V Placet

DMA Delobelle Femto-St

Mechanical properties of bulk polydimethylsiloxane for microfluidics over a large range of frequencies and aging times

Keywords: PDMS bulk material, curing conditions, dynamic mechanical analysis, viscoelastic properties, aging

The dynamic mechanical characterization of polydimethylsiloxane (PDMS) over a large range of frequencies (10 -2 < f < 10 5 Hz) and long aging times at room temperature (4 h < t v < ~60 000 h) has been presented. Three samples with different curing conditions have been studied and three different techniques, dynamic mechanical analysis at different temperatures, nano-indentation and scanning micro-deformation microscopy, have been used. Although the three techniques work at different scales and at different frequencies all the results match the same master curve. As expected, the storage and the loss moduli greatly increase with the frequency. Moreover, these moduli moderately increase with the aging time t v depending on the curing temperature. A simple model which takes the frequency and the aging time into account, and which is based on the Havriliak-Negami model, has been presented and identified. Hence, values of the relaxed and instantaneous moduli at t v = 0 and t v = ∞ are proposed. Only the relaxed moduli depend on the curing conditions and moreover it has been shown that the tangent of the phase lag is independent of the aging time and thus of the curing process.

Introduction

Polydimethylsiloxane (PDMS) has become the most popular building material used in a variety of low-cost aqueous microfluidic devices (values, pumps, fluidic circuits) aimed in particular at single use applications for biological or medical diagnostics [START_REF] Lötters | The mechanical properties of the rubber elastic polymer polydimethylsiloxane for sensor applications[END_REF][START_REF] Unger | Monolithic micro-fabricated valves and pumps by multilayer soft lithography[END_REF][START_REF] Mcdonald | Polydimethylsiloxane as a material for fabricating microfluidic devices[END_REF][START_REF] Malek | Hybrid replication development for construction of polymeric devices Microsyst[END_REF][START_REF] Mata | Characterization of polydimethylsiloxane properties for biomedical micro/ nanosystems[END_REF]. Some applications have also been reported in micro-optical systems (micro-opto-electro-mechanical systems (MOEMS) such as adaptative lenses, tilting mirrors etc) [START_REF] Chen | Variablefocusing microlens with microfluidic chip[END_REF][START_REF] Werber | Thermo-pneumatically actuated, membrane-based micro-mirror devices[END_REF] and in microelectromechanical systems (MEMS) sensors (chemical, medical, tactile sensors etc) [START_REF] Schneider | Mechanical properties of silicones for MEMS[END_REF][START_REF] Du Roure | Force mapping in epithelial cell migration[END_REF]. In fact its extreme ease of use combined with its good mechanical properties (high elasticity), biocompatibility and transparency underpin the widespread use of PDMS. In the microfluidic applications, in order to have low power consumption, many groups use this material for mobile parts (often membranes) in active systems such as micro-valves or micro-pumps [2-4, 10, 11]. However, there are few reported characterizations of the bulk mechanical properties of cross linked PDMS under well defined curing temperature conditions and long aging times [START_REF] Mata | Characterization of polydimethylsiloxane properties for biomedical micro/ nanosystems[END_REF][START_REF] Schneider | Mechanical properties of silicones for MEMS[END_REF][START_REF] Liu | Influences of heating temperature on mechanical properties of polydimthylsiloxane[END_REF][START_REF] Khanafer | Effects of strain rate, mixing ratio and stressstrain definition on the mechanical behavior of polydimethylsiloxane (PDMS) material as related to its biological applications[END_REF][START_REF] Kim | Measurement of non linear mechanical properties of PDMS elastomer[END_REF][START_REF] Johnston | Mechanical characterization of bulk Sylgard 184 for microfluidics and microengineering[END_REF][START_REF] Gupta | Adhesive force significantly affect elastic modulus determination of soft polymeric materials in nanoindentation[END_REF][START_REF] Singh | Rapid prototyping of magnetic valve based on nanocomposite Co/PDMS membrane MicroSyst[END_REF]. However, a limited amount of published data is devoted to the dynamic mechanical properties [START_REF] Lötters | The mechanical properties of the rubber elastic polymer polydimethylsiloxane for sensor applications[END_REF][START_REF] Mata | Characterization of polydimethylsiloxane properties for biomedical micro/ nanosystems[END_REF]. Some analyzing elements of these two different points are presented in this paper.

Materials and experimental procedures

Material

To realize the PDMS samples, the Sylgard 184 PDMS kit manufactured by Dow Corning Corporation [18] and composed of a prepolymer and a curing agent has been used. The prepolymer and curing agent in the ratio 10 : 1 were mixed with an Ultra Turax homogenizer for 30 s. The solution was poured into a Petri dish up to a thickness of about 3-4 mm and degassed in a dessicator by the application of vacuum for 15 min, with cycles of 2 min between atmospheric pressure and 0.5 Pa. The solution was then cured in an oven at 65 °C for 4 h following the recommendations from Dow Corning [START_REF] Du Roure | Force mapping in epithelial cell migration[END_REF]18]. This procedure has been applied to a plate labelled 'Spec.1'. A second plate (labelled 'Spec.2') with a curing time of 10 h at 65 °C has also been elaborated. Indeed, these two curing conditions are those encountered during the fabrication process of an active micro-valve for microfluidic applications [START_REF] Thuillier | Development of a low cost hybrid Si/PDMS multi-layered pneumatic microvalve Microsyst[END_REF], i.e. step 1: elaboration of the active membrane corresponding to Spec.1, and step 2: bounding under the load of both lower and upper parts of the micro-valve corresponding to Spec.2 [START_REF] Thuillier | Development of a low cost hybrid Si/PDMS multi-layered pneumatic microvalve Microsyst[END_REF]. Then, PDMS was demolded and cut to make rectangular specimens (thickness ~ 3.75 mm, width ~ 14 mm and useful length ~ 35 mm). Some data obtained five years ago on a PDMS specimen cured at 180 °C for 30 min and labelled 'Spec.3' are also presented for comparison with the two precedent ones, as this curing condition gives the maximum hardening of the material [START_REF] Schneider | Mechanical properties of silicones for MEMS[END_REF][START_REF] Johnston | Mechanical characterization of bulk Sylgard 184 for microfluidics and microengineering[END_REF].

Experimental procedures

Specimens 1 and 2 have been characterized thanks to three different techniques (dynamic mechanical analysis (DMA), nano-indentation and scanning micro-deformation microscope (SMM) over long aging times t v (t v ~ 63 000 h ~ 7 years) at room temperature T ~ 20-23 °C.

For each DMA experiment the dimensions of the cross section of the tested specimen have been accurately measured with an accuracy of ±0.05 mm. The distance between the two jaws where the specimen is attached has been measured (precision wedges) with a precision of 0.1 mm. However, due to the standard tensile specimen shape (dog bone shape), the precision of the useful length (homogeneous strain and stress) to determine the strain has been estimated to 0.5 mm (finite element modeling). Finally, taking the dimensions of the tested specimens and these different uncertainties into account, the precision on the modulus is in the range 3-4%.

The DMA measurements with frequencies in the range of 0.01-80 Hz have been performed on a commercial BOSE Electroforce 3200 machine at room temperature for the three specimens and at T = 0, -20, -40 and -60 °C for Spec.1 at t v ~ 4, ~11 000 and ~56 000 h. Thus, for this last sample the time-temperature equivalence has been analyzed over a large domain of frequency: 10 -2 < f < 10 5 Hz. According to the ASTM Guide for Dynamic Testing, the software calculates the values of E′, the storage modulus, E″ the loss modulus and tan(δ) = E″/E′, the tangent of the phase angle.

Nano-indentation tests have been carried out using a Nano-Indenter II S equipped with a Berkovich tip. The study was conducted following the classical method (quasi-static) and continuously with the CSM technique at a frequency of 45 Hz [START_REF] Oliver | An improved technique for determining hardness and elastic-modulus using load and displacement sensing indentation experiments[END_REF]. With this last method E′ and E″ can be calculated thanks to a suitable model [START_REF] Rouzic | Comparison of three different scales techniques for the dynamic mechanical characterization of two polymers[END_REF]. For each tested sample the measurement sequence consists of five indents with a maximum penetration depth of h max = 5 µm and an approximately constant rate equal to 2 × 10 -2 s -1 . For the quasi-static method five unloadings were performed at about 1, 2, 3, 4 and 5 µm and 50% of the unloading curves are considered to calculate the contact stiffness. For the CSM procedure the indenter vibrates at 45 Hz with an amplitude of 2 nm during the indenter penetration.

Some experiments at high frequency have been performed thanks to the SMM, which is a type of ac force contact microscope [START_REF] Vairac | Scanning microdeformation microscopy in reflection mode[END_REF][START_REF] Rouzic | Simultaneous measurement of Young's modulus and Poisson's ratio at microscale with two-modes scanning microdeformation microscopy[END_REF]. The sensor is a micromechanical resonator composed of a silicon cantilever with a small sharp sapphire tip at the end. This cantilever is glued onto a piezoelectric bimorph transducer at the other end. This transducer excites the vibration of the tip-sample system. The tip remains in contact with the sample and vibrates at some kHz with an amplitude of some nanometers. The amplitude and phase of the cantilever vibration are measured with a high sensitivity heterodyne interferometer [START_REF] Vairac | Scanning microdeformation microscopy in reflection mode[END_REF][START_REF] Rouzic | Simultaneous measurement of Young's modulus and Poisson's ratio at microscale with two-modes scanning microdeformation microscopy[END_REF]. This microscope is an effective tool to record images of surfaces or sub-surfaces with heterogeneous local elasticity or to characterize elastic properties of materials. In this study it has been used to measure the complex Young's modulus of the PDMS samples. The excitation frequency is scanned and the first resonant frequency which depends on the static force exerted by the tip onto the sample can be determined. Actually, knowing this resonant frequency and the static force, local contact stiffness can be estimated and then with a well-suited model [START_REF] Rouzic | Comparison of three different scales techniques for the dynamic mechanical characterization of two polymers[END_REF][START_REF] Rouzic | Simultaneous measurement of Young's modulus and Poisson's ratio at microscale with two-modes scanning microdeformation microscopy[END_REF] the storage and the lost moduli are extracted. This has been done on the three samples at different aging times (figure 1) and with two different (but close to each other) cantilevers. So the resonant frequencies are slightly different and in the range 2.6-4.5 kHz.

Results and analysis

E′ and E″ moduli as a function of the frequency

Figures 1(a) and (b) give for Spec.1 and three aging times (t v = 4, 11 000 and 56 000 h) the evolutions of E′ and E″, determined with the three experimental techniques, as a function of the excitation frequency. For this material we took ν = 0.48 for the Poisson's ratio. The William-Landel-Ferry (WLF) model has been applied to calculate the storage and loss moduli master curve (DMA analysis) is shown in figures 1(a) and (b). In this model, for a fixed reference temperature T 0 , the translation parameter a T/To of the frequencies is given by:

= = - - + - = ′ ( ) E T f E T a f a T T T T j ( , ) , , with Ln( ) C ( ) C
and ( ) ( or ") . It is interesting to note that, although the three techniques work at different scales (micro and macro) and at different frequencies, nano-indentation (quasi-static and dynamic (f = 45 Hz)) and SMM (f ~ 3 and 4.5 kHz) results perfectly match the master curves determined from the DMA procedure (0.01-80 Hz) and thus a posteriori validate the application of the WLF model to this material. Note that for these two last experimental methods the calculated uncertainties on E″ are fairly large. Moreover, from figures 1(a) and (b) it is obvious that the two moduli evolve with the aging time.

E′ and E″ moduli as a function of the aging time

For the two studied specimens (Spec.1 and Spec.2), figures 2(a) and (b) show the variations of E′ and E″ (DMA analysis at room temperature) with the aging time t v (4 h < t v < ~60 000 h) and for three frequencies: 0.01, 1 and 80 Hz. Note that the first measurements have been carried out approximately 4 h after the fabrication of the samples. These moduli clearly increase with aging. Due to the additional curing time of Spec.2 (10 h) the initial value of the storage modulus E′ (4 h, 0.01 Hz) = 2.05 MPa is greater than that of Spec.1, E′ (4 h, 0.01 Hz) = 1.7 MPa (figure 2(a)). So, an additional curing time of 6 h at 65 °C sensibly corresponds to an aging of 10 000 h at room temperature. Otherwise, for the two specimens the initial values of E″ are close to each other (figure 2(b)). Moreover, Spec.3 which has been cured at 180 °C and tested at t v ~ 256 h exhibits an initial value at 0.1 Hz equal to 2.9 MPa. These observations will be discussed further.

To quantitatively take these variations into account an Avrami's kinetic have been chosen and the following relation is proposed: For the two studied specimens the relation (2b),

ω ω ω ω - = - = + ∞ - - - = ′ ⎡ ⎣ ⎢ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎤ ⎦ ⎥ t ZT t E t E E E t t j 1 ?( ) exp ( ( ) ) then : ( , ) (0, ) ( ( , ) (0,
ω ω ω ω ω π = + ∞ - - - = ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎤ ⎦ ⎥ E t E E E t t f or ( , ) (0, ) 1 ( , ) (0,
ω ω ′ ′ = E t E h f t ( , ) / (4 , ) ( ) v
v has been represented in figure 3 for three frequencies (0.01, 1 and 80 Hz). Note that E′(0,ω) ~ E′(t v = 4 h, ω). The identified parameters corresponding to the four curves drawn in figure 3 (equation (2b)) are: t 0 = 46 000 h, n = 0.5, E′(∞,ω) / E′(0,ω) = 1.49 and 1.55 (Spec.1) and 1.4 and 1.47 (Spec.2) for f = 0.01 and 80 Hz, respectively. The E′(∞,ω) / E′(0,ω) ratio sensibly increases with the frequency. Note that the fitting of the experimental points in figure 3 allows the two kinetic parameters n and t 0 of the relation (2a) to be determined. Hence, for an infinite aging time the maximum increase of the Young's modulus values is about 50 and 40% for the specimens 1 and 2, respectively. The value n = 1/2 is difficult to explain as it is not directly associated with the physical nature of the evolution of the cross-linking. As an example, for crystal growth in polymer, n is in the range 1-4 depending on the nucleation and the crystallization mechanisms.

Phenomenological modeling: frequency and time aging dependencies

Taking the aging into account, for viscoelastic materials the general equation of the complex modulus is given by:

∑ ∑ ωτ ωτ ωτ * = + - = + = = = ( ) ( ) E t E t E t E t f f p p ( ) ( ) ( ( ) ( ) ) i with i 1 1 i and 1 j j j m j j j m j v i v r v i v 1 1 (3a)
where E i (t v ) and E r (t v ) are the instantaneous and relaxed Young's moduli, respectively and τ j a number m of discrete relaxation times whose ponderation coefficients are p j . From a phenomenological point of view and considering the Havriliak-Negami model [START_REF] Havriliak | A complex plane representation of dielectric and mechanical relaxation processes in some polymers[END_REF], f(iωτ j ) can be written as a continuous complex function of the argument iωτ: Hence, storage and loss moduli are given by: βφ ωτ απ ωτ

ωτ ωτ = + α β ( ) f i 1 (1 (i ) ) j (3b)
′ = + - + + α α β E t E t E t E t ( ) ( ) ( ( ) ( ) ) cos ( ) ( 1 2( ) cos ( / 2) ( ) ) v i v r v i v 2 /2 (3c) βφ ωτ απ ωτ = - + + α α β E t E t E t " ( ) ( ( ) ( ) ) sin ( ) ( 1 2( ) cos ( / 2) ( ) ) v i v r v 2 /2 (3d) with ⎛
⎡ ⎣ ⎢ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎤ ⎦ ⎥ = + ∞ - - - E t E E E t t ( ) (0) ( ( ) (0) ) 1 exp n i v i i i v 0 (3f) ⎡ ⎣ ⎢ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎤ ⎦ ⎥ = + ∞ - - - E t E E E t t ( ) (0) ( ( ) (0) ) 1 exp n r v r r r v 0 (3g)
The model is composed of equations (3c)-(3g). τ is a single relaxation time, α and β two empirical exponents and

E i (0), E r (0), E i (∞), E r (∞) the instantaneous ( = ∞ f
) and the relaxed (f = 0) modulus at t v = 0 and ∞, respectively. As previously shown, t 0 = 46 000 h (1.65 × 10 8 s) and n = 1/2. From the experimental points represented in figures 1(a) and (b) (master curves of Spec.1) the identified values of the model's parameters are β = 1 (like in the Cole and Davidson model), α = 0.236, τ = 2 × 10 -9 s, E r (t v ) = 1.62, 1.91, 2.12 and 2.37 MPa, E i (t v ) = 28.2, 34, 38.2 and 43.1 MPa for t v (h) = 0, 11 000, 56 000 h and ∞, respectively (Spec.1). The corresponding curves are drawn in figures 1(a) and (b). Hence, for Spec.1, E r (0) = 1.62 MPa, E r (∞) = 2.37 MPa and E i (0) = 28.2 MPa, E i (∞) = 43.1 MPa. Note that E r (∞) / E r (0) ~ 1.46 and E i (∞) / E i (0) ~ 1.53 which is close to the values calculated from the kinetics at 0.01 and 80 Hz in figure 3: 1.49 and 1.55, respectively. As shown in figures 2(a) and (b), the curing conditions of Spec.3 are sufficient to assure constant mechanical properties, i.e. no perceptible evolution of the moduli between 240 and 6000 h, and the nano-indentation results (f ~ 0.02 Hz) at t v = 62 000 h is close to those at 0.01 Hz for shorter aging time (figures 2(a) and (b)). Moreover, it is interesting to observe that the calculated values from the model of the storage modulus at 1 and 80 Hz for an infinite aging time, E′(∞,1 Hz) = 2.88 MPa and E′(∞,80 Hz) = 3.86 MPa approximately correspond to those of Spec.3 for the same frequencies, 3.04 and 3.95 MPa, respectively. Hence, as shown in figures 2(a) and (b) for very long aging times, the mechanical properties of Spec.1 and Spec.2 tends to those of Spec.3. From these observations, it can be concluded that the instantaneous moduli at very long aging time for the three studied samples are sensibly identical and that only the relaxed moduli are very dependent on the curing conditions. For the three studied specimens the proposed values for E r (0), E r (∞), E i (0) and E i (∞) are reported in table 1.

In the previous model (equations (3c)-(3g)) it has been assumed that E′(t v ,ω) and E″(t v ,ω) follow the same aging kinetic, thus the tangent of the phase lag, tan(δ) = E″(t v ,ω)/E′(t v ,ω), should be independent of the aging kinetic and thus of the curing conditions. This hypothesis is fairly well validated in figure 4 where tan(δ) for the three studied specimens and the set of experimental conditions describe the same master curve as a function of the frequencies. Note that Curing conditions this conclusion has been established for a given percentage of cross-linker, 10% recommended by the manufacturer. There are only a few results in the literature to confirm this conclusion. However, the nano-indentation results of Mata et al [START_REF] Mata | Characterization of polydimethylsiloxane properties for biomedical micro/ nanosystems[END_REF] obtained on a PDMS with 10% of cross-linker cured during 30 min at 95 °C and reported in figure 4 for 4, 64 and 256 Hz, perfectly match the master curve and thus accredit the previous conclusion. This is equally true for the DMA result at 45 Hz reported by Singh et al [START_REF] Singh | Rapid prototyping of magnetic valve based on nanocomposite Co/PDMS membrane MicroSyst[END_REF] for a PDMS (10 : 1) cured for 15 min at 120 °C (figure 4).

E r (0) (MPa) E r (∞) (MPa) E i (0) (MPa) E i (∞) (MPa)

Discussion with regard to the literature

For the PDMS with 10% cross-linker, contrary to the dynamics tests, many studies have been reported in the literature for quasi-static conditions. The present results (storage relaxed modulus) are compared to those of the literature in table 2 (values) and in figure 5. Hence, in this figure the storage modulus E′ for quasi-static conditions is plotted as a function of the curing time t c and for different curing temperatures T c . It is thus shown that the values obtained for Spec.1 and Spec.2 at T c = 65 °C are in fairly good agreement with those in the literature. Moreover, for this curing temperature and, as previously mentioned, for very long aging times t v , for long curing time (t c > 1000 h) E′ tends to E r (∞) which is in the range 2.6-2.97 MPa [START_REF] Schneider | Mechanical properties of silicones for MEMS[END_REF][START_REF] Johnston | Mechanical characterization of bulk Sylgard 184 for microfluidics and microengineering[END_REF], values corresponding to high curing temperature (150 °C < T c < 200 °C) for relatively short curing times (t c < 1 h). The value obtained for Spec.3 is 2.6 MPa (figure 1(a)). Note that above 200 °C, as mentioned by Liu et al [START_REF] Liu | Influences of heating temperature on mechanical properties of polydimthylsiloxane[END_REF], thermal decomposition of PDMS begins. Note that for a given curing temperature T c and as a function of the curing time t c , a kinetic close to that written in equation (3g) could be applied to E r (0). So, this relation could be rearranged as:

= + ∞ ∞ - - - ⎡ ⎣ ⎢ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎤ ⎦ ⎥ E t t E t E E t t t ( , ) (0, ) ( ( , ) (0, ) ) 1 exp with
n r v c r c r r c v 0 (3h) ⎡ ⎣ ⎢ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎤ ⎦ ⎥ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ Δ = + ∞ ∞ - - - = E t E E E t t t A H n T (0, ) (0, 0) ( ( , ) (0, 0)
is a thermally activated parameter. k is the Boltzmann's constant and ΔH the activation energy. For T c = 65 °C the parameters of the relation (4) have been identified on the experimental points (figure 5, the literature and this study).

We keep n = 1/2 as for the aging time kinetic. Hence, E r (0,0) = 1.1 MPa, E r (∞,∞) = 2.7 MPa and t 0c = 7.2 × 10 4 s (20 h). Then, t 0c has been calculated to adjust the kinetics at 25, 100 and 160 °C (figure 5): t 0c = 1.1 × 10 7 , 3.6 × 10 3 and 180 s, respectively. From the four t 0c values (ln(t 0c ) = f(1/T c ) an activation energy ΔH = 0.4 ± 0.1 eV has been calculated. This value is somewhat uncertain but is in fairly good agreement with that of the β transition, ΔH β ~ 0.5 eV, which it often turns out to be [START_REF] Singh | Rapid prototyping of magnetic valve based on nanocomposite Co/PDMS membrane MicroSyst[END_REF] 15 min at 120 °C 2.05 -in activation barriers of deformation, diffusion, physical aging and solid phase reactions in PDMS material [START_REF] Dusek | Polymer Characterization: Rheology, Laser Interferometry[END_REF].

As a conclusion, the value of the relaxed modulus for an uncured PDMS is about E r (0,0) = 1.1 MPa and is E r (∞,∞) = 2.7 ± 0.1 MPa (× 2.5) for a fully tempered material. From a physical point of view, Sylgard 184 is composed of a monomer and hardener combined at a ratio of 10 : 1 and for an incomplete curing process there is always an excess of hardener. During the curing procedure and the aging at room temperature the excess hardener is further cross-linked with additional fillers from the monomer and thus the mechanical properties increase. Of course, and as shown in figure 5, this cross-linking is a thermally activated phenomenon.

In the fluidic dynamic MEMS applications (fluidic circuits, valves, pumps etc) the frequency range is often lower than 20 Hz and consequently the precise knowledge of the dynamic mechanical properties between the quasi-static conditions and 20 Hz is required (10 -2 < f < 20 Hz). In this range, as previously reported, the storage modulus increases by a ratio of 1.3 from the static conditions (figure 1(a)) and the tangent of the phase lag by a ratio of about 4.6 (2.4 × 10 -2 to 1.1 × 10 -1 , figure 4). Hence, for these kinds of applications the knowledge of the dynamic mechanical properties of this material is suitable. For dynamic sensors applications (acceleration sensors, capacitive accelerometers with PDMS spring, dielectric elastomer actuators, energy harvesting etc) [START_REF] Carpi | Dielectric Elastomers and Electromechanical Transducers[END_REF][START_REF] Lötters | Polydimethylsiloxane as an elastic material applied in a capacitive accelerometer[END_REF][START_REF] Molberg | Frequency dependent dielectric and mechanical behavior of elastomers for actuator applications[END_REF], the frequency range is higher than the previous one, typically between 100 Hz and some kHz and, as previously shown in this paper (figures 1(a), (b) and 4), the mechanical properties greatly evolve and are definitively required to accurately design such active sensors or actuators. Moreover, an interesting result of this study for the design is the independence of the tangent of the phase lag to the curing conditions.

Note that numerous applications have been performed with 10% curing agent, as recommended by the manufacturer, but for certain dynamic PDMS applications [START_REF] Lötters | The mechanical properties of the rubber elastic polymer polydimethylsiloxane for sensor applications[END_REF][START_REF] Unger | Monolithic micro-fabricated valves and pumps by multilayer soft lithography[END_REF][START_REF] Mcdonald | Polydimethylsiloxane as a material for fabricating microfluidic devices[END_REF][START_REF] Malek | Hybrid replication development for construction of polymeric devices Microsyst[END_REF][START_REF] Mata | Characterization of polydimethylsiloxane properties for biomedical micro/ nanosystems[END_REF][START_REF] Chen | Variablefocusing microlens with microfluidic chip[END_REF][START_REF] Werber | Thermo-pneumatically actuated, membrane-based micro-mirror devices[END_REF][START_REF] Carpi | Dielectric Elastomers and Electromechanical Transducers[END_REF][START_REF] Lötters | Polydimethylsiloxane as an elastic material applied in a capacitive accelerometer[END_REF][START_REF] Molberg | Frequency dependent dielectric and mechanical behavior of elastomers for actuator applications[END_REF] it could be interesting to study this further, as for quasi-static conditions [START_REF] Khanafer | Effects of strain rate, mixing ratio and stressstrain definition on the mechanical behavior of polydimethylsiloxane (PDMS) material as related to its biological applications[END_REF], the dynamic modulus changes with different monomer-hardener ratios to produce a softening for easier activation.

Conclusion

The dynamic mechanical characterization of PDMS over a large range of frequencies and long aging times at room temperature has been reported, thanks to three experimental techniques. The storage and the loss moduli greatly increase with the frequency (0.01 < f < 10 5 Hz) and moderately with the aging time. A simple model which takes these two behaviors into account has been presented and identified. Values of relaxed and instantaneous moduli E t E t ( ) , ( ) r v i v at t v = 0 and ∞ are proposed. The calculated values of the relaxed modulus are in good agreement with those reported in the literature. Only the relaxed modulus seems to depend on the curing conditions and moreover that the tangent of the phase lag is shown to be independent of the aging time and thus of the curing process.

Through our investigation into the effect of aging at ambient temperature on the dynamic mechanical properties of Sylgard 184 PDMS we hope to provide quantitative information for researchers designing active sensors and actuators, such as micro-valves, micro-pumps, accelerometers etc, employing this elastomer cured under certain conditions (t c at T c ) as a structural material. Please check the details for any journal references that do not have a link as they may contain some incorrect information.

1 )

 1 In this relation, T 0 = 23 °C, C 1 = 11 and C 2 = 162, whatever the time aging. E′ and E″ increase with the frequency (0.01 < f < 10 5 Hz) from 2 to 7.5 MPa and 0.02 to 1.6 MPa, respectively.

Figure 1 .

 1 Figure 1. (a): Storage modulus (Spec.1 and Spec.3) determined with the three techniques as a function of the frequency and the aging time. The four lower solid lines correspond to the model response (equations (3c)-(3g)) for Spec.1 at t v = 4, 11 000, 56 000 h and for an infinite time. The upper solid curve is the model response for Spec.3 at t v = 240 h. (b): Loss modulus (Spec.1 and Spec.3) determined with the three techniques as a function of the frequency and the aging time. As for (a) the four lower solid lines correspond to the model response (equations (3c)-(3g)) for Spec.1 at t v = 4, 11 000, 56 000 h and for an infinite time. The upper solid curve is the model response for Spec.3 at t v = 240 h.

Figure 2 .

 2 Figure 2. (a): Storage modulus (Spec.1, Spec.2 and Spec.3) as a function of the aging time. (b): Loss modulus (Spec.1, Spec.2 and Spec.3) as a function of the aging time.

  equation (2a) into account:

Figure 3 .

 3 Figure 3. Aging kinetic of the storage modulus (Spec.1, Spec.2 and Spec.3). Model, equation (2b).

Table 1 .

 1 Relaxed and instantaneous moduli at zero and infinite aging times. Model: n = 1/2, t 0 = 1.65 × 10 8 s, β = 1, α = 0.236, τ = 2 × 10 -9 s, 10 -2 < f < 10 5 Hz

Figure 4 .

 4 Figure 4. Tangent of the phase lag (Spec.1, Spec.2 and Spec.3) as a function of the frequency and the aging time. The results of Mata et al[START_REF] Mata | Characterization of polydimethylsiloxane properties for biomedical micro/ nanosystems[END_REF] and Singh et al[START_REF] Singh | Rapid prototyping of magnetic valve based on nanocomposite Co/PDMS membrane MicroSyst[END_REF] are also reported for 4, 64, 256 and 45 Hz, respectively. For the legibility of the figure the points of Spec.2 have not been reported but perfectly match the master curve. Model, equations (3c)-(3g).

Figure 5 .

 5 Figure 5. Evolution of the quasi-static storage modulus as a function of the curing time for different curing temperatures. Present study and literature. Model, equation (4).

Table 2 .

 2 Literature results of relaxed (quasi static tests) modulus as a function of the curing conditions.

		Curing conditions:	E r (0)	E r (∞)
	Authors	10:1 cross-linker	(MPa)	(MPa)
	du Roure et al [9] 4 h at 65 °C	1.5	-
		12 h at 65 °C	2	-
		168 h at 65 °C	2.5	-
	Mata et al [5]	30 min at 95 °C	1.45	-
				(at 4 Hz)
	Gupta et al [16]	336 h at 25 °C	1.5	-
	Schneider et al [8] 15 min at 150 °C	1.82	-
		15 min at 150 °C +	-	2.6
		4 h at 200	°C
	Khanafer et al [13] 12 h at 65 °C	2.1	-
	Johnston et al [15] 48 h at 25 °C	1.32	-
		48 min at 100 °C	2.05	-
		23 min at 150 °C	2.59	-
		18 min at 200 °C	-	2.97
	Singh et al


Pauline Marijon

Chloé Bertolus

PhD; Jean-Philippe Foy

MD; Guillaume Marechal

MD; Jean-Baptiste Caruhel

MD Mourad Benassarou

MD Alexandre Carpentier

PhD Vincent Degos

PhD Aymeric Amelot

PhD Bertrand Mathon
email: bertrand.mathon@aphp.fr

Custom surgical management of invasive malignant tumors of the scalp

Keywords: craniofacial microsurgery, cranioplasty, cutaneous malignancy, free tissue transfer, microsurgical reconstruction, neurosurgery

Background: There is no universal management protocol concerning invasive malignant tumors of the scalp with bone and dura mater invasion. The aim of this study was to report and discuss our experience in the management of these forms of tumors.

Methods:

We retrospectively reviewed all consecutive patients of microsurgical scalp reconstruction performed after resection of invasive cutaneous malignancies of the scalp, calvarium and dura mater from 2017 to 2019, at Pitié-Salpêtrière University Hospital (Paris, France).

Results: Five patients met inclusion criteria. There were 3 squamous cell carcinomas and 2 undifferentiated sarcomas. Mean age at surgery was 63.6 years. The sex ratio male/female was 4. Two received radiation prior to resection and 2 patients had a history of prior scalp tumor surgery. All the patients underwent craniectomy and the mean cranial defect size was 41cm 2 . Cranioplasty was performed in one patient. Soft tissue coverage was provided by free tissue transfer of latissimus dorsi muscle in all patients. In 4 patients, split-thickness skin graft was performed in a second surgical stage few weeks later. There were no intraoperative complications and no complications into the donor site for the tissue transfer or the skin graft. Two patients had flap necrosis that healed after a new free flap of latissimus dorsi.

Conclusions: Wide resection with craniectomy, and reconstruction with microvascular free tissue transfer provides safe and reliable treatment of recalcitrant invasive scalp skin cancers. The surgical management of these complex patients is a challenge that must be conducted by trained, experienced and multidisciplinary teams.

INTRODUCTION

Few data are available in the literature concerning scalp invasive malignancies and their surgical care (excision and reconstruction after large and complex resection of calvarium). Tumor excision with scalp and bone invasion can be a surgical challenge due to the size and the invasiveness of the tumor. Likewise, calvaria reconstruction after excision can be also challenging and is generally performed using skin grafts or large flap depending on the defect size. However, there is no universal management protocol for these forms of tumors. When they are invasive, craniectomy and dura mater excision may be required to obtain full tumor excision, which exposes the patient to a risk of cerebrospinal fluid leak and infection. After these procedures, there is no consensus on the need of cranioplasty after microsurgical reconstruction of the skin.

Reconstruction scalp and skull defect resulting from tumor excision present most difficulties for the surgeon. Wide excision may include skin, periosteum, skull and dura mater, creating a complex wound. The use of radiation therapy and chemotherapy can compromise healing and scalp's vascularization. Flaps used for reconstruction should be durable to support adjuvant treatment. In 1972, free tissue transfer for reconstruction of large scalp defect was used for the first time, with an omentum flap to cover the defect [START_REF] Mclean | Autotransplant of omentum to a large scalp defect, with microsurgical revascularization[END_REF]. Since then, other series have demonstrated the efficacy of this approach [START_REF] Pennington | Free-flap reconstruction of large defects of the scalp and calvarium[END_REF][START_REF] Wackym | Reconstruction of massive defects of the scalp, cranium, and dura after resection of scalp neoplasms[END_REF]. The aim of the present study was to report and discuss our experience in the management of invasive malignant tumors of the scalp with bone and dura mater invasion.

PATIENTS AND METHODS

Patients recruitment

We retrospectively reviewed all consecutive patients of microsurgical scalp reconstruction performed after resection of invasive cutaneous malignancies of the scalp, calvarium and dura mater from February 2017 to July 2019, at Pitié-Salpêtrière University Hospital (Paris, France).

Presurgical evaluation included: complete general and neurological examination, brain MRI and CT-scan, body scanner and in some cases body positron emission tomography (PET). These data were analyzed by a multidisciplinary team, including plastic surgeon, neurosurgeon, anesthetist, oncologist, radiotherapist, dermatologist and radiologist, to determine whether surgical treatment was indicated and whether a postoperative treatment would be required.

Classification of comorbidities and outcomes

Patient's data were collected through review of medical records. Current smoking was defined as smoking within 2 months of surgery. Hypertension and hyperlipidemia were defined by the use of antihypertensive and cholesterol-lowering drugs, respectively. Diabetes was defined by the use of oral hypoglycemic drugs or insulin. Immunosuppression was defined by use of immunosuppressive drugs or diagnosis known to cause an immunocompromised state. Patient's preoperative physical status was classified using the American Society of Anesthesiologists (ASA) score. Patients with a mild systemic disease were classified as ASA II. Patients with a systemic disease which is not incapacitating were classified as ASA III, and patients with an incapacitating systemic disease which is life-threatening were classified as ASA IV.

Measurement of skull defects were performed on postoperative CT-scans. Patients were carefully followed to ensure that no complications were missed. Cerebral spinal fluid (CSF) leak was defined by the presence of fluid collected from the operative site. Partial flap loss was defined as flap necrosis that resulted in subtotal loss of flap, while complete flap loss was defined as requiring total flap replacement.

Standard Protocol Approvals, Registrations and Patient Consents

The database is registered with the Commission Nationale de l'Informatique et des Libertés (n°. 2214386). In accordance with the ethical standards of our hospital's institutional review board, the Committee for the Protection of Human Subjects, and French law, written informed consent was not needed for demographic, physiological and hospital-outcome data analyses because this observational study did not modify existing diagnostic or therapeutic strategies; however, patients were informed of their inclusion in the study. The manuscript was prepared in accordance with the Strengthening the Reporting of Observational studies in Epidemiology (STROBE) statement.

RESULTS

Patient characteristics

We found 5 patients who underwent resection followed by reconstruction surgery for invasive malignant tumors of the scalp extending into the calvarium and the dura mater. Mean age at surgery was 63.6 ± 9.2 years (range 35-84). The sex ratio male/female was 4. Preoperative patient comorbidities are summarized in Table 1. The mean follow-up period was 19 (range 7-36) months.

Tumor characteristics and surgical management

Of the 5 patients included in the study, 3 had stage IVA squamous cell carcinomas and 2 had sarcomas (one grade III undifferentiated sarcoma and one low-grade myofibroblastic sarcoma). The 3 patients with squamous cell carcinomas presented both clinical and histological poor prognostic factors according to the National Comprehensive Cancer Network (NCCN, USA) classification [START_REF] Rowe | Prognostic factors for local recurrence, metastasis, and survival rates in squamous cell carcinoma of the skin, ear, and lip. Implications for treatment modality selection[END_REF]. All patients underwent wide scalp resection, craniectomy and dural electrocoagulation (Table 2). The mean cranial defect size was 41 cm 2 (3.4 -155). The 3 patients who had a dural resection required primary dural repair. Cranioplasty was performed on the same surgical stage in one patient (#2, Fig. 3 c). Soft tissue coverage was provided by free tissue transfer of latissimus dorsi muscle in all patients (#1, Fig. 1 c, e; #2, Fig. 3 d; #3, Fig. 4 e). The recipient vessels were the superficial temporal artery and vein in all patients. Mean surgical duration was 7.3 (range 5 -10.5) hours. Mean intraoperative blood loss was 780 (range 400 -1500) ml. In 4 patients (80%) split-thickness skin graft was performed in a second surgical stage few weeks later (Fig. 1 d,e; Fig. 3 d; Fig. 4 e).

Postoperative management

In the postoperative period, all patients underwent CT-scan to detect potential early intracranial bleeding and then received subcutaneous heparin. Four patients had been staying for only 24 hours in the intensive care unit, and one patient had been staying for 7 days (#1). The median total days of hospitalization was 9 (range 8 -35) days. Four patients were discharged home and 1 was discharged to a rehabilitation center before returning home.

Patients operated on for squamous cell carcinomas did not receive any adjuvant treatment, while adjuvant radiotherapy has been performed in patients with sarcoma. A patient with systemic metastases of sarcoma (#3) received additional chemotherapy (adriamycin 60mg/m 2 ). Of the 5 patients, 1 patient who experienced an early local tumor recurrence died 7 months after surgery (#4), and 1 patient died within 10 months of surgery because of multiple systemic metastases (#3). The remaining 3 patients had no recurrence at 12, 34 and 36 months after the surgery, respectively.

Complications

There were no intraoperative complications and no complications into the donor site for the tissue transfer or the skin graft. All free tissue transfers achieved 100% soft tissue coverage.

Postoperative complications included one CSF leak that required lumbar punctures (#2). Two patients had flap necrosis due to arterial thrombosis (#3 and #5) 7 and 3 days after surgery respectively, that healed after a surgical management consisting of a new free flap of latissimus dorsi. One patient experienced flap dehiscence (#4) which was treated using topical skin care. There were no complications in the 4 patients who had skin grafts.

DISCUSSION

The common cutaneous neoplasms of the scalp (squamous cell carcinoma, basal cell carcinoma, and malignant melanoma) are usually successfully treated with soft tissue excision, and scalp reconstruction using primary repair, local flaps or skin grafting [START_REF] Brandt | Nonmelanoma Skin Cancer[END_REF][START_REF] Richmond | Primary and metastatic malignant tumors of the scalp: an update[END_REF]. All of our patients showed invasion of their cutaneous malignancies into the cranium. This was evidenced preoperatively on imaging and confirmed intraoperatively. These patients were first recused by other local or regional hospitals because of the risk of postoperative complications and then referred to our tertiary center.

Reconstruction was considerably challenging in these patients given their tumor size, and because the quality of the surrounding tissue was compromised by prior surgery, radiation or infection. Our management of these cutaneous malignancies of the scalp with deep invasion was therefore guided by the balance between obtaining clear surgical margins while avoiding neurological complications. A multidisciplinary approach has been successful in achieving complete surgical excision and immediate single-stage soft tissue reconstruction.

Post-operative defect was covered by free tissue transfer of latissimus dorsi muscle with splitthickness skin graft. Other flap techniques such as rectus abdominis muscle or antero-lateral thigh were also eligible. However, free flaps are optimal for reconstruction of large skin defects [START_REF] Hussussian | Microsurgical scalp reconstruction in the patient with cancer[END_REF]. In cases which require postoperative irradiation, free flap might be more reliable than local pedicle flaps. Furthermore, the skin graft at the donor site may not tolerate postoperative irradiation [START_REF] Earley | A critical appraisal of the use of free flaps in primary reconstruction of combined scalp and calvarial cancer defects[END_REF][START_REF] Furnas | Scalp reconstruction by microvascular free tissue transfer[END_REF][START_REF] Pennington | Free-flap reconstruction of large defects of the scalp and calvarium[END_REF].

Latissimus dorsi muscle is a large, broad and flat muscle providing stable soft tissue coverage, particularly if additional scalp needs to be resected in order to remove substrate for potential tumor recurrence [START_REF] Sood | Latissimus Dorsi Flap in Breast Reconstruction: Recent Innovations in the Workhorse Flap[END_REF]. The thoracodorsal artery and the thoracodorsal vein collected with this flap are of ideal caliber and length to perform the microsurgical vascular anastomosis [START_REF] Serletti | Thoracodorsal vessels as recipient vessels for the free TRAM flap in delayed breast reconstruction[END_REF]. The latissimus dorsi muscle flap also has the advantage of mimicking native scalp thickness and contour. Although secondary to the control of the malignancy, the aesthetic result of the reconstruction is an important consideration. Free flaps achieve good aesthetic results both in non-hearing and hearing areas [START_REF] Lutz | Reconstruction of scalp defects with free flaps in 30 cases[END_REF].

Complications after surgery for reconstruction of scalp and calvaria in skin cancers are frequent. [START_REF] Wang | Free flap reconstruction of the scalp and calvaria of major neurosurgical resections in cancer patients: lessons learned closing large, difficult wounds of the dura and skull[END_REF] They are more frequent when skin cancer involves bone and dura mater, and complications include CSF leak, sinus and cortical veins thrombosis, and neurological impairment.

Our patients had voluminous and invasive tumors, and had poor general health condition and serious comorbidities, that can explain the high rate of flap necrosis. The two cases of flap loss requiring revision surgery occurred in patients with poor general status and active smoking, the latter being a well-known risk factor for complication after free flap tissue transfer. [START_REF] Chao | Microsurgical reconstruction of composite scalp and calvarial defects in patients with cancer: a 10-year experience[END_REF][START_REF] Ehrl | Does Cigarette Smoking Harm Microsurgical Free Flap Reconstruction?[END_REF][START_REF] Thomas | Clinical Factors Associated With Reoperation and Prolonged Length of Stay in Free Tissue Transfer to Oncologic Head and Neck Defects[END_REF] Special attention should therefore be paid to smoking patients in the postoperative period. We recommend a systematic intervention for smoking cessation before the surgery. [START_REF] Chen | Smoking cessation interventions for adults aged 50 or older: A systematic review and meta-analysis[END_REF] Moreover, chronic use of corticosteroids does not increase rates of wound complications, reoperation, or readmission. However, this population of patients may be at increased risk for major bleedings requiring blood transfusion following free flap reconstruction. [START_REF] Weisberger | Analysis of the impact of chronic corticosteroid use on free flap reconstruction[END_REF] Superior longitudinal sinus thrombosis can occur especially in patients with tumoral involvement of the dura mater and requires pre-and/or postoperative anticoagulation therapy.

Finally, patients whose surgery is delayed have a poor prognosis, suggesting that early detection and treatment are crucial. [START_REF] Afifi | Lessons learned reconstructing complex scalp defects using free flaps and a cranioplasty in one stage[END_REF] Despite flap loss in two of our patients, we obtained local healing after surgical revision. The three other patients are currently in remission. Given the severity of the initial clinical presentation, these outcomes can be considered as good. Indeed, oncological and esthetical benefits were major in patients with no other therapeutic resources. Our experienced, shared by other teams, with these advanced lesions is that even when resection is likely to be incomplete because of either sagittal sinus involvement or the magnitude of the lesion, palliative resection and reconstruction are worth considering [START_REF] Ioannides | Reconstruction for large defects of the scalp and cranium[END_REF][START_REF] Mccombe | Free flaps in the treatment of locally advanced malignancy of the scalp and forehead[END_REF]. We therefore advocate for an aggressive and customized surgical management involving multidisciplinary collaboration and strategic thinking.

For patients with large skull defects, the indications for cranioplasty are both functional, in terms of cerebral protection, and aesthetic, to maintain the contour of the calvarium. Nevertheless, performing a cranioplasty during the immediate reconstruction remains controversial, because it increases the risk of postoperative infection while it does not require a second surgery. In our series, we performed immediate cranioplasty in only one patient who experienced no serious postoperative adverse events. The other patients presented exophytic ulcerated tumors with concomitant contamination or infection, that are contraindications for subsequent cranioplasty. In a study evaluating alloplastic cranioplasty, an infection rate of 40% was reported when calvarium was reconstructed with cranioplasty in the same surgical stage [START_REF] Cho | Recalcitrant Invasive Skin Cancer of the Scalp: Combined Extirpation and Microsurgical Reconstruction Without Cranioplasty[END_REF]. Several studies reported high complication rates (from 33 to 57%), when cranioplasty was performed during the immediate reconstruction, which required in most cases the removal of the prothesis [START_REF] Afifi | Lessons learned reconstructing complex scalp defects using free flaps and a cranioplasty in one stage[END_REF][START_REF] Chao | Microsurgical reconstruction of composite scalp and calvarial defects in patients with cancer: a 10-year experience[END_REF][START_REF] Sosin | Microsurgical Reconstruction of Complex Scalp Defects: An Appraisal of Flap Selection and the Timing of Complications[END_REF]. However, these studies were not exclusively on patients with cranial defects due to resection of malignant tumors. Regardless of this, the higher complication rates reported in all studies about concomitant cranioplasty and scalp reconstruction have to be considered with caution. Several factors appeared to be important in these complications: the presence of residual devascularized or infected bone after excision, inadequate coverage of the cranioplasty with well-vascularized free tissue, and the nature of the material used for the cranioplasty [START_REF] Kozák | The management of extensive tumours of the scalp[END_REF][START_REF] Mccombe | Free flaps in the treatment of locally advanced malignancy of the scalp and forehead[END_REF]. Preoperative and intraoperative recognition of the extent of the bony abnormality appears therefore fundamental to guide both tumor resection and cranioplasty.

Cho et al. reported 11 cases of reconstruction without cranioplasty for recalcitrant invasive skin cancer

of the scalp and proposed to avoid cranioplasty during the immediate reconstruction [START_REF] Cho | Recalcitrant Invasive Skin Cancer of the Scalp: Combined Extirpation and Microsurgical Reconstruction Without Cranioplasty[END_REF]. Moreover, in the Bethesda Cranial Facial Reconstruction Protocol delayed skull reconstruction resulted in a lower complication rate than previously described [START_REF] Kumar | Advanced cranial reconstruction using 13 intracranial free flaps and cranial bone grafts: an algorithmic approach developed from the modern battlefield[END_REF]. This approach is based on the two major advantages theoretically provided if cranioplasty is avoided at first excision. First, as seen in patients without dural repair, having a vascularized graft immediately on the dura mater can limit the creation of epidural fluid collections [START_REF] Kumar | Advanced cranial reconstruction using 13 intracranial free flaps and cranial bone grafts: an algorithmic approach developed from the modern battlefield[END_REF]. The second advantage of delaying cranioplasty is evidenced in patients who underwent dural repair using xenograft material and in whom the contact with a well-vascularized flap seemed to aid in healing and decreased CSF leaks [START_REF] Cho | Recalcitrant Invasive Skin Cancer of the Scalp: Combined Extirpation and Microsurgical Reconstruction Without Cranioplasty[END_REF]. Finally, although experience with cranioplasty is limited, the use of titanium mesh satisfies our requirements of a malleable but sturdy implant, which can be fixed rigidly.

Patient photographic authorization:

All patients included in this study had given photographic authorization.

FIGURE LEGENDS

Fig. 1 (patient #1)

A 35-year-old man with a history of sarcoma (diagnosed in 2000 in Algeria with 5 resection surgeries between 2000 and 2017), arrived in France in July 2017. He presented a wide skin, bone and dura mater damage (a and b) with a left extra and subdural empyema (a). A three-stage surgical management was performed. First, skin and pericranium resection with craniectomy and empyema evacuation have been performed. Postoperatively, intravenous antibiotics and anticoagulation therapy were started to threat both empyema and a cortical vein thrombosis. The second surgical stage was the reconstruction of cranial and skin defects, using a free flap of latissimus dorsi muscle (c), performed one month after the first surgical time. Finally, two weeks after the second surgical stage, a skin graft (donor-site was anterior thigh face) allowed to cover the flap (d). Lateral view of follow-up at 6 weeks demonstrating stable soft tissue coverage (e).

Fig. 2 (patient #4)

A 70-year-old man who developed recurrent invasive squamous cell carcinoma after immunosuppression for living-related kidney transplant. Preoperative T1-weighted magnetic resonance imaging (a) and CT scan (b) show an ulcerated tumor with invasion through the scalp into the cranium with superior longitudinal sinus tumoral involvement (c). Scalp and calvarial defect after wide local excision (d). A large Surgicel sheet was used to repair the dural defect (d). The defect was then reconstructed by a latissimus dorsi muscle free flap.

Fig. 3 (patient #2)

A 61-year-old woman, with a long history of recurrent squamous cell carcinoma (diagnosed in 2012, treated by chemotherapy and radiotherapy between 2012 and 2015, with follow-up break between 2015 and 2017) arrived in September 2017. She presented a wide skin, bone and dura mater damage with tumor invasion of the superior longitudinal sinus (a). We performed skin (b), bone (c) and dura mater excision. In the same time, the defect was reconstructed with tailored titanium cranioplasty (c) 16 and free flap of latissimus dorsi muscle. The patient had a little CSF leak after surgery resolved with lumbar punctures. After three weeks, a skin graft (donor-site was antero-internal thigh face) allowed to cover the flap. View of follow-up after free latissimus dorsi muscle flap with split-thickness skin graft and postoperative radiation (d).

Fig. 4 (patient #3)

A 68-year-old man presented in February 2018 with a giant retro-auricular left undifferentiated sarcoma (diameter was 29 cm) involving skin, bone and dura mater (a and b). A preoperative embolization was performed to limit bleeding during the surgery. Two days after embolization, resection surgery was achieved (c and d). The defect was reconstructed in the same time with a free flap of latissimus dorsi muscle. Patient had a flap necrosis, one week after surgery, requiring a revision surgery with a new latissimus dorsi muscle free flap. This second surgery led to local healing (e).

  Two patients were treated by radiation therapy prior to resection (#1 and #2) and 2 patients had a history of prior scalp tumor surgery (#1 and #5). None of the patients had adjuvant lymph node excision. Two patients had active scalp infections at the time of presentation: one had methicillinresistant Staphylococcus aureus (MRSA) plus Proteus mirabilis plus Enterococcus Faecalis extra-and subdural empyema with skull necrosis that required surgical evacuation and intravenous antibiotics prior to reconstruction surgery (#1, Fig. 1 a, b), and the second one had methicillin-susceptible Staphylococcus aureus superficial infection treated with oral antibiotics (#4, Fig. 2 a-c). Three patients had tumor-related cerebral venous thrombosis before surgery: two had partial superior sagittal sinus thrombosis (#2 and #4, Fig. 2 a) and had one cortical vein thrombosis (#1, Fig. 1 A) that needed postoperative anticoagulation therapy.

Funding:

No funding was received for this research.

CONCLUSION

Wide resection with craniectomy, and reconstruction with free tissue transfer with or without cranioplasty provide safe and reliable treatment for invasive malignant tumors of the scalp. The free tissue transfer of latissimus dorsi muscle is the mainstay to cover complex scalp defects and tolerate radiation. The place of cranioplasty during the immediate reconstruction remains controversial and is still debated. Anyhow, the surgical management of these complex patients is a challenge that must be conducted by trained, experienced and multidisciplinary teams comprising neurosurgeons, plastic surgeons and dedicated anesthetists.

Conflict of Interest:

All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers' bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.

Ethical approval:

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee (name of institute/committee) and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

For this type of study formal consent is not required.

Informed consent was obtained from all individual participants included in the study.


Diana Brasselet

Laurent Chouchana

Thierry Vial

Marlène Damin-Pernik

Bénédicte Lebrun-Vignes

Drug-induced retroperitoneal fibrosis: a case/non-case study in the French PharmacoVigilance Database

Keywords: Adverse Drug Reactions Pharmacovigilance Pharmacoepidemiology Drug safety confidence interval CT, computed tomography DEA, derivatives of ergot alkaloids Information Classification, General

Background: The potential role of drugs in the onset of retroperitoneal fibrosis (RPF) is poorly understood.

Aim: To identify drugs that may cause RPF.

Method: We used the case/non-case method in the French PharmacoVigilance Database (FPVD).

Results: Among the 722992 reports recorded, we identified 73 cases of RPF. 67% were men and the median age was 60 years (range 26-87). In these 73 cases, 176 drugs were 'suspect'. Derivatives of ergot alkaloids (DEA) presented the most significant association with RPF. To a lesser extent, significant associations are found with many drugs used in cardiology -eg, beta-blockers, platelet Information Classification: General (TNF-α antagonist) or not known (some hypnotics, antiepileptic drugs, antipsychotics, anxiolytics, and antidepressants) to induce such an adverse drug reaction (ADR). Finally, these data could contribute to realize prospective studies to confirm these signals.

Introduction

Retroperitoneal fibrosis (RPF) is a rare disease characterized by a fibro-inflammatory tissue, which usually surrounds the abdominal aorta and/or the iliac arteries and extends into the retroperitoneum to envelop neighboring structures-eg, ureters [START_REF] Vaglio | Retroperitoneal fibrosis[END_REF]. RPF is generally idiopathic. The clinical manifestations of RPF are nonspecific. Manifestations can be localized -eg, pain, deep vein thrombosis, leg edema (by compressive effects of the fibro-inflammatory tissue on inferior vena cava), urinary disorders until acute renal failure (by ureters compression) and/or systemic (related to the immune nature of the disease) -eg, anorexia, fatigue, fever [START_REF] Vaglio | Retroperitoneal fibrosis[END_REF][START_REF] Vaglio | Idiopathic Retroperitoneal Fibrosis[END_REF][START_REF] Estrade | Retroperitoneal fibrosis][END_REF]. Because of the non-specific clinical manifestations, there is often an important delay between the first symptoms occurrence and RPF diagnosis. This explains why the diagnosis is often realized at an advanced stage of RPF especially in the context of end-stage renal failure [START_REF] Vaglio | Retroperitoneal fibrosis[END_REF]. Also, these clinical manifestations are insufficient to make the diagnosis of RPF. The diagnosis is made based on computed tomography (CT) or magnetic resonance imaging (MRI) [START_REF] Vaglio | Idiopathic Retroperitoneal Fibrosis[END_REF]. Biopsy and histological examination of the retroperitoneal tissue are necessary when the mass presents atypical localizations-eg pelvic, peripancreatic and/or when the clinical signs and laboratory values suggest an infectious or malignant disease to exclude these aetiologies. Apart from surgical management such as ureterolysis, steroids are often used to treat idiopathic RPF as first-line treatment [START_REF] Vaglio | Prednisone versus tamoxifen in patients with idiopathic retroperitoneal fibrosis: an open-label randomised controlled trial[END_REF], also other practices in patients with relapsing RPF-eg, association with methotrexate [START_REF] Alberici | Methotrexate plus prednisone in patients with relapsing idiopathic retroperitoneal fibrosis[END_REF], other immunosuppressants or tamoxifen-are sometimes realized.

The pathogenesis of idiopathic RPF is unclear and probably multifactorial: environmental factors (asbestos, tobacco) [START_REF] Swartz | Retroperitoneal Fibrosis and Asbestosis-A Plausible Association?[END_REF][START_REF] Goldoni | Asbestos and Smoking as Risk Factors for Idiopathic Retroperitoneal Fibrosis: A Case-Control Study[END_REF], systemic autoimmune process especially in the context of IgG4-related disease [START_REF] Rossi | Idiopathic retroperitoneal fibrosis and its overlap with IgG4-related disease[END_REF] and although thought to be small there would exist a genetic component [START_REF] Klein | The HLA System[END_REF]. Idiopathic RPF accounts for more than two thirds of the cases of RPF. The remaining third being secondary to infections, cancer, trauma, surgery, radiotherapy or drug intake [START_REF] Vaglio | Retroperitoneal fibrosis[END_REF][START_REF] Palmisano | Chronic periaortitis: a fibro-inflammatory disorder[END_REF].

Iatrogenic RPF is poorly understood except for derivatives of ergot alkaloids (DEA) which represent the most common drugs associated with this adverse drug reaction (ADR). Indeed, no clear data are available about its prevalence and incidence and iatrogenic RPF is described mostly through case reports in the literature. Thus, the aim of this study was to investigate associations between drugs and reports of RPF, using the case/non-case method in the French PharmacoVigilance Database (FPVD).

Methods

Case/non-case method

The case/non-case method measures disproportionality between the combination of a drug and a particular ADR, here RPF, in a pharmacovigilance database [START_REF] Moore | French pharmacovigilance database system: examples of utilisation[END_REF][START_REF] Rochoy | Antidepressive agents and hyponatremia: A literature review and a case/non-case study in the French Pharmacovigilance database[END_REF][START_REF] Montastruc | Drugs and dilated cardiomyopathies: a case/noncase study in the French PharmacoVigilance Database[END_REF]. Cases are reports corresponding to the ADR of interest (here RPF) and non-cases are all reports of ADRs other than the ADR of interest. The method permits a comparison of drug exposure among cases and non-cases. The strength of the association between drug exposure and the occurrence of the ADR of interest is evaluated by the calculation of an ADR reporting odds ratio (ROR) with its 95% confidence interval (CI).

Source: the French PharmacoVigilance Database (FPVD)

The French Pharmacovigilance system consists of a network of 31 regional pharmacovigilance centers [START_REF] Moore | French pharmacovigilance database system: examples of utilisation[END_REF]. The FPVD was created in 1985 to record any reports of ADR spontaneously notified by health professionals and since 2011 also by patients. For each report, information about patient (age, gender, medical history) and drug intake are recorded in the FPVD. ADRs are coded according to the Medical Dictionary for Drug Regulatory Activities (MedDRA) [START_REF] Brown | The medical dictionary for regulatory activities (MedDRA)[END_REF]. A causality assessment for each drug is performed using the french method ("Begaud" method) [START_REF] Miremont-Salamé | Causality assessment in pharmacovigilance: The French method and its successive updates[END_REF]. Clinical details are summarized at the end of each report. Evolution can be updated later, after further investigations. Seriousness was defined as an ADR leading to death, hospitalization (or prolongation of hospitalization), persistent or significant disability or incapacity, being life-threatening or other medical justification [START_REF] Ewans | Statistics: analysis and presentation of safety data[END_REF].

Selection of 'cases' and 'non-cases'

As a first step, we identified all spontaneous reports registered between 1 January 1985 and 27 December 2017 under the MedDra terms « peritoneal and retroperitoneal fibrosis and adhesions » (High Level Term). All these reports were carefully reviewed by 2 pharmacovigilance specialists (DB, LC). Second, we included in the study only reports with imaging confirmed RPF (CT scan or MRI). When the report was not sufficiently informative or when another aetiology was finally found (oncological, infectious) according to the detailed summary of the clinical description, the case was not included in the analysis.

'Non-cases' were defined as all other reports (i.e. all other reports not defined as retroperitoneal fibrosis) recorded during the same period in the FPVD.

Statistical analysis

Collected data were compared between cases (reports defined as RPF) and non-cases. We calculated the ROR to compare the risk of drug exposure among cases and non-cases. The RORs are given with their 95% CI (16).

Results

Characteristics of cases

Of the 722992 reports recorded in the FPVD between 1 January 1985 and 27 December 2017, 125 were reported as « peritoneal and retroperitoneal fibrosis and adhesions », 7 cases were excluded because another aetiology was finally found, 44 cases were excluded because of lack of information or RPF not confirmed by imaging and one duplicate has been identified. 73 reports were finally validated as RPF. Among them, 67% (N=49) were men. Median age was 60 years (mean: 59 years; min: 26 years; max: 87 years). Nearly half of the patients had a medical history of hypertension (45.2%; N=33), 17.8% had dyslipidaemia (N=13) and 15.1% were diabetic (N=11). 28.8% of the patients were diagnosed in a context of renal failure (RF), isolated RF or associated with lower limb damage or abdominal and/or low-back pain. A ureteral obstruction was objectified at imaging for 32.9% of patients and 6.8% had hydronephrosis. Among patients treated by antihypertensive drugs, 21% had ureteral obstruction. One patient had an aortic aneurysm and one patient had a fibrosis extended into the mediastinal zone (without details) (Table 1). Dosage of IgG4 was specified for only 4 patients: no elevation of IgG4 was found for 3 of them and one patient had a subnormal values. All cases were serious: 81% with hospitalization (N=59), 12% with "other medical justification" (N=9), 3% "persistent or significant disability or incapacity" (N=2), 3% were life-threatening (N=2) and one was fatal.

Among the 73 cases, 176 drugs were "suspect". Median delay to RPF occurrence was 2 years and 7 months (mean: 5 years and 4 months; min: 22 days; max: 33 years and 9 months). This delay has been calculated from the date of introduction of the suspected drugs. Modification in drug intake after RPF diagnosis was known for 143 suspected drugs: drug withdrawal (N=82), unchanged dosage (N=60), decreased dosage (N=1). RPF therapeutic management was specified for 29 cases: corticosteroids (N=27) and tamoxifen (N=2). Evolution was known for only 21 cases: death (N=1), RPF extension (N=2), RPF stability (N=6), incomplete (N=11) or complete (N=1) regression.

[Table 1]

Case/non-case comparison

The case/non-case analysis included the 73 patients ("cases") and the 722867 reports ("noncases"). The 52 excluded cases were not included in the analysis. In the 73 cases, 176 drugs were defined as "suspect" (i.e. possibly associated with the occurrence of ADR whatever the level of imputability), with a mean of 2.4 drugs per case. Table 2 indicates the values of ROR between the occurrence of RPF and drug exposure in the FPVD. Derivatives of ergot alkaloids (DEA) presented the most significant association with RPF. To a lesser extent, significant association was found with many drugs used in cardiology -eg, beta-blockers, platelet antiaggregant, statins, antihypertensive drugs including inhibitors of the renin-angiotensin system and diuretics, drugs used in neuropsychiatry-eg, hypnotics, antiepileptic drugs, anxiolytics, antipsychotics, and antidepressants and with others therapeutics classeg TNF-alpha, antidiabetics... Concerning beta-blockers, when the indication was known, it was always hypertension except for one case where the indication was myocardial infarction with a probable heart failure.

[Table 2]

Discussion

As far as we know, this is the first large systematic study that investigates drugs involved in RPF and the characteristics of iatrogenic RPF through a pharmacovigilance database. Previously, RPF has been acknowledged mainly by case reports (Table 3). Our study suggests that, in usual medical practice, many drugs, not only DEA, can be suspected in RPF occurrence.

The case/non-case method is a relevant method for detecting associations between a rare adverse drug effect such as RPF and drug exposure in real conditions of use. This study presents some mandatory methodological drawbacks as with all other studies using a pharmacovigilance database -eg, underreporting [START_REF] Hazell | Under-reporting of adverse drug reactions : a systematic review[END_REF][START_REF] Thiessard | Trends in spontaneous adverse drug reaction reports to the French pharmacovigilance system (1986-2001)[END_REF] and other biases detailed below. From a methodological point of view, our study is original compared with previous publications about iatrogenic RPF. It improve our knowledge about the circumstances of occurrence of this rare ADR in real clinical practice.

Epidemiological characteristics of iatrogenic RPF are not established in literature. Compared to patients with idiopathic RPF, the characteristics of our cases were similar in terms of age or sex ratio. Men are approximately affected twice as often as women by RPF [START_REF] Vaglio | Retroperitoneal fibrosis[END_REF][START_REF] Lioger | Retroperitoneal fibrosis in adults: Main characteristics and relevance of the diagnostic procedures based on a retrospective multicenter study on 77 cases[END_REF]. The mean age is around 50-60 years [START_REF] Vaglio | Retroperitoneal fibrosis[END_REF][START_REF] Lioger | Retroperitoneal fibrosis in adults: Main characteristics and relevance of the diagnostic procedures based on a retrospective multicenter study on 77 cases[END_REF]. Nearly half of our patients suffering from hypertension which corresponds to the proportion described by Estrade and al. [START_REF] Estrade | Retroperitoneal fibrosis][END_REF]. Hypertension in patients with RPF is probably of renal origin by a rise in renin release secondary to urinary tract obstruction or entrapment of the renal artery [START_REF] Van Bommel | Retroperitoneal fibrosis[END_REF]. The considerable delay between the suspected drug introduction and RPF diagnosis can be explained by the nonspecific clinical manifestations of the disease and by the slow chronic process of fibrosis. Lugosi and al. found a mean delay between the first symptoms and the diagnosis of 258 days with extremes ranging from one to 1422 days [START_REF] Lugosi | Long-term follow-up of a French cohort of retroperitoneal fibrosis[END_REF]. RPF evolution is variable: complete or partial regression, persistence or extension despite corticosteroid therapy [START_REF] Estrade | Retroperitoneal fibrosis][END_REF][START_REF] Lugosi | Long-term follow-up of a French cohort of retroperitoneal fibrosis[END_REF], which is found in our study.

We found a very important disproportionality signal with DEA that can be explained partly by a notoriety bias. Indeed, DEA including antiparkinsonian drugs, inhibitors of prolactin secretion and antimigraine drugs, is the pharmacological class most frequently associated with iatrogenic RPF in literature [START_REF] Bilici | Retroperitoneal fibrosis caused by pergolide in a patient with Parkinson's disease[END_REF][START_REF] Shaunak | Pericardial, retroperitoneal, and pleural fibrosis induced by pergolide[END_REF] with a fibrotic risk widely described in valvular heart disease [START_REF] Antonini | Fibrotic heart-valve reactions to dopamine-agonist treatment in Parkinson's disease[END_REF][START_REF] Zanettini | Valvular heart disease and the use of dopamine agonists for Parkinson's disease[END_REF][START_REF] Corvol | Pergolide associated valvulopathy: critical analysis of the literature and practical recommendations[END_REF]. The pharmacological explanation is that DEA induced activation of the 5-HT2B serotonin receptor, lead to fibroblast proliferation and increased extracellular matrix, including collagen and glycosaminoglycan [START_REF] Zanettini | Valvular heart disease and the use of dopamine agonists for Parkinson's disease[END_REF][START_REF] Smith | Role of Serotoninergic Pathways in Drug-Induced Valvular Heart Disease and Diagnostic Features by Echocardiography[END_REF].

The association between beta-blockers and RPF is currently not well established. Various cases were published in the 70s-80s [START_REF] Bullimore | Retroperitoneal fibrosis associated with metoprolol[END_REF][START_REF] Laakso | Retroperitoneal fibrosis associated with sotalol[END_REF][START_REF] Doherty | Retroperitoneal fibrosis after treatment with atenolol[END_REF]. Some authors suggested that beta-blockers were probably being used to treat hypertension associated with RPF rather than to induce directly RPF [START_REF] Pryor | Do beta-adrenoceptor blocking drugs cause retroperitoneal fibrosis?[END_REF]. Indeed, Information Classification: General hypertension induced by RPF is probably diagnosed and treated by antihypertensive drugs before the underlying disease diagnose. The hypothesis of a protopathic bias (and also a notoriety bias, for the same reasons than DEA) could explain the important signal found with beta-blockers and especially because beta-blockers indication in our study is always hypertension excepting for one patient with a medical history of myocardial infarction and probable heart failure. However, in our study patients treated by antihypertensive drugs including beta-blockers were not more affected by ureteral obstruction. This doesn't support the protopathic bias hypothesis. A significant disproportionality was also found with other antihypertensive drugs-eg diuretics, agents acting on the renin-angiotensin system and lipid-modifying agents, platelet antiaggregants and antidiabetic drugs. Except for some cases described in the 70s-80s with prolonged treatment of an analgesic dose of aspirin [START_REF] Lewis | Analgesic abuse, ureteric obstruction, and retroperitoneal fibrosis[END_REF] and a case reported with hydrochlorothiazide [START_REF] Waters | Hydralazine, hydrochlorothiazide and ampicillin associated with retroperitoneal fibrosis: case report[END_REF] there is no association between these drugs and RPF occurrence in literature. It's should be noted that benfluorex has a known risk of valvular lesions explained by 5-HT2B serotonin receptor activation by one of its metabolites, norfenfluramine. Lugosi and al. describe that near than half patients with RPF have at least two cardiovascular risk factors [START_REF] Lugosi | Long-term follow-up of a French cohort of retroperitoneal fibrosis[END_REF] that could explain the use of these drugs in patients with RPF even if the absence of a significant signal found with calcium antagonists limits this hypothesis.

According to current data available in the literature, the role of TNF-α antagonist in RPF genesis is not clear. Some cases are described, mainly with etanercept [START_REF] Couderc | Retroperitoneal Fibrosis During Etanercept Therapy for Rheumatoid Arthritis[END_REF][START_REF] Shiber | Retroperitoneal fibrosis: case series of five patients and review of the literature[END_REF][START_REF] Manganelli | Retroperitoneal fibrosis following a long-standing etanercept treatment in a spondyloarthritis patient[END_REF]. Occurrence of a pathology for which a treatment is known to be effective called a paradoxical effect, has been notified for TNF-α antagonist-eg sarcoidosis (37), psoriasis, scleritis, uveitis, large-vessel vasculitis, and inflammatory bowel diseases. Lupus-like syndrome is also reported with these drugs. This dysregulation of the immune system induced by TNF-α antagonists could promote immune-mediated RPF occurrence. Less likely, RPF might be a manifestation of a systemic autoimmune or inflammatory disease (1) treated by TNF-α antagonist with therefore a possible indication bias. Moreover, we found a case of refractory RPF to combined glucocorticoid and methotrexate therapy treated effectively with infliximab [START_REF] Catanoso | Efficacy of infliximab in a patient with refractory idiopathic retroperitoneal fibrosis[END_REF].

A significant disproportionality was found with antihistamines drugs, ferrous sulfate and various drugs used in neuropsychiatry such as antipsychotics, antiepileptics, hypnotics, anxiolytics, and antidepressants. According to our knowledge, there is no association between these drugs and RPF in literature. It is therefore difficult to conclude about a new association between the occurrence of RPF and these drugs brought to light in this study or on the existence of unidentified biases. There is also no pharmacological explanation that could explain this signal. More generally about fibrotic risk, currently data shown that elevation of serotonin blood and/or synaptic cleft concentrations resulting from the pharmacological effects of antidepressant is not sufficient to increase the fibrotic risk and that 5-HT2B serotonin receptor stimulation seems to be the main factor [START_REF] Lapi | Use of antidepressant serotoninergic medications and cardiac valvulopathy: a nested case-control study in the health improvement network (THIN) database: Cardiac valvulopathy and antidepressant serotoninergic medications[END_REF]. None of these drugs have a significant 5-HT2B agonist activity and further studies are needed to understand this signal.

Some authors in the 70s-80s suggested that excessive and prolonged consumption of analgesics (codeine, aspirin, phenacetin) could be associated with the occurrence of RPF [START_REF] Lewis | Analgesic abuse, ureteric obstruction, and retroperitoneal fibrosis[END_REF]. In our study, there is no significant signal found with analgesics apart from aspirin.

No significant signal was found with proton pump inhibitors, platinum salts or calcium antagonists.

[Table 3]

Conclusion

In conclusion, the association between RPF occurrence and drugs is difficult to establish because of a history of drug intake sometimes arduous to reconstitute due to late diagnosis and difficulty to conclude on the evolution at the suspected drug withdrawal (unsystematic regression of fibrosis or potentially related to corticosteroids efficacy). Despite the compulsory limits of the case/non-case study • This is the first case reported of RPF induced by TNF-alpha antagonist (etanercept) in literature supporting the hypothesis of a dysregulation of the immune system in the process of iatrogenic RPF.

Total LLD (isolated or associated with pain or RF)

Total 'pain' (isolated or associated with LLD or RF) 17 (

Table 1 :

 1 Circumstances of diagnostic and the localization of fibrosis on imaging (n=73)

	RF: renal failure	WP: without particularities. RPF with involvement of abdominal aorta and/or vena
	LLD: lower limb damage (edema or deep vein thrombosis)	cava and/or iliac arteries and/or renal arteries and/or mesenteric arteries without
	*abdominal and/or low-back pain	specific mention about involvement of urinary tract, kidneys or mediastinal zone or
	FD: fortuitous diagnosis	aortic aneurysm.
	NS: circumstances of diagnostic are not specified	UO: ureteral obstruction
		FK: fibrosis surrounding kidneys
		H: hydronephrosis
		M: involvement of mediastinal zone (without details)
		AA: aortic aneurysm

Table 2 :

 2 Risk of exposure to drugs in cases (n = 73 with 176 suspect drugs) and noncases (n = 722867) and occurrence of RPF in the French PharmacoVigilance Database (FPVD)

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Information Classification: General (confounding factors, underreporting…), these data represent a pharmacovigilance signal and can contribute to perform prospective studies in order to confirm these signals. A significant association has been found between various drugs and RPF reports. This concerns not only drugs known to be associated with this ADR (DEA) but also other drugs less known (TNF-α antagonist) or not known (some antipsychotics, antiepileptics, hypnotics, anxiolytics, and antidepressants) to induce such an ADR. Our results emphasize that, in clinical practice, a systematic anamnesis about drug intake should be performed in each aetiological research of RPF and especially because no aetiology is found for more than two-thirds of the cases of RPF.

Declaration of interests

All authors have no conflict of interest to disclose in relation to this study.

Information Classification: General Information Classification: General Information Classification: General