_REF] observed for both ABS/ZnO and ABS/ZIF-8 samples), thus corroborating XRD results. The wide band around 3200-3600 cm -1 was assigned to surface -OH groups.

Fig. 10: A) TGA and B) DSC analyses of ABS filters before and after ZIF-8 synthesis. Graphs a) as-printed ABS filter, b) ABS filter + ALD ZnO at 60 °C, and c) ZIF-8 coated ABS filters (hydrothermal conversion at 60 °C, 4 h).

Finally, TGA (in air flow) and DSC analyses were conducted (Fig. 10) in order to determine the stability of ABS matrix before and after MOF synthesis. As observed in Fig. 10A graph a, total thermal degradation of bare ABS occurs in a wide range of temperatures (275°C -500°C) with two decomposition steps. The first DTG (Derivative Thermogravimetry) peak at 365°C corresponds to butadiene and styrene, while the second at 475°C is related to monomeric acrylonitrile [START_REF] Dong | Thermal degradation of acrylonitrile-butadiene-styrene terpolymer in bean oil[END_REF][START_REF] Yang | Thermal analysis of an acrylonitrilebutadiene-styrene/SWNT composite[END_REF][START_REF] Wojtyla | Is 3D printing safe? Analysis of the thermal treatment of thermoplastics: ABS, PLA, PET, and nylon[END_REF]. Comparing the virgin ABS and the support after ALD process and hydrothermal synthesis treatment, no significant difference was observed (DTG peak ±10°C). ZIF-8 degradation was not observed because the range of decomposition temperatures is similar to ABS. DSC analyses (Fig. 10B) confirmed no significant changes in the glass transition temperature after either ABS support exposure to ALD or solvothermal synthesis conditions. Glass transition temperatures (Tg) at 110 °C, and 138°C were attributed to styrene and acrylonitrile, respectively. The main melting temperature peaks at 420 °C were same for all studied samples, thus confirming virtually zero influence of the selected synthesis conditions on the filters stability.

The developed synthesis protocol based on low temperature ALD of ZnO and its hydrothermal conversion to ZIF-8 at low temperature too, permits a total and very uniform coverage of the ABS filter with a high surface area adsorbing material (ZIF-8), without any external deformation or damage of the filter. These 3D printed filters functionalized with an active ZIF-8 layer have been subsequently tested for removing traces of toxic gases from a gas stream by filtration/adsorption.

Adsorption studies

Dimethyl methylphosphonate (DMMP) was selected as a model compound to test the efficacy of the functionalized 3D ABS/ZIF-8 filters by adsorption in gas phase. Indeed, DMMP is a simulant of the Sarin gas, with much lower toxicity. According to molecular simulation (Fig. S1), the size of the DMMP molecule is 5.17 × 4.24 Å. In a typical experiment the adsorption reactor has been loaded with four ABS/ZIF-8 filters having in total ~12 mg of the ZIF-8 active material deposited on their 3D structure. The efficiency of the filtering system has been then evaluated by flowing through it 32 ppm of DMMP (162 mg•m -3 ) diluted in N2 stream at a flow rate of 10 mL•min -1 . negligible and undesirable pressure drop. Hence, an optimum filter architecture offering a good compromise of adsorbent accessibility vs. pressure drop should always be found to allow their efficient integration into real air filtration systems [START_REF] Ma | Metal-Organic Framework Films and Their Potential Applications in Environmental Pollution Control[END_REF].

Fig. 11: Adsorption of DMMP by ABS filter (cDMMP 32 ppm), uncoated (blank), ZnO-coated and ZIF-8 coated. Table 1: Main parameters derived from the breakthrough curves (time, volume and mass of DMMP adsorbed at the breakthrough point (5%) and at the equilibrium point (95%

Indeed, both contact efficiency and adsorption capacity of the ZIF-8 could be easily increased by modifying the filter architecture (increasing mesh of the grid), improving filter design (crosslinked-tortuous mesh), adapting filter loading in the system. and/or by modifying the ZIF-8 surface with catalytic species such WO3, TiO2 or MoO2 [START_REF] Kanan | Dual WO3 based sensors to selectively detect DMMP in the presence of alcohols[END_REF][START_REF] Panayotov | Uptake of a Chemical Warfare Agent Simulant (DMMP) on TiO2: Reactive Adsorption and Active Site Poisoning[END_REF][START_REF] Head | Dimethyl methylphosphonate adsorption and decomposition on MoO2 as studied by ambient pressure x-ray photoelectron spectroscopy and DFT calculations[END_REF] for in situ degradation of DMMP. Hence, it is expected that such optimised 3D printed filters functionalised with MOF layers could be advantageously applied in air purification integrated systems for the capture/elimination of VOCs [START_REF] Bible | 3D-Printed Acrylonitrile Butadiene Styrene-Metal Organic Framework Composite Materials and Their Gas Storage Properties, 3D Print[END_REF][START_REF] Kreider | Toward 3D printed hydrogen storage materials made with ABS-MOF composites[END_REF][START_REF] Ma | Metal-Organic Framework Films and Their Potential Applications in Environmental Pollution Control[END_REF] and pesticides [START_REF] Liu | Adsorption and removal of organophosphorus pesticides from environmental water and soil samples by using magnetic multi-walled carbon nanotubes @ organic framework ZIF-8[END_REF].

In a wider context, such adsorbing filters could be also implemented with other 3D printed polymers like HIPS (High Impact PolyStyrene), polyamide, polyethylene terephthalate, polycarbonate, polypropylene or PEEK (PolyEther Ether Ketone). They could be advantageously modified also with other MOFs such as MIL-53(Al) [START_REF] Bechelany | Highly crystalline MOF-based materials grown on electrospun nanofibers[END_REF], UiO-66 (Zr) [START_REF] Moon | Instantaneous Hydrolysis of Nerve-Agent Simulants with a Six-Connected Zirconium-Based Metal-Organic Framework[END_REF], eventually ZIF-8 with graphene oxide [START_REF] Zhou | Preparation of zeolitic imidazolate framework-8 /graphene oxide composites with enhanced VOCs adsorption capacity[END_REF] or ordered mesoporous carbons [START_REF] Huynh | Dimethyl Methylphosphonate Adsorption Capacities and Desorption Energies on Ordered Mesoporous Carbons[END_REF], providing that they can be effectively integrated in 3D printed systems and the applied reaction conditions remain compatible with the thermal and chemical stability of the polymer.

Conclusions

In this work we present an effective and robust protocol for surface functionalization of 3D printed polymer filters with a uniform layer of a metal organic framework (ZIF-8) by conversion of a metal oxide layer (ZnO) prepared by ALD. The growth of the MOF crystals was carried out by an environment friendly synthesis protocol in aqueous media without any organic solvent in order to prevent any polymer deformation. ALD temperature was found to be a crucial parameter for the deposition of ZnO layers by ALD on ABS supports and it was feasible at temperatures as low as 60 °C without altering the support architecture. The hydrothermal conversion of the deposited ZnO layers led to uniform layer of well intergrown ZIF-8 crystals on the whole filter surface. DMMP adsorption experiments revealed promising performance for the prepared ABS/ZIF-8 filters (20.35 mg of DMMP per g of ZIF-8) with a system efficiency of 15.5%. Taking into account the robustness of the synthesis protocol (reactive seeding by ALD followed by the hydrothermal conversion of the metal oxide to MOFs), the concept of polymer filter functionalization could be easily implemented to other MOFs and 3D printed supports, providing that the synthesis parameters are compatible with the physico-chemical stability of the FFF polymers. Finally it should be mentioned that, besides of relatively high capital and operational costs of ALD vacuum based technology, the 3D printing and solvothermal conversion of ALD oxide layers do not represent any significant financial or operational bottleneck for process upscaling. In addition, the possibility of simultaneous ALD on a huge number of substrates, together with the recent development of low temperature processes, roll-to-roll or other special ALD reactors, will certainly enable economically viable ALD coatings on polymeric supports. Such type of 3D printed filters with an active MOF layer could then be possibly implemented in a large variety of environmental applications such as adsorption systems for removing toxic gases or water pollutants, and last but not least also as pre-concentrators or pre-filters in sensing systems.

Fig. 3

 3 Fig.3shows FESEM images of ZIF-8 crystals grown by hydrothermal conversion (at different

  d) are shown for comparison. The structural characteristics of the ZIF-8 material obtained by conversion of ZnO ALD layers deposited at either 60 °C or 100 °C have been studied by XRD. As shown on Fig. 4, all the ZIF-8 characteristic diffraction peaks are well detected, thus confirming the good crystallinity of the formed materials. The large diffraction band in the graph b of Fig. 4 pattern (33-35°, 2) was attributed to residual (unconverted) ZnO.

  ).

	Filter	t5% (min) t95% (min)	V5% (mL)	V95% (mL)	m5% (µg)	m95% (µg)
	ABS (Blank)	30.2	163.7	302	1637	48.9	265.1

Acknowledgments

Financial support from Gobierno de Navarra (grants PC052-23) and from the Spanish Ministerio de Ciencia, Innovación y Universidades and the European Regional Development Fund (ERDF/FEDER) (grant RTI2018-096294-B-C31) is gratefully acknowledged. I. P. thanks Obra Social la Caixa, Fundación Caja Navarra and UPNA for his research contracts in the framework of the programs "Ayudas Postdoctorales" and "Captación Talento". L. M. G. wishes to thank Banco de Santander and Universidad Pública de Navarra for their financial support under "Programa de Intensificación de la Investigación 2018" initiative. Authors would like to acknowledge the "Servicio de Difracción de Rayos X y Análisis por Fluorescencia del Servicio General de Apoyo a la Investigación-SAI", Universidad de Zaragoza. The microscopy work has been conducted in the "Laboratorio de Microscopias Avanzadas" at "Instituto de Nanociencia de Aragón -Universidad de Zaragoza". Authors acknowledge the LMA-INA for offering access to their instruments and expertise.

The characteristic breakthrough curves are shown in Fig. 11. In spite of its large size, the DMMP molecules is able to diffuse within the ZIF-8 framework because the elastic behavior of ZIF-8 (flipping motion of ligands) allows possible opening of pore apertures up to ~4.2 Å [START_REF] Krokidas | Molecular Simulation Studies of the Diffusion of Methane, Ethane, Propane, and Propylene in ZIF-8[END_REF].

Different parameters can be determined (Table 1): time t5%, volume V5% and mass m5% of analyte adsorbed at the breakthrough point (when outlet concentration Ct is 5% of inlet concentration C0), and t95%, V95% and m95% at the equilibrium point (when Ct and C0 are nearly identical ~95% of C0).

After subtracting the blank experiment, the mass of DMMP adsorbed by the ABS/ZnO filter (mt95% (ABS/ZnO)) was estimated at ~62 µg although it reached ~305 µg for the ABS/ZIF-8 filter (total weight of ZIF-8 mt95% (ABS/ZIF-8) = 12 mg). With these parameters, the adsorption capacity (We) of the filter system (mg of analyte adsorbed per g of adsorbent) was calculated both with and without the effect of the support in order to evidence the activity of the ZIF-8 layer. The total adsorption capacity of the system was We = 0.14 mg of DMMP per g of the filter. However, when considering only the ZIF-8 mass (12 mg) and the amount of adsorbed analyte (mt95% (ZIF-8) = 242 µg), the adsorption capacity of the ZIF-8 layer was WZIF-8 = 20.4 mg of DMMP per g of ZIF-

8. The efficiency of the whole system (ABS/ZIF-8 filters + reactor) calculated as W5% / W95% was 15.5%, revealing a high dead volume for the experimental set-up. Removing the contribution of the intrinsic system adsorption (blank experiment), the filters efficiency is ~20%. For the sake of comparison, a set of adsorption experiments has been also conducted with an equivalent amount of ZIF-8 single crystals packed in a bed microconcentrator (Fig. S2). The adsorption equilibrium was determined to be ~ 145 mgDMMP/gZIF-8. This experimental value has been further crosschecked by adsorption simulation of DMMP inside the ZIF-8 pores (Fig. S3): a comparable value of ~ 170 mgDMMP/gZIF-8 was found at equivalent partial pressure ~ 162 mg of DMMP•m -3 . The higher adsorption capacity measured for the ZIF-8 crystals (in comparison with the studied 3D ABS/ZIF-8 filters) is attributed to both higher accessible surface area of individual ZIF-8 crystals and lower bypassing of the gas in the packed bed microconcentrator, though at the expense of a non-

Conflicts of interest

The authors declare no conflicts of interest.

Appendix A. Supplementary Information

Supplementary material related to this article can be found in the online version.


Pierre Bourdillon
email: pierre.bourdillon@neurochirurgie.fr

Sylvain Rheims

Hélène Catenoix

Alexandra Montavont

Karine Ostrowsky-Coste

Jean Isnard

Marc Guénot

Surgical techniques: Stereoelectroencephalography-guided radiofrequencythermocoagulation (SEEG-guided RF-TC)

Stereoelectroencephalography-guided radiofrequency-thermocoagulation (SEEG-guided RF-TC) consists of coupling SEEG investigation with RF-TC stereotactic lesioning directly through the recording electrodes. In this systematic review the surgical technique, indications, and outcomes are described. Maximum accuracy is reached when a frame-based procedure with a robotic assistance and a per-operative vascular X-ray imaging are performed. Monitoring of the lesioning procedure based on the impedance, a sharp modification of which indicates that the thermocoagulation has reached its maximum volume, allows the optimization of the lesion size. The first indication concerns patients in whom a SEEG is required to determine whether surgery is feasible and in whom resection is indeed possible. Even if surgery is performed owing to insufficient efficacy of SEEG-guided RF-TC, the procedure remains interesting owing to its high positive predictive value for good outcome after surgery.

The second indication concerns patients in whom phase I non-invasive investigations have concluded to surgical contraindication and who may still undergo SEEG in a purely therapeutic perspective (small deep zones inaccessible to surgery and network nodes of large epileptic networks). Lastly, SEEG-guided RF-TC can be considered as a first-line treatment for periventricular nodular heterotopia (PNH). Independently of indication, the overall seizure-free rate is 23% and the responder rate is 58%. The best results are obtained for PNH (38% seizure-free and 81% responders), while the worst results have been reported for temporal lobe-epilepsy in a dedicated study. The overall complication rate is 2.5%. More evidence is needed to help determine the exact place of SEEG-guided RF-TC in the surgical management algorithm.

Introduction

Stereotactic neurosurgery was initially developed in the second half of the XX th century as a solution to the high morbidity of invasive open surgery [START_REF] Spiegel | Stereotaxic Apparatus for Operations on the Human Brain[END_REF]. Historically, in epilepsy surgery stereotactic procedures were mainly used to localize the seizure onset zones through the recording of intracranial-EEG by stereoelectroencephalography (SEEG) [START_REF] Talairach | Functional stereotaxic exploration of epilepsy[END_REF]. More recently, following the growing accuracy of the delineation of the seizure onset zone and epileptic networks, the surgical management of drug-resistant epileptic patients faced new challenges [START_REF] Bourdillon | Similarities and differences in neuroplasticity mechanisms between brain gliomas and nonlesional epilepsy[END_REF][START_REF] Englot | A modern epilepsy surgery treatment algorithm: Incorporating traditional and emerging technologies[END_REF][START_REF] Ryvlin | Epilepsy surgery in children and adults[END_REF]. Alongside surgical advances in conventional epilepsy surgery, stereotactic lesioning has emerged as an alternative when surgery is not an option. Several stereotactic approaches have been developed, including radiosurgery [START_REF] Mcgonigal | Radiosurgery for epilepsy: Systematic review and International Stereotactic Radiosurgery Society (ISRS) practice guideline[END_REF], Laser Interstitial Thermotherapy (LiTT) [START_REF] Willie | Real-Time Magnetic Resonance-Guided Stereotactic Laser Amygdalohippocampotomy for Mesial Temporal Lobe Epilepsy[END_REF][START_REF] Curry | MR-guided stereotactic laser ablation of epileptogenic foci in children[END_REF][START_REF] Donos | Laser ablation for mesial temporal lobe epilepsy: Surgical and cognitive outcomes with and without mesial temporal sclerosis[END_REF][START_REF] Hoppe | Laser interstitial thermotherapy (LiTT) in epilepsy surgery[END_REF][START_REF] Jermakowicz | Laser thermal ablation for mesiotemporal epilepsy: Analysis of ablation volumes and trajectories[END_REF][START_REF] Kang | Laser interstitial thermal therapy for medically intractable mesial temporal lobe epilepsy[END_REF][START_REF] Tao | Stereotactic EEG-guided laser interstitial thermal therapy for mesial temporal lobe epilepsy[END_REF][START_REF] Gross | Less is more: novel less-invasive surgical techniques for mesial temporal lobe epilepsy that minimize cognitive impairment[END_REF][START_REF] Youngerman | Laser ablation is effective for temporal lobe epilepsy with and without mesial temporal sclerosis if hippocampal seizure onsets are localized by stereoelectroencephalography[END_REF], High Intensity Focal Ultrasound (HIFU) [START_REF] Quadri | High-intensity focused ultrasound: past, and future in neurosurgery[END_REF][START_REF] Macdonell | Robotic Assisted MRI-Guided Interventional Interstitial MR Guided Focused Ultrasound Ablation in a Swine Model[END_REF], and SEEG-guided Radiofrequency Thermo-Coagulation (SEEG-guided RF-TC) [START_REF] Guénot | SEEG-guided RF thermocoagulation of epileptic foci: feasibility, safety, and preliminary results[END_REF][START_REF] Zhao | Stereotactic bilateral transfrontal minimal radiofrequency thermocoagulation of the amygdalohippocampal complex for bilateral medial temporal lobe epilepsy: a retrospective study of 12 patients[END_REF][START_REF] Bourdillon | Stereotactic Electroencephalography Is a Safe Procedure, Including for Insular Implantations[END_REF][START_REF] Moles | SEEG-guided radiofrequency coagulation (SEEG-guided RF-TC) versus anterior temporal lobectomy (ATL) in temporal lobe epilepsy[END_REF][START_REF] Cossu | Stereoelectroencephalography-guided radiofrequency thermocoagulation in the epileptogenic zone: a retrospective study on 89 cases[END_REF][START_REF] Cossu | Stereo-EEG-guided radio-frequency thermocoagulations of epileptogenic grey-matter nodular heterotopy[END_REF][START_REF] Mirandola | Stereo-EEG: Diagnostic and therapeutic tool for periventricular nodular heterotopia epilepsies[END_REF][START_REF] Cossu | RF-ablation in periventricular heterotopia-related epilepsy[END_REF][START_REF] Dimova | Radiofrequency thermocoagulation of the seizure-onset zone during stereoelectroencephalography[END_REF][START_REF] Luo | Bilateral stereotactic radiofrequency amygdalohippocampectomy for a patient with bilateral temporal lobe epilepsy[END_REF]. The present review focuses on the latter, which aims at coupling SEEG investigation with RF-TC stereotactic lesioning directly through the recording electrodes. This technique was first reported in 2004 in a feasibility study [START_REF] Guénot | SEEG-guided RF thermocoagulation of epileptic foci: feasibility, safety, and preliminary results[END_REF] with the initial aim to produce a lesion in the seizure onset zone, as an alternative to surgery. The subsequent publications highlighted the limits of this approach as a purely curative procedure and developed its use for palliative and diagnostic purposes [START_REF] Luo | Bilateral stereotactic radiofrequency amygdalohippocampectomy for a patient with bilateral temporal lobe epilepsy[END_REF][START_REF] Catenoix | SEEG-guided thermocoagulations: a palliative treatment of nonoperable partial epilepsies[END_REF][START_REF] Guénot | SEEG-guided RF-thermocoagulation of epileptic foci: a therapeutic alternative for drug-resistant non-operable partial epilepsies[END_REF]. More recently, the publication of several large series with long-term outcomes helped clarify the indications of SEEG-guided RF-TC. Nevertheless a metaanalysis recently highlighted the high heterogeneity of these studies [START_REF] Bourdillon | SEEGguided radiofrequency thermo-coagulations in patients with focal epilepsy: A systematic review and meta-analysis[END_REF].

Only one study has specifically focused on children [START_REF] Cossu | Stereo-EEG-guided radiofrequency thermocoagulations[END_REF], but large published series include a considerable pediatric population [START_REF] Cossu | Stereoelectroencephalography-guided radiofrequency thermocoagulation in the epileptogenic zone: a retrospective study on 89 cases[END_REF][START_REF] Bourdillon | Stereo electroencephalography-guided radiofrequency thermocoagulation (SEEG-guided RF-TC) in drug-resistant focal epilepsy: Results from a 10-year experience[END_REF] and do not report significantly different results in such patients.

Surgical technique

Materials

All of the studies investigating SEEG-guided RF-TC were conducted using a radiofrequency lesion generator system Cosman G3 or G4 (Cosman Medical, Burlington, MA, USA) connected to the SEEG electrodes through a dedicated electrode selector. The most frequently used SEEG electrodes are the Microdeep electrodes (Dixi Medical, Besançon, France) which have a diameter of 0.8 mm, and include 5 to 18 recording contacts that are 2 mm in length and 1.5 mm apart [START_REF] Guénot | SEEG-guided RF thermocoagulation of epileptic foci: feasibility, safety, and preliminary results[END_REF][START_REF] Cossu | Stereo-EEG-guided radio-frequency thermocoagulations of epileptogenic grey-matter nodular heterotopy[END_REF]; Alcis SEEG electrodes (Alcis -Temis Santé, Besançon, France) are also approved for coagulation procedures.

Electrode placement

As SEEG-guided RF-TC is an additional procedure performed during an SEEG, electrode placement technique of the latter procedure is crucial (see example in Figure 1). To be appropriate for RF-TC the electrode needs to reach the targeted anatomical structure with a good geometric accuracy in order to allow safe coagulations in rich vascular environments. These two aspects can be assessed through a common measure called the effective accuracy [START_REF] Bourdillon | Effective accuracy of StereoElectroEncephaloGraphy (SEEG): robotic tridimensional vs Talairach orthogonal approaches[END_REF]. Very few studies provide comparison between SEEG techniques, but the use of a stereotactic-frame, a surgical robot, and a per-operative angiography (specifically compared to pre-operative angio-MRI) seems to be associated with a better effective accuracy [START_REF] Bourdillon | Effective accuracy of StereoElectroEncephaloGraphy (SEEG): robotic tridimensional vs Talairach orthogonal approaches[END_REF][START_REF] Mullin | Stereo-Electro-Encephalo-Graphy (SEEG) With Robotic Assistance in the Presurgical Evaluation of Medical Refractory Epilepsy: A Technical Note[END_REF]. Concerning the overall safety of electrode insertion, a meta-analysis recently found SEEG to be safe [START_REF] Mullin | Is SEEG safe? A systematic review and meta-analysis of stereo-electroencephalography-related complications[END_REF]. Evidence is still, however, lacking to clearly identify technical risk factors associated with poor safety as there are many biases that could hamper interpretation of results, but the best safety is obtained by teams performing a frame-based procedure coupled to a per-operative angiography or angio-CT [START_REF] Bourdillon | Stereotactic Electroencephalography Is a Safe Procedure, Including for Insular Implantations[END_REF][START_REF] Mullin | Stereo-Electro-Encephalo-Graphy (SEEG) With Robotic Assistance in the Presurgical Evaluation of Medical Refractory Epilepsy: A Technical Note[END_REF][START_REF] Mullin | Is SEEG safe? A systematic review and meta-analysis of stereo-electroencephalography-related complications[END_REF][START_REF] Taussig | Stereoelectroencephalography (SEEG) in 65 children: an effective and safe diagnostic method for pre-surgical diagnosis, independent of age[END_REF][START_REF] Munari | Depth electrode implantation at Hôpital Sainte Anne[END_REF][START_REF] Mullin | 373 Is Stereotacticelectroencephalography Safe? A Systematic Review and Meta-Analysis of Stereoelectroencephalography-Related Complications[END_REF][START_REF] Cardinale | Stereoelectroencephalography: surgical methodology, safety, and stereotactic application accuracy in 500 procedures[END_REF].

Radiofrequency thermocoagulation

The French national guidelines on SEEG include a section dedicated to SEEG-guided RF-TC and are, to date, the only recommendations published on this topic [START_REF] Isnard | French guidelines on stereoelectroencephalography (SEEG)[END_REF][START_REF] Bourdillon | SEEG-guided radiofrequency thermocoagulation[END_REF]. They slightly differ from the initial description of the technique [START_REF] Guénot | SEEG-guided RF thermocoagulation of epileptic foci: feasibility, safety, and preliminary results[END_REF] as more recent laboratory investigations have found that two parameters determine the volume of the lesion: the dipole selection and the radiofrequency parameters [START_REF] Bourdillon | Stereo-electroencephalography-Guided Radiofrequency Thermocoagulation: From In Vitro and In Vivo Data to Technical Guidelines[END_REF][START_REF] Staudt | Radiofrequency Energy and Electrode Proximity Influences Stereoelectroencephalography-Guided Radiofrequency Thermocoagulation Lesion Size: An In Vitro Study with Clinical Correlation[END_REF].

The French guidelines recommend to create the dipole by selecting adjacent electrode contacts on a single electrode [START_REF] Isnard | French guidelines on stereoelectroencephalography (SEEG)[END_REF]. These contacts are conventionally located in cortical areas showing either a low amplitude fast pattern or spike-wave discharges at the onset of the seizures. Interictal paroxysmal activities can help to locate the ictal onset zone but when isolated are classically insufficient to be considered as a SEEG-guided RF-TC target [START_REF] Catenoix | SEEG-guided thermocoagulations: a palliative treatment of nonoperable partial epilepsies[END_REF]. Nevertheless, because of the high specificity of the spiking signature of type II focal cortical dysplasias [START_REF] Chassoux | Stereoelectroencephalography in focal cortical dysplasia: a 3D approach to delineating the dysplastic cortex[END_REF], SEEG-guided RF-TC can be exceptionally performed where this signature is observed, even if no seizure has been recorded. Functionally evaluating all the possible targets by bipolar electrical stimulations (low and high frequency pulse stimulation up to 3mA) during the video-SEEG recording session is recommended to avoid side effects [START_REF] Isnard | French guidelines on stereoelectroencephalography (SEEG)[END_REF]. RF-TC may be reconsidered on dipoles whose stimulation provokes a neurological deficit. The direct electric stimulations for functional mapping are similar to those used for triggering seizures during the SEEG or when studying the functional connectivity within a network, and can be either low frequency shock or high frequency train stimulations. Train stimulations are particularly relevant for functional mapping of primary areas. The parameters for shock stimulations are 1 Hz frequency; a shock duration of 0.5 to 3 ms; an intensity of 0.5 to 4 mA (progressively increased); a stimulation duration of 20 to 60 s. The parameters for train stimulation are 50 Hz frequency; a shock duration of 0.5 to 1 ms; an intensity of 0.5 to 5 mA (progressively increased); a stimulation duration of 3 to 8 s [START_REF] Isnard | French guidelines on stereoelectroencephalography (SEEG)[END_REF].

Parameters of radiofrequency current can be adapted to optimize the size of the lesion. For each coagulation site, the maximal lesion volume may be reached by using parameters adjusted according to the impedance (which indicates when coagulation occurs), whereas the lesion size obtained using standard fixed parameters is usually smaller (see Figure 2) [START_REF] Bourdillon | Stereo-electroencephalography-Guided Radiofrequency Thermocoagulation: From In Vitro and In Vivo Data to Technical Guidelines[END_REF]. The power delivered (in practice, the voltage, since the intensity is automatically adapted by the radiofrequency generator) is therefore increased until the impedance suddenly increases (usually within a few seconds); in vitro data suggest that this is related to the coagulation of proteins producing a sudden modification of the resistance that the tissue applies to the delivered sinusoidal current [START_REF] Bourdillon | Stereo-electroencephalography-Guided Radiofrequency Thermocoagulation: From In Vitro and In Vivo Data to Technical Guidelines[END_REF]. After this impedance change, any extra current delivered does not produce any increase of the size of the lesion.

Indications

There is no high-level evidence concerning the indication of SEEG-guided RF-TC, and therefore the points presented in this section must not be interpreted as recommendations but as insight about frequent clinical situations and when to consider this procedure in the diagnostic/therapeutic algorithm.

The first historical indication of SEEG-guided RF-TC, which is still the most frequent one, concerns patients in whom a SEEG is needed to determine whether surgery is feasible and in whom resection finally appears to be indeed possible [START_REF] Catenoix | SEEG-guided thermocoagulations: a palliative treatment of nonoperable partial epilepsies[END_REF]. When the seizure onset zone is small enough to be entirely covered by the sum of the coagulation volumes, SEEG-guided RF-TC may be sufficient to cure the patient. In other cases, when only a portion of the seizure onset zone can be targeted by the RF-TC, the procedure remains a very helpful prognostic tool: the improvement of the epileptic status of the patient, even partial and time-limited, predicts with a high positive predictive value (93%) good outcome after conventional surgery [START_REF] Bourdillon | Stereo electroencephalography-guided radiofrequency thermocoagulation (SEEG-guided RF-TC) in drug-resistant focal epilepsy: Results from a 10-year experience[END_REF]. It is worth mentioning that the negative predictive value is low; the absence of improvement of epilepsy after an SEEG-guided RF-TC does not therefore constitute an argument against surgical resection if the phase II investigations conclude to the presence of an accessible seizure onset zone. Very rarely, a partial lesion of a type II focal cortical dysplasia can lead to an increase of the seizure frequencyin which case rapid conventional surgery can be necessary if the adjustment of the drug regimen is insufficient to stabilize the situation. Concerning the very specific situation of temporal lobe epilepsy (TLE), for which LiTT is becoming increasingly popular as an alternative to amygdalohippocamtectomy, evidence indicates that SEEG-guided RF-TC is inferior to conventional surgery, and therefore SEEG must not be only justified by the possibility of performing stereotactic lesions in such cases [START_REF] Moles | SEEG-guided radiofrequency coagulation (SEEG-guided RF-TC) versus anterior temporal lobectomy (ATL) in temporal lobe epilepsy[END_REF].

More recently, some patients in whom phase I investigations concluded to a surgical contraindication underwent SEEG in a purely therapeutic perspective to perform SEEG-guided RF-TC. This situation can correspond to that of a small, but clearly inaccessible to resective surgery, suspected ictal onset zone requiring a SEEG confirmation. A good example would be an anterior insular focal cortical dysplasia covered by a language dominant operculum in a patient who cannot sustain awake surgery, or a periventricular nodular heterotopia sited along the tapetum [START_REF] Mirandola | Stereo-EEG: Diagnostic and therapeutic tool for periventricular nodular heterotopia epilepsies[END_REF][START_REF] Cossu | RF-ablation in periventricular heterotopia-related epilepsy[END_REF]. Moreover, when the surgical contraindication found on phase I investigations is a large multi-lobar epileptic network, SEEG-guided RF-TC can be indicated as a palliative option. The goal is then to target crucial nodes within the epileptic network in order to limit the organization and the propagation of seizures [START_REF] Catenoix | The combination of stereo-EEG and radiofrequency ablation[END_REF]. Nevertheless, the definition of the network's nodes remains largely empirical (network modelling is not yet usable in clinical practice).

Concerning hypothalamic hamartomas, a preliminary study using SEEG-guided RF-TC has recently been published [START_REF] Wei | Stereoelectroencephalography-Guided Radiofrequency Thermocoagulation for Hypothalamic Hamartomas: Preliminary Evidence[END_REF]. However, this type of lesional epilepsy exceptionally requires phase II investigations. Moreover, the good results obtained by surgery and radiosurgery [START_REF] Régis | Gamma knife surgery for epilepsy related to hypothalamic hamartomas[END_REF], the better volume control of LiTT, and the difficulty to perform a meaningful functional mapping in this area makes it harder to justify SEEG-guided RF-TC in this condition. When economic constraints limit the access to the reference techniques, standard monopolar RF-TC remain an option [START_REF] Tandon | Stereotactic Radiofrequency Thermocoagulation of Hypothalamic Hamartoma Using Robotic Guidance (ROSA) Coregistered with O-arm Guidance-Preliminary Technical Note[END_REF].

The last very empirical and restricted indication is the use of SEEG-guided RF-TC as a diagnostic tool when multiple hypotheses are difficult to differentiate during a SEEG. In this specific case, the electrodes are left in place after the RF-TC and the SEEG continues. The analysis of the consequences of the lesions performed within the network can help to check the validity of the different hypotheses.

Outcome

A recent meta-analysis that included 6 studies [START_REF] Zhao | Stereotactic bilateral transfrontal minimal radiofrequency thermocoagulation of the amygdalohippocampal complex for bilateral medial temporal lobe epilepsy: a retrospective study of 12 patients[END_REF][START_REF] Cossu | Stereoelectroencephalography-guided radiofrequency thermocoagulation in the epileptogenic zone: a retrospective study on 89 cases[END_REF][START_REF] Mirandola | Stereo-EEG: Diagnostic and therapeutic tool for periventricular nodular heterotopia epilepsies[END_REF][START_REF] Dimova | Radiofrequency thermocoagulation of the seizure-onset zone during stereoelectroencephalography[END_REF][START_REF] Luo | Bilateral stereotactic radiofrequency amygdalohippocampectomy for a patient with bilateral temporal lobe epilepsy[END_REF][START_REF] Bourdillon | Stereo electroencephalography-guided radiofrequency thermocoagulation (SEEG-guided RF-TC) in drug-resistant focal epilepsy: Results from a 10-year experience[END_REF] and a total of 296 patients found a seizure-free rate of 23% and a responder rate of 58% one year after the SEEG-guided RF-TC, responders being defined as those with 50% or greater reduction in seizure frequency as compared to the pre-procedure period [START_REF] Bourdillon | SEEGguided radiofrequency thermo-coagulations in patients with focal epilepsy: A systematic review and meta-analysis[END_REF]. At the study level, meta-regression failed to identify factors impacting the seizurefree rate, including the number of RF-TC. Furthermore, therapeutic efficacy was very heterogeneous, limiting the interpretation of the pooled result. It also remained high in sub-group analyses which indicates that heterogeneity was not only related to variable efficacy with regards to the underlying lesion and that it could, in part, be related to variations in the management of SEEG-guided RF-TC across centers.

It is worth mentioning two specific etiologies in which the results differ from the general results and must lead to specific indications or contraindications. On the one hand, results of SEEG-guided RF-TC in periventricular nodular heterotopias are particularly interesting: among such patients treated with SEEG-guided RF-TC it has been reported that 38% were seizure free and 83% were responders [START_REF] Bourdillon | SEEGguided radiofrequency thermo-coagulations in patients with focal epilepsy: A systematic review and meta-analysis[END_REF].

On the other hand, in TLE a controlled study found that SEEG-guided RF-TC was inferior to anteriortemporal lobectomy (ATL); none of the patients who underwent SEEG-guided RF-TC was seizure-free at one year (versus 75% in the ATL group), and only 48% percent were responders (versus 100% in the ATL group) [START_REF] Moles | SEEG-guided radiofrequency coagulation (SEEG-guided RF-TC) versus anterior temporal lobectomy (ATL) in temporal lobe epilepsy[END_REF]. No other published study has focused specifically on other etiologies and sub-group analysis has failed to identify other etiologies as having good or bad prognosis [START_REF] Bourdillon | SEEGguided radiofrequency thermo-coagulations in patients with focal epilepsy: A systematic review and meta-analysis[END_REF].

Emergence of SEEG-guided RF-TC has led to extend indications of SEEG for patients who are not eligible for conventional surgery. The implantation strategies may sometimes have also deeply evolved, for instance in over implanting a suspected well-limited seizure onset zone in order to perform a curative RF-TC procedure or to target network nodes for patients in whom only a palliative approach may be considered. Nevertheless these modifications of SEEG implantation strategies are very difficult to assess and none of the published studies investigating SEEG-guided RF-TC provide an evaluation of this.

Concerning safety, the previously mentioned meta-analysis found homogeneous results across studies.

The pooled complication rate of SEEG-guided RF-TC was 2.5% and, among those reported, only one (0.3%) was a non-expected deficit with minimal consequences on the patient's daily life (hypoesthesia of the right thumb) [START_REF] Bourdillon | SEEGguided radiofrequency thermo-coagulations in patients with focal epilepsy: A systematic review and meta-analysis[END_REF]. The other complications were neurological deficits that were anticipated during the pre-procedure functional mapping by direct electric stimulation and corresponding to a very well circumscribed seizure onset zone situated in the primary motor area. The RF-TC was thus carefully discussed with the patients who were aware that the deficit noticed during functional mapping would occur after the lesion and may permanently remain [START_REF] Bourdillon | SEEGguided radiofrequency thermo-coagulations in patients with focal epilepsy: A systematic review and meta-analysis[END_REF].

Conclusion

The increasing number of studies investigating SEEG-guided RF-TC have established that this surgical procedure is remarkably safe. More evidence is necessary to determine the exact place of SEEG-guided RF-TC in the surgical management of drug-resistant epilepsies, but two situations emerge in which SEEG-guided RF-TC may be considered in clinical practice. First, in patients requiring a SEEG in their presurgical evaluation, and when SEEG succeeds in delineating an accessible seizure onset zone, SEEGguided RF-TC should be considered; although the probability of avoiding surgery by curing the patient is low the positive predictive value of a possible effect, even partial, of the RF-TC is valuable in the decision to proceed or not with surgery. Secondly, when the suspected seizure onset zone is not surgically accessible, but limited in size, a SEEG could be justified for the purpose of performing a SEEG-guided RF-TC, even if surgical resection is not an option. A good illustration of this indication is periventricular nodular heterotopias; these are rarely accessible to surgery and require SEEG to determine the respective contribution of the heterotopia and neocortex in the epileptic network.

Subsequent SEEG-guided RF-TC allows a one-step procedure providing very encouraging results and might therefore be considered as an effective first-line treatment for periventricular nodular heterotopias -related drug-resistant epilepsy, LiTT being an interesting alternative option. Concerning TLE, SEEGguided RF-TC is inferior to the gold standard technique and should not be used as an alternative.

Disclosure

None of the authors have any conflict of interest to disclose. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.

  Consequently, we chose to include all published studies investigating SEEG-guided RF-TC. For this, three electronic databases were searched (MEDLINE, Cochrane (CENTRAL) and the Web of Science) using the following search strategy: [Stereoelectroencephalography OR Stereo-electroencephalography OR SEEG] OR [Stereotactic AND ("Electroencephalography" OR "EEG")] AND [Thermocoagulation OR TC OR radiofrequency OR RF OR radiofrequency-thermocoagulation OR RT-TC] OR [thermos-SEEG OR thermoSEEG] AND [Epilepsy OR Epileptic], limited to the period from January 1, 2004 to December 31, 2017. We will nonetheless provide additional insights about specific pediatric situations.

Figure 1 :

 1 Figure 1: Procedure workflow.

Figure 2 :

 2 Figure 2: a. in vitro aspect of multiple thermocoagulation on a single electrode; b. MRI aspect of multiple

Acknowledgements

We thank Dr Philip Robinson (DRCI, Hospices civils de Lyon, France) for help in manuscript preparation and English editing.