Zixian Jia

Youcef Fermi

Abdoulaye Siby

Ovidiu Brinza

Khaled Hassouni

Swaminathan Prasanna
email: swaminathan.prasanna@lspm.cnrs.fr

Enhanced gas-phase nucleation of diamond nanoparticles in a microplasma torch

Keywords: microplasma, microwave plasma, diamond nanoparticles, carbon nanostructures

In this work, we present and characterize a microwave plasma microtorch that can support the formation of diamond nanoparticles (DNPs) in the gas phase when using hydrogen-methane as a precursor gas. We first showed how the non-equilibrium character of the plasma obtained in this microtorch ensures enhanced H-atom, C 2 and CH 3 production at moderate gas temperatures levels that which provides thermal stability of DNPs. The material characeterization by TEM, Raman and EELS spectra confirm the presence of substantial amount of diamond phase in the carbon dust particles collected from the plasma phase. Investigation of the effects of injected MW power on the process characteristics showed a direct correlation between enhanced C 2 production and appearance of diamond phase, which points out to the critical role of C 2 as far as gas-phase nucleation of DNPs is concerned.

Introduction

Nano-diamonds or Diamond nanoparticles (DNPs) are known to have outstanding properties such as superior hardness, high chemical stability and inertness to 1 chemical environments, high thermal conductivity, good biocompatibility and easy surface functionalization, making it suitable for a wide range of applications in the fields of medicine, tribology, catalysis, electronics and energy [START_REF] Vadym N Mochalin | The properties and applications of nanodiamonds[END_REF][START_REF] Nunn | Nanodiamond: a high impact nanomaterial[END_REF][START_REF] Shenderova | Synthesis, properties, and applications of fluorescent diamond particles[END_REF][START_REF] Kumar | Nanodiamonds: Emerging face of future nanotechnology[END_REF][START_REF] Yu Dolmatov | Detonation nanodiamonds: new aspects in the theory and practice of synthesis, properties and applications[END_REF]. Further, doping the DNPs with elements such as N, Si or B in order to produce color-centers hold tremendous promise to transcend the limits of different applications in biomedical, microelectronics and quantum technologies [START_REF] Shenderova | Synthesis, properties, and applications of fluorescent diamond particles[END_REF][START_REF] Kumar | Nanodiamonds: Emerging face of future nanotechnology[END_REF].

As a consequence, the synthesis of DNPs has attracted lot of attention and can be approached by several strategies such as detonation [START_REF] Krüger | Unusually tight aggregation in detonation nanodiamond: identification and disintegration[END_REF][START_REF] Yu Dolmatov | Detonation nanodiamonds: new aspects in the theory and practice of synthesis, properties and applications[END_REF], milling of natural or high-pressure high-temperature (HPHT) single diamond crystals, laser ablation [START_REF] Amans | Nanodiamond synthesis by pulsed laser ablation in liquids[END_REF], high-pressure high-temperature technique (HPHT) [START_REF] Qian | Graphitization of diamond powders of different sizes at high pressure-high temperature[END_REF], ion irradiation of graphite [START_REF] Tl Daulton | Production of nanodiamonds by high-energy ion irradiation of graphite at room temperature[END_REF]etc. However, major bottlenecks remain with respect to the extended adoption of DNPs as most applications demand DNPs with high quality, i.e. a low surface coverage of sp 2 hybridized carbon, and a narrow size distribution around 10 nm [START_REF] Shenderova | Synthesis, properties, and applications of fluorescent diamond particles[END_REF][START_REF] Kumar | Nanodiamonds: Emerging face of future nanotechnology[END_REF]. The traditional methods of producing DNPs such as detonation [START_REF] Yu Dolmatov | Detonation nanodiamonds: new aspects in the theory and practice of synthesis, properties and applications[END_REF] and milling suffer from large particle size distributions, low crystalinity and large amount of undesirable impurities [START_REF] Shenderova | Synthesis, properties, and applications of fluorescent diamond particles[END_REF]. Furthermore, applications based on color centers demand homogeneous distributions of dopant impurities which remains a consistent limitation of these traditional production methods [START_REF] Shenderova | Synthesis, properties, and applications of fluorescent diamond particles[END_REF].

In this regard, a bottom-up production strategy has been recongnized to meet the specific demands where DNPs can be nucleated from a careful selection of hydrocarbon and dopant precursors [START_REF] Shenderova | Synthesis, properties, and applications of fluorescent diamond particles[END_REF]. Plasma based processes can facilitate conditions supporting mild nucleation and growth of DNPs making them suitable for bottom-up strategy of producing not only pure but also doped DNPs. CVD diamond has been classically grown under microwave (MW) assisted plasmas through heterogeneous growth on a substrate that has been pre-treated or directly on single diamond crystals. These processes have been further exploited to grow micrometric sized high-quality diamond crystals with and without doping [START_REF] Shenderova | Synthesis, properties, and applications of fluorescent diamond particles[END_REF][START_REF] Michael Nr Ashfold | Nitrogen in diamond[END_REF][START_REF] De Feudis | Large-scale fabrication of highly emissive nanodiamonds by chemical vapor deposition with controlled doping by siv and gev centers from a solid source[END_REF][START_REF] Tallaire | High nv density in a pink cvd diamond grown with n2o addition[END_REF]. Further investigations in the recent past have been able to demonstrate homogeneous nucleation of DNPS with a wide variety of hydrocarbon precursor gases such as methane [START_REF] Gries | Diagnostics in dusty c-h-o plasmas with diamond and graphitic nanoparticle generation[END_REF], ethanol [START_REF] Kumar | Formation of nanodiamonds at near-ambient conditions via microplasma dissociation of ethanol vapour[END_REF][START_REF] Iqbal | Impact of carrier gas flow rate on the synthesis of nanodiamonds via microplasma technique[END_REF], halogenated hydrocarbons [START_REF] Mrdwke Frenklach | Homogeneous nucleation of diamond powder in the gas phase[END_REF][START_REF] Mwdjker Frenklach | Induced nucleation of diamond powder[END_REF] etc. The carbon nanostructures produced using these gas phase nucleation processes were basically complex hybrid aggregates of graphitic carbon, amorphous carbon, and crystals of DNPs of 2-10 nm. This demonstrates that plasma process enabling homogeneous nucleation of DNPs has tremendous potential to meet the critical challenges with regard to purity, crystallinity and size distribution.

While the growth of CVD diamond films in a moderate-pressure CVD process is quite well known and understood [START_REF] Gicquel | Cvd diamond films: From growth to applications[END_REF], no such dedicated process and fundamental understanding of homogeneous nucleation and early growth of DNPs in plasma exists to date. As a matter of fact, while CH 3 and H-atom have been recognized as key species in the growth of CVD diamond [START_REF] Gicquel | Cvd diamond films: From growth to applications[END_REF][START_REF] Heintze | Mechanism of c 2 hydrocarbon formation from methane in a pulsed microwave plasma[END_REF], there is no such agreement in the litterature on the key species that induce early nucleation of nanodiamond in the gas phase or on solid substrates [START_REF] Gicquel | Cvd diamond films: From growth to applications[END_REF]. Early studies on nanocrystalline diamond (NCD) deposition mentionned the key-role that may be played by C 2 as a driver for the nucleation of nanocristalline sp 3 domain [START_REF] Dieter M Gruen | Carbon dimer, c2, as a growth species for diamond films from methane/hydrogen/argon microwave plasmas[END_REF] . On the same line, Dolmatov et al. [START_REF] Yu Dolmatov | Detonation nanodiamonds: new aspects in the theory and practice of synthesis, properties and applications[END_REF] speculate that C 2 may be the key primary radical responsible for the nucleation and growth of DNPs by detonation process. According to these authors, the highly reactive C 2 radicals react with themselves to form cyclo-hexane which subsquently undergo further molecular growth involving methyl radical and H-atom, just like in the case of classical CVD diamond, to form sp 3 carbon diamondoid structures. In summary, these investigations show that the production of diamond nanoparticles in the gas phase requires a nucleation step that is likely driven by C 2 along with early growth step that requires CH 3 radical and H-atom. Besides, the production of DNPs in the plasma phase requires local plasma characterisrtics that meet the stability conditions for DNPs once nucleated. In particular, the gas temperature should be moderate, typically below 2000k, in order to avoid graphitization [START_REF] De | Graphitization of diamond (111) studied by first principles molecular dynamics[END_REF].

It appears therefore that the production of diamond nanoparticles in the plasma phase requires plasma conditions that enhance C 2 , CH 3 and H-atom production while keeping moderate gas temperature. Such conditions can be hardly met in classical diamond plasma CVD systems that were specifically designed to achieve large CH 3 /H densities with a little attention to C 2 production and where the gas temperature level is generally not optimal as far as DNPs stability is concerned. This point partly motivates our objective to develop a novel plasma process that is suitable for gas phase production of DNPs. Our strategy is to use the peculiarity of microplasmas that provide very high electron density thus enhancing the electron driven production of a variaty of H-C radicals and H-atom under fairly moderate gas temperatures. More specifically, we report the preliminary results on the synthesis of DNPs in a MW assisted microplasma torch using a gas mixture of hydrogen and methane. We will first present the MW-plasma device developed in this work and its characterization in terms of gas temperature, electron and H-atom densities. Then, in the second part of the paper, we will discuss the characteristics of the carbanaceous materials synthesized in the torch and discussing its characteristics which clearly shows the gas phase nucleation of diamond nanoparticles. We will eventually end up the paper by discussing the obsreved correlations between the plasma and material characteristics

MW plasma torch characterization

The 2.45 GHz MW plasma microtorch (Figure 1) was developed in-house and is based on coaxial transmission line resonator (CLTR) coupling techniques [START_REF] Choi | Microwave-excited atmospheric-pressure microplasmas based on a coaxial transmission line resonator[END_REF]. The length of the coaxial resonator is approximately equal to 3λ/4 which facilitates the electromagnetic resonance at one end of the torch. Further, a small gap distance of 250 µm at the end of the torch, between the central brass rod and the annular grounded head, coupled with effect of resonance ensures a strong electromagnetic field that sustains the plasma. The as created plasma volume is very small (O(mm -3 ) resulting in MW power densities as high as 5 × 10 9 W/m -3 , which is three orders of magnitude greater than in a conventional CVD reactor used for diamond growth [START_REF] Prasanna | Self-consistent simulation studies on effect of methane concentration on microwave assisted h2-ch4 plasma at low pressure[END_REF]. This provides a distinct advantage for producing highly non-equilibrium discharges with high electron densities and moderate gas temperatures which can realize a wide range of reactive environments with charged species, radicals and photons over dimensions limited to few millimetres. The present torch can be operated with different gas mixtures such as argon, hydrogen, methane, acetylene and ethanol, and over a wide range of pressure conditions (10 mbar -1 atm). Nevertheless, in this paper we will restrict our discussion to a precursor gas mixture composition of H 2 (96%) and methane (4%) and an operating pressure of 100 mbar where substantial amount of diamond nanoparticles were produced.

The local plasma conditions such as rotational temperature, electron and H-atom density, were characterized using optical emission spectrocopy (OES) with the help of spectrometers having 1 m (THR1000) and 2m (SOPRA) focal lengths. The latter spectrometer was used only for electron density measurements where highly resolved line profile monitoring is required. The gas temperature, which may be inferred from the rotational temperature, and the electron density and temperature are the main driver of the gas phase chemistry. In our experiments, the Q-branch of H 2 Fulcher-α band was used to estimate the H 2 rotational temperatures from the slope of the Boltzmann plot corresponding to the selected rotational lines Q1, Q2, Q3 and Q6 (Fig. S1 ). It is expected that for the moderate pressure conditions used in our experiments, the upper excited states d 3 u of Fulcher band would undergo collisions before raditative de-excitation. Therefore, the use of the lower state rotational constant for determining the rotational temperature as commonly used for very low pressure plasma is questionable. On the otherhand the use of the upper state rotational constant to determine the temperature requires full equilibriation of the rotational mode of the excited levels which is not guaranted in our conditions. Also, other spectroscopic bands such as the R branch of the transition G 1 g , ν = 0

→ B 1 +
u of H 2 or Swann band of C 2 which can also give information on the gas temperature, were unfortunately not accessible for the discharge conditions of interest. We therefore estimated upper-and lower-limit values of the rotational temperature corresponding to two asymptotic situations : negligible collision effect and full rotational equilibration of the excited states, respectively. The actual gas temperature is expected between these two limits.

In fact, taking into account the collisional cross-section of d 3 u 2×10 -15 cm -2 [START_REF] Rk Garg | Gas temperature measurements in a microwave plasma by optical emission spectroscopy under single-wall carbon nanotube growth conditions[END_REF], the collisional and radiative lifetimes are comparable i.e. 50 ns and 60 ns respectively . This means that the excited states are likely to undergo few collisions before radiative deexcitation. Consequently, the gas temperature is likely to be much closer to the upper limit. As shown in Fig. 2, the lower and upper limits fo the rotational temperature increase from 525 to 760 K and from 1000 to 1500 K respectively, when the injected power increases from 30 to 90 W. Most importantly, the gas temperature remains below 1500 K which avoids graphitization of nanodiamond in the plasma phase. The limited gas heating that characterize our system is due to the large surface to volume ratio and the effective cooling of the torch.

Electron density were estimated from the stark broadening of the H β line [15] that was captured using the 2m focal length spectrometer SOPRA (c.f. Figure S2). For our plasma conditions, the broadening of H β line is well represented by a voigt profile where the Gaussian component comprises of the instrumental and Doppler broadening while the Lorentzian component includes Van der Waals and, more importantly, Stark broadenings. The Doppler and Van der Waals line-widths were estimated using the H 2 rotational temperature. Typical values for these were 9 and 5 pm respectively. Using the mercury line at 576 nm, the instrumental broadening was estimated as 7.66 pm. The Stark linewidth of H β line obtained by eliminating the instrument, Doppler and Van der Waals line-widths is about 10 pm. It is clear that the broadening is dominated by Stark effect and therefore the uncertainty in the gas temperature does not affect the electron density substantially. The electron densities remained constant with injected MW power with typical values of 2×10 13 cm -3 (c.f. Figure S3). This is two orders of magnitude greater than the values measured in a conventional resonating MW reactor used for CVD growth of diamond [START_REF] Hassouni | Modelling of diamond deposition microwave cavity generated plasmas[END_REF]. The direct consequence of such high electron densities is enhancement of electron impact processes.

H-atom density was determined by performing H-atom actinometry [START_REF] Gicquel | Validation of actinometry for estimating relative hydrogen atom densities and electron energy evolution in plasma assisted diamond deposition reactors[END_REF] where trace amounts of Ar (1%) are added without altering the plasma char- acterisitics. The H-atom densities are inferred directly from the ratio of line intesities of H α to Ar line at 750 nm using the calibration method thoroughly described in [START_REF] Gicquel | Validation of actinometry for estimating relative hydrogen atom densities and electron energy evolution in plasma assisted diamond deposition reactors[END_REF]. Large concentrations of H-atom of about 6% (c.f. Fig. 2) was found in our conditions which are of the same order of magnitude to that obtained in the resonant MW cavity used for diamond CVD. However, unlike in the case of resonant cavity MW reactor where the thermal processes are responsible for the dissociation of hydrogen , the gas temperature in our conditions is not high enough to induce thermally driven dissociation, and the H-atom production is mainly ensured by the electron impact processes. As for hydrocarbon molecules, we could detect strong emission from the Swann molecular band of C 2 , the intensity of which increased with the input MW power. Unfortunately, we could not detect the emission lines from CH molecules that could be used as an indicator for the concentration of CH 3 in our conditions. Therefore, we adapted the 0D model described in [START_REF] Hassouni | Investigation of chemical kinetics and energy transfer in a pulsed microwave h2/ch4 plasma[END_REF] to our conditions in order to estimate the densities of key hydrocarbon species, especially C 2 and CH 3 . The results from the simulations indicate a substantial increase in the concentrations of CH 3 and C 2 with MW power as seen in Figure 3(b). H-atom and electron rich conditions are responsible for the formation of CH 3 radical through both the H-shifting and electron impact reactions of CH 4 that produces CH 3 radical. The production of C 2 through thermal processes is very unlikely for the gas temperature values that characterize our plasmas. However, the very high electron densities in our conditions strongly enhance the formation of this radical through electron impact on intermediate hydrocarbon species such as C 2 H 2 and C 2 H 4 that are effectively produced thermally in the gas temperature range that characterizes our plasma [START_REF] Hassouni | Modelling of diamond deposition microwave cavity generated plasmas[END_REF]. As a result, the concentration of C 2 is comparable with that of CH 3 at 90 W which shows that these species can indeed play a key-role in hydrocarbon molecular growth and subsequent carbon nanoparticle nucleation as suggested by several studies and discussed in the next section for our discharge conditions. The carbon nanostructures produced in our discharge were collected on a substrate placed down stream the reactor. A variety of carbon nanostructures have been collected for different precursor feed gas compositions (Argon + Hydrogen and methane), working pressure and injected MW power. Scanning electron microscopy (SEM) (Zeiss Supra 40 VP SEM-FEG) was used to characterize the morphology of carbon nanoparticles while TEM observation was performed in a JEOL 200 kV microscope with the carbon nanoparticles suspension drop-casted onto a carbon TEM grid. The Raman spectra of the samples at room temperature were measured in the backscattering configuration using a HR800 spectrometer equipped with a Peltier-cooled CCD detector (Horiba Jobin Yvon) with spectral and spatial resolutions of 0.25 cm -1 and 5 µm, respectively. Raman measurements were carried out by focusing the probe beam at 473 nm with a 50x objective on the carbon particles.

Material characterization using SEM (Figure S4) and TEM indicated an agglomerated nanostructures deposit with particle size around 10 nm. EDX shows that the nanostructures contain predominantly C with traces of O and Si due to pollution from the substrate during the characterization(c.f. Fig S4). Figure 4 shows the TEM images and their corresponding electron diffraction pattern for the carbon nanostructures collected at power of 90 W. Under these conditions, hybrid nanostructures containing both graphitic carbon and diamond were observed as seen in with diamond (111) planes and graphite (002) planes, respectively. The crystaline nature of the collected carbon nanostructres is further confirmed by the observation of concentric rings in the electron diffraction patterns with strong indication of diamond phase as seen in Fig 4b . EELS spectra in Fig. 5 show the 1s σ * bond which correspond to sp 3 -hybridised carbon. The presence of some amounts of sp 2 carbons is also indicated by 1s π * feature. This establishes that the diamond nanoparticles along with impurities of sp 2 hybridized and other amorphous carbon are formed in these conditions.

Figure 6 shows the evolution of the raw and fitted Raman spectra (obtained with a laser excitation at 473 nm) of the carbon nanostructures collected on a silicon substrate placed at a distance of 1 cm from the plasma for different absorbed MW power. The quantity of collected carbon nanostructures increases with the MW power. Prominent peaks of sp 2 carbon i.e. D peaks at 1280 and 1385 cm -1 and G peak at 1598 cm -1 are present for all the conditions [START_REF] Andrea | Resonant raman spectroscopy of disordered, amorphous, and diamondlike carbon[END_REF]. The diamond peak located around 1332 cm -1 appeared in the Raman spectra of the carbon nanostructures only for MW powers above 70 W. For bulk diamond with high crystalline quality, this peak is usually sharp. The diamond nanoparticles obtained in our conditions are however of nm-sized grains. As a consequence, due to phonon confinement, an asymmetrical broadening was observed on this diamond peak which was blue shifted to 1329 cm -1 [START_REF] Vitaly I Korepanov | Carbon structure in nanodiamonds elucidated from raman spectroscopy[END_REF]. In addition to this, the Raman spectra shows a peak around 1129 cm -1 which is associated with the µ 1 mode of transpolyacetylene [START_REF] Andrea | Resonant raman spectroscopy of disordered, amorphous, and diamondlike carbon[END_REF] and the ratio of this peak to the G-peak increased with MW power.

From the Raman spectra, the ratio of Diamond peak at 1332 cm -1 and sp 2 carbon G-band is used to access the trends of sp 3 /sp 2 ratio. Figure 7 shows the The power has a positive effect on sp 3 carbon formation. In addition, the I D /I G ratio decreases from 0.53 to 0.33 with increasing power. Similar results reported in the literature also show that I D /I G ratio decreases with increasing sp 3 fraction in the amorphous carbon films [START_REF] Paik | Raman and xps studies of dlc films prepared by a magnetron sputter-type negative ion source[END_REF] [START_REF] Cai | Investigation of (ti: N)-dlc coatings prepared by ion source assisted cathodic arc ion-plating with varying ti target currents[END_REF].

Growth of carbon nanostructures can follow two different pathways (i) homogeneous nucleation in the gas phase or (ii) heterogeneous growth at the surface of the substrate. While homogeneous nucleation is driven by gas phase molecular growth and nucleation, the heteregeneous growth is driven by surface stabilization offered by the substrate as observed during the CVD growth of diamond. The signature of carbon nanostructures produced in our experiments was insensitive to the nature of the different substrates used such as silicon and fibre glass. Moreover, there was no pre-treatment of the substrate whose temperature was almost constant ( 340 K) in all the experiments. The signature of the Raman spectra of the carbon nanostructures collected were very similar when the distance between the substrate and the plasma was varied in the range 2.5 mm and 2 cm. Similarly, this was true with respect to the carbon nanostructures collected over a wide range of deposition duration, i.e., 5 to 60 minutes. All these experimental facts clearly show that the the nucleation of DNPs in our discharge occurs in gas phase. However, we cannot rule out the possibility of subsequent evolution of homogeneously nucleated DNPs containing nanostructures, once deposited on the susbtrate.

When correlating the results of plasma and material characterization, one can clearly see that the production of DNPs in the gas phase takes place when C 2 and CH 3 densities are strongly enhanced, i.e. basically for a power value above 70 W. In particular, no DNPs were observed despite the existence of significant amount of H-atom and substantial amount of CH 3 at low MW injected power. Therefore, our results tend to confirm that the existence of a substantial amount C 2 is required to nucleate DNPs in gas phase. Most probably, C 2 radical is responsible for the formation of small hydrocarbon structures which act as seeds that further grow through the classical mechanism involving CH 3 and H. Examples of such structures and their formation mechanism from C 2 have been suggested in [START_REF] Yu Dolmatov | Detonation nanodiamonds: new aspects in the theory and practice of synthesis, properties and applications[END_REF]. Unfortunately, the increase in C 2 is accompanied by the enhancement of transpolyacetylene which is initiated by C 2 radical through polymerization that results in unsaturated carbon chains such as (-C*=C*-). Moreover, the H-atom rich conditions further stabilize these large transpolyacetylene structures. This shows that optimal conditions of DNPs gas phase production requires finding the discharge conditions that inhibits the formation of polyacetylene structures while still ensuring large enough C 2 density to produce molecular structures that lead to nucleation of DNPs.

Conclusion

We have constructed a MW microplama source which can produce conditions that favour the homogeneous nucleation of DNPs. Specifically, the high-electron densities that characterize our microplasma provided H-atom, C 2 and CH 3 rich conditions with moderate gas temperatures that can support molecular growth pathways which end up with nucleation of DNPs. The correlation between the results of plasma diagnostics and material characterization clearly indicates the importance of carbon radicals C 2 and CH 3 . Particularly, the direct correlation between the appearance of diamond phase and enhanced concentrations of C 2 at high MW power points to the critical role played by C 2 during the early molecular growth that leads to DNPs nucleation. Further research is needed to identify the intermediate HC species that are involved in the molecular growth mechanism leading to DNPs. This may be achieved using molecular beam mass spectrometry. Also, substantial improvement of the plasma phase DNP production proces is possible by limiting the competetive formation of transpolyacetylene under C 2 rich conditions that are also required for DNPs nucleation.

Figure 1 :

 1 Figure 1: MPCVD reactor and OES set up

Figure 2 :

 2 Figure2: Lower and upper limits of H 2 rotational temperature measured from the OES spectra of Q-branch of H 2 Fulcher-α band and concentration of Hatoms measured by actinometry[START_REF] Gicquel | Validation of actinometry for estimating relative hydrogen atom densities and electron energy evolution in plasma assisted diamond deposition reactors[END_REF] as a function of input microwave power.

Figure 3 :

 3 Figure 3: (a) The relative abundance of C 2 radical to H-atom concentration evaluated from the ratio of intensities of I(C 2 Swann) and H-alpha and (b) Concentrations of C 2 and CH 3 from 0D-global model as a function of input power

  Fig 4a. The d-spacings of 0.21 nm and 0.35 nm are consistent (111) Diamond

Figure 4 :

 4 Figure 4: TEM image of carbon nanoparticles at resolution of 1.8 A and its corresponding (b) selected area electron diffraction with blue circle refers to the (111) plane of diamond phase. (Experimental conditions: CH4 = 4 sccm, H2 = 96 sccm, Pressure of reactor = 100 mbar, Injected power = 90 W)

Figure 5 :

 5 Figure 5: EEEL spectra of carbon nanoparticles (Experimental conditions: CH4 = 4 sccm, H2 = 96 sccm, Pressure of reactor = 100 mbar, Injected power = 90 W)

Figure 6 :

 6 Figure6: The Raman spectra of the samples obtained with different powers: 30W, 70W and 90W Also shown are the different components of the spectra: Diamond peak at 1330 cm -1 (blue line), D-peak at 1280 (Green line) and 1385 cm -1 (Orange line), G peak at 1598 cm -1 (red line) and peak at 1129 cm -1 corresponding to transpolyacetylene.

Figure 7 :

 7 Figure7: the sp 3 /sp 2 (left) and I D /I G (right)ratio as a function of input power (sp 3 /sp 2 bonds ratio is calculated from the intensity of the diamond peak at 1328 cm -1 , the intensity of the G band about 1583 cm -1 )

  e n s i t y ( a . u ) R a w H β s i g n a l H β f i t S e c o n d p e a k f i t C u m u l a t i v e F i t P e a k W a v e l e n g t h ( n m )

Figure S2 :

 S2 FigureS2: Example of raw H β spectrum and its corresponding fits obtained at 90 W and pressure of 100 mbar. Also seen is a parasitic line caused by Cu 2+ which must be present in the gas phase due to etching of the brass pin by hydrogen.

Figure S3 :

 S3 Figure S3: Electron densities measured from Stark broadening of H β as a function of input MW power.

Figure S4 :

 S4 Figure S4: Material characterization of carbon nanoparticles deposited on Si substrate. a) SEM image carbon nanoparticles, b) its corresponding EDX spectra, and c)Micro Raman spectra. (Experimental conditions: CH4 = 4 sccm, H2 = 96 sccm, Distance between plasma torch and substrate = 5mm, Frequency of microwave =2480 Hz, Pressure of reactor = 100 mbar, Injected power = 90 W, Deposition time = 60 min)

Acknowledgements

"ANR (Agence Nationale de la Recherche) and CGI (Commissariat à l'Investissement d'Avenir) are gratefully acknowledged for their financial support of this work through Labex SEAM (Science and Engineering for Advanced Materials and devices), ANR-10-LABX-0096 and ANR-18-IDEX-0001". One of the author (Khaled Hassouni) acknowledge the support of the Institut Universitaire de France.

Supplementary Material


Aurélien Perrot
email: rennes.aurelien.perrot@cea.fr

Jean-François Coulon
email: jean-francois.coulon@ecam-rennes.fr

François Cheviré
email: francois.chevire@univ-rennes1.fr

PLA) and bioactive glasses (BG) composite: influence of gold coated of BG powder on mechanical properties and chemical reactivity

Keywords: polylactic acid, bone tissue engineering, composite materials, Polymer-ceramic composite, Bioactivity, Degradable polymers, Bioactive glass, mechanical properties Bioactive glass(es): BG Fused filament fabrication: FFF Poly-lactic acid: PLA Computer Aided

Bioresorbable polylactic acid (

Introduction

The world of biomaterials has progressed tremendously in recent years. Biocompatibility is no longer a sufficient requirement and now these materials must exhibit a bioactive character. This means that they should induce a specific biological response involving surface reactions thus leading to the creation of intimate bonding at the interface with the host tissue.

Bioactive glasses (BG) include different compositions of synthetic inorganic amorphous silicabased materials. The first bioactive glass, Bioglass 45S5®, was discovered in 1969 by Larry Hench [START_REF] Hench | The Story of Bioglass[END_REF].

Since then, several other formulations have been reported [START_REF] Gritsch | Combining Bioresorbable Polyesters and Bioactive Glasses: Orthopedic Applications of Composite Implants and Bone Tissue Engineering Scaffolds[END_REF] and a silica content lower than 60 wt% appears to be the requirement for sufficient bioactivity [START_REF] Välimäki | Molecular Basis for Action of Bioactive Glasses as Bone Graft Substitute[END_REF]. The chemical reactivity of BG is usually demonstrated by the formation of a hydroxyapatite layer on the surface of the material upon prolonged contact with a simulated body fluid (SBF).

Poly-lactic acid (PLA) is a biodegradable polyester obtained by polymerization of lactic acid itself made by fermentation of sugars derived from renewable resources [START_REF] Singhvi | Biomass to Biodegradable Polymer (PLA)[END_REF] such as corn starch that is considered an environmentally friendly thermoplastic polymer [START_REF] Nduko | Microbial Production of Biodegradable Lactate-Based Polymers and Oligomeric Building Blocks From Renewable and Waste Resources[END_REF]. The United States of America Food and Drug Administration approved its use in contact with human biological fluids in 1970.

Consequently, due to its biocompatibility and degradability in the human body, PLA has been adopted in various medical applications such as interference screws for the hand, ankle and knee [START_REF] Dejong | In Vivo Comparison of a Metal versus a Biodegradable Suture Anchor1 1Implants and Implant Instrumentation for This Project Were Provided via Material Transfer Agreement by Arthrex, Naples, Florida[END_REF] or surgical threads [START_REF] Benicewicz | Review : Polymers for Absorbable Surgical Sutures-Part I[END_REF]. Upon contact with body fluids, PLA undergoes a hydrolytic degradation into lactic acid that can be then eliminated by the body as CO 2 and H 2 O [START_REF] Snežana | Polymeric Matrix Systems for Drug Delivery[END_REF].

J o u r n a l P r e -p r o o f

As both BG and PLA are biocompatible materials, a fruitful research has been carried out to combine them. The suitability of such composite as a scaffold for bone replacement has been investigated on various biological tests: bioactivity, in vitro degradation, … [START_REF] Blaker | In Vitro Evaluation of Novel Bioactive Composites Based on Bioglass®-Filled Polylactide Foams for Bone Tissue Engineering Scaffolds[END_REF] [10] [START_REF] Blaker | Long-Term in Vitro Degradation of PDLLA/Bioglass® Bone Scaffolds in Acellular Simulated Body Fluid[END_REF] [12] [START_REF] Yang | Evaluation of Human Bone Marrow Stromal Cell Growth on Biodegradable Polymer/Bioglass® Composites[END_REF]. The PLA/BG composite exhibits the specific properties of both the polymer and the BG; The thermoplastic matrix is radiolucent and bioresorbable while the BG promotes osteosynthesis, stimulates angiogenesis and accelerates osteosynthesis.

Prior to the late 2000s and the emergence of additive manufacturing, the use of composites as orthopaedic biomaterials was very limited, mainly due to issues associated with their design and manufacture [START_REF] Evans | Composite Technology in Load-Bearing Orthopaedic Implants[END_REF]. Tissue engineering has therefore recently focused on the production of composites by additive manufacturing, allowing the creation of objects with complex shapes and geometries through the use of CAD/CAM (Computer Aided Design and Manufacturing) software. The most common processes are extrusion, photopolymerization, powder bed fusion and material jetting. P.

Stavropoulos and P. Foteinopoulos have recently listed and classified the additive manufacturing

techniques [START_REF] Stavropoulos | Modelling of Additive Manufacturing Processes: A Review and Classification[END_REF].

The FFF (Fused Filament Fabrication) approach is based on the extrusion of a fusible filament and its deposition layer by layer using a printing nozzle driven by a 3-axis system. Once a layer is completed, the scaffolding under construction is vertically offset to allow the next layer to be extruded, until the final object is obtained. The layer thickness determines the quality of the 3D printing with resolution usually between 50 and 200 μm [START_REF] Wang | 3D Printing of Polymer Matrix Composites: A Review and Prospective[END_REF].

Regarding biomedical experiments, PCL (polycaprolactone), PLGA (poly(lactic-co-glycolic acid)), PLA (polylactic acid) and their combination with other biomaterials are among the preferred alternatives to produce scaffolds by FFF [START_REF] Zhang | Three-Dimensional (3D) Printed Scaffold and Material Selection for Bone Repair[END_REF]. PLA is one of the most widely used polymers for biomedical and general applications [START_REF] Murariu | PLA Composites: From Production to Properties[END_REF], due to its low temperature extrusion which is between 190 and 230°C [START_REF] Guvendiren | Designing Biomaterials for 3D Printing[END_REF] thus allowing the use of low cost printers [START_REF] Gregor | Designing of PLA Scaffolds for Bone Tissue Replacement Fabricated by Ordinary Commercial 3D Printer[END_REF]. However, it is essential to control the extrusion mechanism so that PLA is not exposed to excessive shear stresses, which could subsequently affect its biocompatibility and cause its degradation. Using size exclusion chromatography (SEC), Grémare et al have shown the possibility of a drop-in polymer chain size from 100 to 54 kDa using the FFF process [START_REF] Grémare | Characterization of Printed PLA Scaffolds for Bone Tissue Engineering[END_REF].

J o u r n a l P r e -p r o o f

Yet, the mechanical properties of a polymer composite, which derived from the polymer chain size, are essential in the orthopaedic field to withstand the stresses exerted on the implant. Engelberg et al. showed that a doubling of the mass average chain size of PLA from 𝑀 ̅ w of 50,000 to 100,000 g.mol - 1 doubled its tensile strength [START_REF] Engelberg | Physico-Mechanical Properties of Degradable Polymers Used in Medical Applications: A Comparative Study[END_REF]. For PLA/Bioglass 45S5® composites, N. Ginsac demonstrated that their mechanical strength was lower than the polymer or Bioglass 45S5® alone [START_REF] Ginsac | Caractérisation de matériaux composite polyacide lactique-bioverre pour application dans la réparation osseuse[END_REF], as a result of degradation of the polymer chain by ionic species released from the BG on the PLA.

Therefore, the objective of this study is to reduce this phenomenon by coating the BG with a thin layer of gold by sputtering to limit its ionic release into the polymer. Although gold is sometimes associated with a cytotoxic metal, it is actually used in many medical applications [START_REF] Dykman | Gold Nanoparticles in Biology and Medicine: Recent Advances and Prospects[END_REF]. Its remarkable physico-chemical properties, including biocompatibility, are mostly induced by the size, charge and shape of the gold particles used [START_REF] Dykman | Gold Nanoparticles in Biology and Medicine: Recent Advances and Prospects[END_REF] [START_REF] Daniel | Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology[END_REF]. It is important to note that the focus of this study is on the mechanical properties of the composite and not on in vitro cellular testing. Indeed, the nano-size of the deposited gold particles necessarily leads to cell death [START_REF] Hatakeyama | Effects of Sputtering Conditions on Formation of Gold Nanoparticles in Sputter Deposition Technique[END_REF]. This new cathodic sputtering method allows a simple and inexpensive first approach in the laboratory.

Materials and methods

a. Manufacture of the PLA-bioactive glass (BG)_Au composite

Raw materials: The PLA powder used for the experiments is the RXP 7501 PLA from Resinex with a mechanical strength of 48 MPa and a density of 1.240 according to the manufacturer's specs.

The 40-70 μm soda-lime silicate glass beads come from Hd Tooling and have the following overall composition (SiO 2 : 70.00 -75.00 wt.%; Na 2 O: 12.00 -15.00 wt.%; CaO: 7.00 -12.00 wt.%; MgO: max. The synthesis of bioactive glass is a multistep process. First, the starting materials are weighed, mixed and crushed in an agate bowl. The mixture is then heated up to 850°C for 90 minutes in a platinum crucible, placed in a muffle furnace and dwelled for 20 minutes to allow decarbonation.

Finally, the mixture is heated at 1350°C for 45 minutes to reach the melting and fining step. The resulting liquid is poured onto a stainless-steel support and then cooled in ambient laboratory atmosphere. The BG is then ground using a mechanical grinder with an agate bowl. The resulting powder is finally sieved to a particle size smaller than 100 μm and checked by laser granulometry measurements. The polymer/glass composite precursor is then obtained by mixing the as-prepared glass powder with the PLA Resinex powder using a turbula until a homogeneous mixture is obtained.

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Gold coating by sputtering: Gold deposition is achieved by sputtering with a Leica plasma sputterer under Argon with a gold target. The BG, previously reduced to powder, is placed on a support and spread to form a monolayer. A high vacuum operation is applied followed by diffusion metallization for 150 seconds. The resulting metallized powder is then mixed with a spatula and the process is repeated three times to obtain a powder with a thin layer of gold. As this process does not provide a perfect gold coating on the BG powder, the attack of organic liquids on the BG is partially retained, allowing the chemical reactivity of the composite.

Setting up filament production: Composite filaments were fabricated using a Noztech Touch extruder equipped with a hopper, an auger and a 1.75mm nozzle paired with a lab-made monitored spooler. A fine control of the extrusion flow and the winding speed allowed to obtain composite filaments with a homogenous diameter of 1.75 ± 0.05mm.

Object design:

The 3D specimens for mechanical characterizations and 3D parts for SBF tests are first modelled using a the open-source software FreedCAD [START_REF]FreeCAD: Your Own 3D Parametric Modeler Available online[END_REF]. The 3D file is then converted to a tessellation STL file that can be fed to the slicer software, here the Cura software [START_REF] Cura | Powerful, Easy-to-Use 3D Printing Software Available online[END_REF].The CNC programming language (G-code) for the 3D printer is obtained using the following parameters: 100% gyroid filling, 150 µm layer height, 50 °C bed temperature and 200 °C for the printing head. The printer used is a Prusa i3 modified RepRap-style 3D printer based on Anet A8 parts equipped with a 1mm nozzle in order to avoid clogging during extrusion of the glass particles charged PLA.

Tensile tests:

Tensile tests are performed using the Instron 3369 tensile testing machine and an extensometer. The specimens used are according to the ISO -527-2 (5A standard). The working speed is 1 mm/min. The positioning of the specimen is checked to ensure that the specimen is straight.

At the start of each test, a calibration is carried out and the extensometer is removed at Rp0.2. Groups of 5 specimens are taken for each parameter to obtain an average and to observe the variability of the measurements.

SEM:

To observe the topology and determine the qualitative composition of the resulting 3D pieces, two SEMs were used at room temperature: the JEOL JSM 7100 F and the JEOL IT 500 HR, all coupled to an EDS unit. The images were taken at a voltage varying between 5 and 15keV. When necessary, the samples were metallized with carbon using a Leica plasma metallizer sputter. Opus software (4.2.37) [START_REF] Blond | Ready-to-Use Germanium Surfaces for the Development of FTIR-Based Biosensors for Proteins[END_REF].

ICP-OES: Chemical analyses on P, Si and Ca elements concentrations in the SBF solution containing composites were performed using an iCAP 7000 series optical plasma emission spectroscopy (ThermoFischer Scientific). The wavelengths studied for the elements Si, Ca and P are available in Annexes. A calibration curve was performed with control samples framing the concentration range. The analysed samples were dissolved in 2% fluoride acid before being inserted into the nebulizer [START_REF] Mosbahi | Risedronate Effects on the In Vivo Bioactive Glass Behavior: Nuclear Magnetic Resonance and Histopathological Studies[END_REF].

SBF (Simulated Body Fluid) preparation:

For the SBF realization, two calcium and phosphorus rich ionic solutions were prepared at 37°C under stirring. For the first solution, 50mM tris(hydroxymethyl)aminoethane, 5 mM CaCl 2 and 3 mM MgCl 2 .6H 2 O were introduced into a 1L volumetric flask, supplemented with distilled water. For the second solution, 50mM tris(hydroxymethyl)aminomethane, 2 mM K 2 HPO 4 , 8.4 mM NaHCO 3 , 6 mM KCl and 275.6 mM NaCl were inserted into another 1 L volumetric flask, also completed with distilled water. The pH of these solutions was then adjusted to 7.4 with 6N HCl. Finally, to obtain the SBF, the two solutions were mixed with a 50-50 volume ratio [START_REF] Rocton | Fine Analysis of Interaction Mechanism of Bioactive Glass Surface after Soaking in SBF Solution: AFM and ICP-OES Investigations[END_REF].

J o u r n a l P r e -p r o o f

The solid samples were made from used tensile specimens with dimensions of 12.5 mm side and 2 mm thickness. Their surface area was 4.12 cm 2 and the ratio of sample surface area to SBF volume of the sample was 6.86.10 -2 cm -1 . The surfaces were polished with abrasive paper FEPA 4000 (5µm). The samples were then cleaned with distilled water and ethanol, dried with an air compressor and placed in a desiccator for 24 hours. They were then immersed in 60 mL of SBF and placed in an incubator at 37 °C under agitation at 60 rpm for the desired time. A control group consisting exclusively of SBF was also carried out. The immersion times were 3, 14, 28 and 57 days. Once removed from the SBF, the samples were rinsed with ethanol and distilled water and then placed in an oven at 50°C for 24 hours, followed by 48 hours in a desiccator.

Results and discussion

a. Characterization of printed polymer composites

In order to observe the adhesion of the gold particles to the BG during the metallization sputtering process, Keyence optical microscopy images were taken of metallized glass beads (for better visualization) as well as of non-metallized beads. The behaviour observed for glass beads is identical to that observed for bioactive glass powders. These shots are shown in Figure 1. A difference in color is well observed between the metallized and non-metallized beads demonstrating good adhesion of the gold particles to the glass. However, the colors are not uniform with darker or lighter shades. This is due to an imperfect homogenization of the gold deposit on all the glass beads. This observation does not necessarily have any consequence on this study. Indeed, the objective is to allow the growth of the hydroxyapatite on the surface of the BG and too much gold could alter its development. Once the production of the composite filaments (1.75 mm diameter) is achieved, we checked the homogeneity of the distribution of the mineral fillers into the PLA matrix. A digital analysis by image processing was carried out using a SEM image of a longitudinal section (1.20*0.8mm) of a PLA (70%wt)/BG (30%wt) filament. A Python program and the Scikit-image library were used for this purpose. The image was converted into a PGM format and then binarized into black and white by choosing a suitable threshold. The white and black pixels were then counted and the percentage of white pixels was calculated. The experimental result shows a volume fraction of 18% in BG while the calculus gives 17.6%. The results are therefore consistent and confirm the good homogeneity of the distribution of mineral particles in the polymer matrix.

In order to verify the presence or persistence of the gold coating on the BG particles once inserted in PLA, a secondary electron SEM picture and an EDS analysis were carried out on the same area and their superposition is presented in figure 2.a. A concentration of gold is observed (orange spots) around the BG grains confirming the good adhesion of the gold on the BG and its resistance to the two extrusion steps (filament production and FFF 3D print). Optically, we observe on the figures 2.b and 2.c, a differentiation of colors for the specimens whose BG underwent a metallization. They have completely blackened due to the diffusion of light by gold in the composite. On the other hand, the specimens blackened homogeneously over the entire material, again indicating good dispersion. When composites are used as bone substitutes, the study of their mechanical characteristics is necessary. The final construction must necessarily approximate the physico-chemical properties of the tissue to be regenerated in order to maintain the functionality of the tissue [START_REF] Puppi | Polymeric Materials for Bone and Cartilage Repair[END_REF]. In our study, six groups of mechanical specimens were printed by FFF technic in the laboratory, in order to understand the influence of the structural conditions of the composites on their mechanical properties. Their compositions are presented in Table 1. The influence of the mass fraction, shape, composition and metallization of the mineral filler in the polymer matrix was studied.

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According to Figure 3, all the produced composites presented a tensile strength curve typical of a ductile fracture except for the untreated PLA/BG composite (green curve #5). They are representative of plastic materials because they present a linear part, a yielding point, a hardening and a necking phases. Indeed, they all showed the ability to deform plastically without breaking until the first defect appeared. The five tested mechanical PLA/BG specimens, on the other hand, fractured as soon as their elastic limit was reached. SEM images of the fracture surfaces of the specimens were taken to illustrate this hypothesis. Those of the PLA/BG (#5) and PLA/Gold-metallized BG (#6) composites are shown in Firstly, the mechanical strength of the extruded pure PLA (39 MPa) has decreased by 9 MPa compared to that given by the supplier (48 MPa). This is probably partly due to the shear stresses experienced during extrusion. However, it was still in agreement with the values found in the literature [START_REF] Chacón | Additive Manufacturing of PLA Structures Using Fused Deposition Modelling: Effect of Process Parameters on Mechanical Properties and Their Optimal Selection[END_REF].

When looking at the variation of the mineral filler fraction in the composite, it was observed that the composite (#3) with 30wt% glass bead filler had a lower maximum strength (16 MPa) than the composite (#2) with only 10wt% glass bead filler (26 MPa). This result demonstrates the importance of minimizing the mineral reinforcement content in PLA matrices to maximize its tensile strength.

In addition, the shape of the mineral filler inserted into the matrix also seems to be a determining factor. For the same weight glass fraction, the composite (#2) with glass beads (isotropic particles) shows a higher maximum mechanical strength (26 MPa) than the composite (#4) with glass powder (18 MPa). The sharp corners of the powder particles certainly degraded the PLA polymer chains and promoted shear by stress concentration.

When designing high-strength PLA-glass composites, it therefore seems wise to control the extrusion parameters and to use glass reinforcement in the form of beads and with the lowest possible mass fraction.

J o u r n a l P r e -p r o o f

However, the chemical composition of the mineral filler also has a significant effect on the mechanical tensile properties of the composites. The use of BG, with a chemical composition closed to 45S5®, as a mineral filler alters the ion exchange at the interface between the PLA matrix and the glass reinforcement. The high contents of Ca, Na and the presence of P decreases the maximum mechanical strength (9 MPa) when compared to a conventional glass composition (18MPa). In addition, the gold metallization of the BG particles also leads to a change in the interface between the BG grains and the PLA matrix. The mechanical properties of the composite including the glass metallized with gold (#6)

are close to those of the composite including the soda lime silicate glass (#4). This seems to have a positive effect as an enhance in the maximum mechanical strength is observed (19 MPa). J o u r n a l P r e -p r o o f

In order to interpret these results, size exclusion chromatography (SEC) measurements were carried out. These analyses made it possible to determine the number and mass average molecular weight of the PLA chains for the different composites. SEC results are shown in Figure 5. Once extruded, the PLA chains have a number and mass molecular mass of approximately Mn = 53,000 g.mol -1 and Mw = 120,000 g.mol -1 . When 10wt% BG powder was added, the size of the polymer chains decreased by about 2 (Mn = 28,000 g.mol -1 and Mw = 65,000 g.mol -1 ). Several properties can be blamed. Firstly, for the same reasons as mentioned above, the fact that the BG in crushed powder form leads to cleavage of certain PLA chains. Secondly, the increase in Na content compared to conventional glass increases the formation of a basic catalysis and therefore the hydrolysis of the PLA chains, thus reducing the molecular weight of the polymer and its mechanical strength [START_REF] Grémare | Characterization of Printed PLA Scaffolds for Bone Tissue Engineering[END_REF]. Gold metallization of BG seems to limit this last phenomenon. The number and mass average molecular weight for the treated BG increased compared to the untreated compound (Mn = 36,000 g.mol -1 and Mw = 81,000 g.mol -1 ). As gold is theoretically located at the interface between PLA and BG, it seems to restrict the release of Na + ion and therefore restrict the hydrolysis of the polymer chains, allowing an enhance for mechanical strength. The chemical reactivity of a biomaterial is traditionally associated with the mechanism of hydroxyapatite formation after immersion of samples in a biological medium. Hydroxyapatite, a mineral species and the main component of tooth enamel, dentin and bone, can significantly affect cell viability and proliferation [START_REF] Kong | A Study on the Bioactivity of Chitosan/Nano-Hydroxyapatite Composite Scaffolds for Bone Tissue Engineering[END_REF]. Generally, in vitro tests in biological fluids are performed before in vivo tests to ensure the formation of hydroxyapatite. In SBF, the thickness of the hydroxyapatite layers increases with immersion time [START_REF] Hupa | Comparison of Bioactive Glasses in Vitro and in Vivo[END_REF]. The layers consist mainly of silica, but hydroxyapatite and a mixed layer of silica and hydroxyapatite are formed during prolonged immersion. In in vivo experiments, the formation of hydroxyapatite follows the same pattern as in vitro. However, in vivo, the layers are more diffuse and mixed layers of silica and hydroxyapatite appear. PK. Vallittu et al. showed that after 5 months of implantation of bioactive glasses into calvarial defects, the formation of viable, noninflammatory mesenchymal tissue with newly formed mineralised woven bone was observed, as well as non-mineralised connective tissue with larger capillaries and blood vessels. The presence of osteocytes was also detected in the newly generated bone matrix [START_REF] Vallittu | Biomaterial and Implant Induced Ossification: In Vitro and in Vivo Findings[END_REF].

The approach used here to determine the chemical reactivity is an immersion in SBF of the composites with immersion times of 0, 7, 14, 28 and 57 days. The tests were carried out on a pure PLA structure, a PLA-BG composite, a PLA-BG metallized with gold composite and a control solution containing only SBF.

ICP AES allows the measurement of ionic concentration of an aqueous solution with a high degree of accuracy (uncertainty of 1 ppm). In this work, it allows us to observe the ionic exchanges that take place between the SBF and the different compounds over time. The results obtained are presented in Figure 6.a, b. and c. They show, as expected, almost no variation in Si, P and Ca concentrations after 57 days for the control test and the SBF containing only pure PLA (red disk on graphs). However, for the composites containing a mineral filler, an increase in Si concentration and a decrease in P and Ca are observed. The increase in Si concentration can be attributed to the partial dissolution of the glass in the SBF. This dissolution not being congruent, it allows the formation of a layer of silica gel on the surface of the partially dissolved BG grains. Crystals of calcium phosphates can then grow on this gel layer [START_REF] Rocton | Fine Analysis of Interaction Mechanism of Bioactive Glass Surface after Soaking in SBF Solution: AFM and ICP-OES Investigations[END_REF]. Furthermore, for these three elements, the increase in Si concentration and the decrease in Ca and P were limited for the composite where the BG underwent metallization compared to that which did not receive any coating treatment. The gold metallization of the BG grains therefore moderates the chemical reactivity of the compound.

It should be noted that no gold concentration was detected for the metallized compound even after 57 days of immersion in SBF.

J o u r n a l P r e -p r o o f

The decrease in Ca and P therefore suggests the transformation of these trace elements into calcite or even hydroxyapatite. To verify this hypothesis, SEM images of the two composites were taken after 57 days of immersion. They are presented in figure 7. On the surface of the sample, numerous hydroxyapatite clusters were observed for both composites. They were easily identifiable as they were all located in cracks in the composite caused by the mechanical stresses experienced by the compounds. Numerous spheres stuck together were observed with hydroxyapatite crystals developing on their surface. These observations are only qualitative and do not allow for a quantification of the hydroxyapatite in the samples. However, from the ICP AES study, it appears that the hydroxyapatite layer appears to grow more prominently for the unmetallized bioglass (#5) because gold metallization reduces ionic releasing, the main factor influencing hydroxyapatite formation.

Conclusion

Mechanical specimens of PLA/glass or PLA/BG composites were successfully prepared by FFF 3D printing. For gold metallization, Keyence images showed that the gold particles adhered well to the BG but not homogeneously. However, the production of the printing filaments as well as the printed parts showed a good distribution of the mineral fillers within the polymer matrix.

The mechanical tests showed that the mechanical properties of the composites depend on several physical factors including the fraction of mineral filler inserted in the matrix and its shape. The composition of the mineral filler is also an important factor because the substitution of glass beads by BG resulted in a strong decrease in the tensile strength due to the degradation of PLA chains by the ionic release of BG. On the other hand, the presence of gold particles at the PLA-BG interface limited ionic releasing and therefore maximized the mechanical tensile properties. The addition of gold appeared to slightly reduce the kinetics of hydroxyapatite formation in contact with SBF but did not inhibit it. The present work suggests that the insertion of gold particles on the BG surface in a PLA-BG composite can enhance its mechanical properties without irreversibly affecting its chemical reactivity.

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  Optical microscopy: For optical imaging, a Keyence VHX-7000 series digital optical microscope was used. It allows 2D and 3D digital modelling of samples with a magnification range of x100 to x1000 and a maximum resolution: 4800 x 3600. J o u r n a l P r e -p r o o f b. Gold coating effects on the polymer chains of PLA-BG composite Size Exclusion Chromatography: Weight average molar mass (Mw) and dispersity (Đ = Mw/Mn) values were measured by size exclusion chromatography (SEC) in tetrahydrofuran (THF) at 40 °C (flow rate = 1.0 mL/min) on a GPC2502 Viscotek apparatus equipped with a refractive index detector Viscotek VE 3580 RI, a guard column Viscotek TGuard, Org 10 x 4.6 mm, a LT5000L gel column (for samples soluble in organic medium) 300 x 7.8 mm and a GPC/SEC OmniSEC Software [29]. The polymer samples were dissolved in THF (2 mg/mL). All elution curves were calibrated with polystyrene standards. c. Characterization of Ca -P layer and solution ion concentration XRD: Structural determination of the compounds was performed by X-ray diffraction (XRD) using a Panalytical X' Pert Pro'' diffractometer (Cu-L2, L3 radiation, λ =1.5418 Å, 40 kV, 40 mA, PIXcel 1D detector). They were collected at room temperature in the 2θ range 5-120° with a step size of 0.0131° and a scan time per step of 200 s. Data Collector and HighScore Plus software were used for data recording and analysis. FTIR-ATR: Attenuated Total Reflection Fourier Transform Infrared spectra were recorded at room temperature using a Bruker Equinox 55 FTIR spectrophotometer, continuously purged with air.

Figure 1 :

 1 Figure 1 : Glass beads: a. Non-metallized beads at 300x magnification; b. Metallized beads at 300x magnification

Figure 2 :

 2 Figure 2 : Microscopic analysis of a metallized PLA/BG composite filament: a. Superposition of a SEM image and gold presence map (orange spots); b. Keyence shot of the sample at 100x magnification; c. Keyence shot of the sample at 300x magnification J o u r n a l P r e -p r o o f

Figure 4 .

 4 b and 4.c. A typical brittle fracture surface is observed for the composite with untreated BG, whereas a ductile fracture is obtained for the composite with goldcoated BG particles. The results are therefore concordant. r n a l P r e -p r o o f

Figure 3 :

 3 Figure 3 : Typical stress-strain curves of the different composites

Figure 4 .

 4 Figure 4.a shows the maximum mechanical strength of the different produced composites.

Figure 4 :

 4 Figure 4 : (a) Maximum mechanical strength of composites; (b) SEM image of brittle fracture surface of PLA/BG (#5); (c) SEM image of ductile fracture surface of PLA/BG metallized with gold (#6)

Figure 5 :

 5 Figure 5 : Number and weight average molecular weight of extruded PLA (#1), PLA/BG (#5) and PLA/ BG metallized with gold (#6)

Figure 6 :

 6 Figure 6 : ICP measurements to determine the concentration of the elements (a) Si; (b) P and (c) Ca of the composites after immersion in SBF for 0, 7, 14, 28 and 57 days

Figure 7 :

 7 Figure 7 : SEM images of (a) (c)PLA/BG and (b) (d) PLA/BG metallized with gold composites

synthesis are SiO 2 from Quartz et Silice (99,9% purity), Na 2 CO 3 from Acros Organics (99.5% purity), CaCO 3 from Alfa Aesar (99,0% purity) and NaH 2 PO 4 from Acros Organics (99+% purity).

	5.00 wt.%; Al 2 O 3 : max. 2.50 wt.%; K 2 O: max. 1.50 wt.%; Fe 2 O 3 : max. wt.0.50 %). Raw precursors for
	45S5® BG (BG)

Table 1 :

 1 Composition (wt. %) of the 3D FFF printed samples used for the mechanical tests

Acknowledgment

The authors wish to thank Sandrine Cammas-Marion (ISCR -ENSCR) for her technical assistance during the SEC, as well as Bertrand Lefeuvre for his technical assistance during the ICP-AES. This publication is supported by the European Union through the European Regional Development Fund (ERDF), the Ministry of Higher Education and Research, the French Region of Brittany and Rennes Métropole.

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