Brieuc Guillerm

Sophie Monge

Vincent Lapinte
email: vincent.lapinte@univ-montp2.fr

Jean-Jacques Robin

Well-defined poly(oxazoline)-b-poly(acrylate) amphiphilic copolymers: from synthesis by polymer-polymer coupling to self-organization in water

Keywords: atom transfer radical polymerization (ATRP), ring-opening polymerization, self-assembly, polyoxazolines, synthesis

In this contribution, we report on the self assembly in water of original amphiphilic poly(2-methyl-2oxazoline)-b-poly(tert-butyl acrylate) copolymers, synthesized by copper catalyzed azide-alkyne cycloaddition (CuAAC) reaction. For such purpose, polyoxazoline (poly(2-methyl-2-oxazoline)) and polyacrylate (poly(tert-butyl acrylate)) are first prepared by cationic ring-opening polymerization (CROP) and atom transfer radical polymerization (ATRP), respectively. Well-defined polymeric building blocks, ω-N 3 -P(t-BA) and -alkyne-P(MOx), bearing reactive chain end groups, are accurately characterized by MALDI-Tof spectroscopy. Then, P(MOx) n -b-P(t-BA) m are achieved by polymer-polymer coupling and are fully characterized by DOSY NMR and size exclusion chromatography, demonstrating the obtaining of pure amphiphilic copolymers. Consequently, the latter lead to the formation in water of well-defined monodisperse spherical micelles (R H = 40-60 nm) which are studied by fluorescence spectroscopy, SLS, AFM and TEM.

INTRODUCTION

The increasing interest of amphiphilic copolymers mainly arose from their self-organization in water leading to the formation of micelles [START_REF] Gohy | Block Copolymers II[END_REF][START_REF] Guillerm | [END_REF] where hydrophobic core is surrounded by a hydrophilic corona. This property was employed for the encapsulation of chemicals using polymeric drug carriers, 3 as well as for the synthesis of membranes, 4 for instance. Many routes led the synthesis of amphiphilic copolymers using sequential polymerization of CROP and CRP 5 and especially CROP and RITP. 6 Dual initiators 7 and dual terminating agents 8 have also employed to the synthesis of such architectures. Among all the possible synthesis pathways, the polymer-polymer coupling appeared to be an attractive approach providing notably straightforward access to block copolymers with the control over each block in terms of molecular weight, and functionality.

Over the last few years, polymer-polymer coupling using "click" chemistry showed enormous potential for material science. [START_REF] Lahann | Click Chemistry for Biotechnology and Materials Science[END_REF][START_REF] Fournier | [END_REF][11] One of the great advantages of this concept was for example the possibility to combine homopolymers preliminary prepared by various living polymerization techniques, such as ringopening polymerization (ROP), ring-opening metathesis polymerization (ROMP), cationic polymerization, nitroxide-mediated radical polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer polymerization (RAFT). Thus, various orthogonal and efficient coupling reactions such as thiol-ene or thiol-ene based Michael addition reactions, [12][13] and Diels-Alder reactions 14 were successfully carried out. Additionally, Huisgen's cycloaddition catalyzed by copper (CuAAC) was investigated to produce block copolymers but only few examples of CuAAC polymer-polymer coupling dealt with the synthesis of amphiphilic copolymers. Indeed, in general, another strategy was used for such purpose involving the Huisgen's cycloaddition with two hydrophobic segments followed by a post-polymerization reaction onto one block to afford amphiphilic copolymers. [15][16] Only limited examples reported the direct CuAAC polymer-polymer coupling between hydrophobic and hydrophilic homopolymers as this reaction is not privileged due to the antagonist properties of both blocks, and high reactivity of polymer end chains was required to efficiently connect the building blocks. Examples described in the literature reported the use of poly(ethylene oxide) (PEO) as hydrophilic block associated with polystyrene, poly(methyl methacrylate), 17 or poly(3hydroxyalcanoate)s. 18 In another work, poly(N-isopropylacrylamide) was associated to poly (6-[4-(4methoxyphenylazo)phenoxy]hexyl methacrylate). 19 The coupling between two hydrophilic homopolymers was described using PEO and poly(2-methyl-2-oxazoline) 20 or 2-hydroxyethyl acrylate and 2-oxazoline. 21 Finally, an example dealing with poly(2-methyl-2-oxazoline) in amphiphilic copolymer reported in the literature dealt with poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-b-poly(2-methyl-2oxazoline) diblock copolymers 22 but no study of the self-organization of such amphiphilic structures was carried out. The only work detailing the synthesis and the self-organization of amphiphilic copolymer based on polyoxazoline and obtained by polymer-polymer coupling is in the recent article of Theogarajan. 23 However the copolymer is triblock copolymer PMOx-b-PDMS-b-PMOx.

In this contribution, we reported on the self-assembly of original amphiphilic diblock copolymers based on poly(2-methyl-2-oxazoline) (P(MOx)) and poly(tert-butyl acrylate) (P(t-BA)). P(MOx) can be regarded as alternative to PEO due to its hydrophilicity, 24 biocompatibility, and low toxicity which made it an appropriate candidate for biomedical applications. [25][26][27][28][29][30] On the other hand, hydrophobic P(t-BA) can be easily functionalized by trans-esterification 31 or converted into biocompatible hydrophilic poly(acrylic acid). 32 The P(MOx) and P(t-BA) building blocks were synthesized by CROP and ATRP processes, respectively. The alkyne and azide terminal functionalizations were carefully characterized by MALDI-Tof spectroscopy and polymer-polymer coupling was then achieved using Huisgen's cycloaddition reaction. Self-assembly of resulting amphiphilic diblock copolymers in water was studied by AFM, TEM, DLS and fluorescence spectroscopy.

EXPERIMENTAL

Materials

Propargyl tosylate, diethylether, methanol, chloroform, potassium hydroxide, calcium hydride (CaH 2 ), sodium, ethyl 2-bromoisobutyrate, tetrabutylammonium fluoride (TBAF) solution (1.0 M in THF), potassium fluoride, azidotrimethylsilane and N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA) were purchased from Aldrich and were used as received. Acetonitrile was dried and distilled according to standard procedures. [START_REF] Perrin | Purification of Laboratory Chemicals[END_REF] Tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were distilled under vacuum over sodium. 2-Methyl-2-oxazoline (MOx) was dried, distilled from CaH 2 and stored under a dry nitrogen atmosphere. Tert-butyl acrylate (t-BA) (Aldrich, 98%) was passed through a column of alumina to remove inhibitor. Copper (I) bromide (Aldrich, 98%) was purified according to standard procedures. [START_REF] Keller | [END_REF] Deuterated solvents (CDCl 3 , CD 3 CN and DMF-d 7 ) were purchased from SDS and were used without further purification.

Analytical techniques

1 H NMR spectra were recorded using a Bruker AMX 300 operating at 300 MHz. Chemical shifts were referenced to the peak of residual non-deuterated solvent. DOSY experiments were performed on a Bruker Advance 3 at 20 °C in 2.5 mm microtubes operating at 600 MHz with DMF-d 7 as solvent. The software used was Topspin 2.1 using the exponential method with a logarithmic scale. Size exclusion chromatography (SEC) of P(MOx) was performed on a PL-GPC 50 Plus equipped with a RI refractive index detector in a mixture of H 2 O/CH 3 OH 7/3, v/v (+ 0.1 M LiNO 3 ). Three PL aquagel-OH columns (25, 7.5 and 4.6 mm ID) were used at 40 °C with a 0.8 mL.min -1 flow rate, calibrated using PEO standards. Size exclusion chromatography (SEC) of P(t-BA) was performed on a PL Mixed D equipped with a RI refractive index detector in THF. Three PLGel 500, 10 3 , 10 4 columns were used at 30 °C with a 1 mL.min -1 flow rate, calibrated using PMMA standards. Size exclusion chromatography (SEC) of P(MOx) n -b-P(t-BA) m samples was performed on a Waters apparatus equipped with an RI refractive index detector in DMF. Three TSK columns (G2000, G3000, G4000 HHR) were used at 70 °C with a 0.8 mL.min -1 flow rate, calibrated using PMMA standards. Mass spectrometry analyses were conducted with a Bruker Ultra-Flex MALDI-Tof mass spectrometer, equipped with a nitrogen laser (LSI, 337 nm, 10 ns pulse length) and one detector. Mixture of peptides was used for external calibration. The ions were accelerated by a potential of 25 kV and reflected with a 26.3 kV potential. P(MOx) samples were recorded in the reflection mode using αcyano-4-hydroxycinamic acid (HCCA) matrix with CF 3 COOK salt. P(t-BA) samples were recorded in the reflection mode using 2,3-dihydrobenzoic acid and NaI. For each spectrum 300 transients were accumulated. P(MOx) and P(t-BA) were dissolved in acetonitrile and THF, respectively, at a concentration of 10 mg.mL -1 . Static Light Scattering (SLS) and Dynamic Light Scattering (DLS) measurements were performed on a Malvern Zetasizer Nano ZS equipped with a 4 mW He-Ne laser (λ = 632.8 nm). Measurements were carried at a scattering angle of 173°. Samples were introduced into cells (pathway: 10 mm) after filtration through 0.45 μm PTFE microfilters. The correlation function was analyzed via the general purpose method (NNLS) to obtain the distribution of diffusion coefficients (D) of the solutes. For the dynamic properties, the apparent equivalent hydrodynamic radius (R H ) and the dispersity were determined from the cumulant algorithm using the Stokes-Einstein equation:

R H = k B T 6π q 2 = k B T 6πD 0
where k B : Boltzmann constant, T: temperature of the sample, : relaxation frequency, q: wave vector, : viscosity of the medium, and D 0 : translational diffusion coefficient. Mean radius values were obtained from three runs. Standard deviations were evaluated from hydrodynamic radius distribution. The polymer concentration was 50 mg.L -1 . SLS experiments allowed the determination of the Critical Micellar Concentration (CMC) which was taken as the intersection of lines plotted on the graph corresponding to the intensity as a function of the copolymer concentration. Polymer concentrations varied from 110 -3 to 0.2 mg.mL -1 . Fluorescence measurements were achieved using a LS45 spectrometer (Shimatzu). Aggregate formation was examined over a wide range of polymer concentrations (110 -3 -0.2 mg.mL -1 ) using pyrene as a fluorescent probe. Diblock copolymers were prepared in Milli-Q water, filtered using 0.45 µm PTFE microfilters, and diluted as needed. To prepare the samples for fluorescence experiment, 0.25 mL of a pyrene solution (1.910 -3 M in methanol) was added to 24.75 mL of Milli-Q water. 75 µL of pyrene solution was then added to the polymer solution (1.5 mL) in the vial, bringing the final pyrene concentration to 910 -7 M. All spectra were recorded from air-equilibrated solutions. The samples were excited at 340 nm and emission was recorded from 330 to 450 nm. The average fluorescence values at 383 and 373 nm were used for subsequent calculations. The CMC, determined by plotting the 373/384 ratio against the polymer concentration (log mg.mL -1 ), was taken as the intersection of regression lines calculated from the linear portions of the graph. Atomic force microscope (AFM) experiments were performed at room temperature using a Nanoscope 5 Veeco. Copolymers dissolved in Milli-Q water were deposited on silicon wafers treated with RCA (1/7 H 2 O 2 , 1/7 NH 3 , 5/7 H 2 O) at 100 °C. Samples were analyzed after complete evaporation of the solvent at room temperature. Images were acquired in air in tapping mode with a silicon tip (tip radium: 7 nm, height: 10-15 µm), a spring constant of 42 N.m -1 , a resonance frequency equal to 330 kHz, and a radius of curvature in the 10-15 nm range. Processing and analysis of AFM images were performed with WSxM 5.0 Develop 3.2 software. 35 Transmission electronic microscopy (TEM) experiments were performed at room temperature using a JEOL 1200 EXII. The samples were prepared by placing a drop of an aqueous solution (0.025% w/v) of the diblock copolymer onto a carbon-coated copper grid. Samples were analyzed after complete evaporation of the solvent. Differential scanning calorimetry (DSC) was performed using a DSC1 apparatus (Mettler Toledo). Samples were heated from -100 to 100 °C at a 10 °C.min -1 heating rate. Glass transition temperatures (T g ) were measured at the third heating.

Typical procedure of MOx polymerization using propargyl tosylate initiator: -alkyne-P(MOx)

The polymerization was carried out using procedures described in the literature. Typical procedure for the synthesis of ω-N 3 -P(t-BA)

The polymerization was carried out using procedures described in the literature [37][38] with ethyl-2bromoisobutyrate as initiator to afford -Br-P(t-BA). Typical procedure for the preparation of aggregates P(MOx) n -b-P(t-BA) m (40 mg) was dissolved in DMF (65 mL). Water (65 mL) was added dropwise with a rate of 10 mL.h -1 to achieve a concentration equal to 0.3 mg.mL -1 . The resulting solution was dialyzed (membranes cut-off: 3000 g.mol -1 ) during three days using MilliQ water which was changed every 12 hours. Water solution was finally filtered through 0.45 μm PTFE microfilters.

RESULTS AND DISCUSSION

Synthesis of -alkyne-P(MOx) and -P(t-BA) functionalized homopolymers Functionalized -alkyne-poly(2-methyl-2-oxazoline) and ω-azido-poly(t-butyl acrylate) homopolymers were first prepared by cationic ring-opening polymerization (CROP) and atom transfer radical polymerization (ATRP), respectively, in order to lead to amphiphilic P(MOx) n -b-P(t-BA) m block copolymers by polymer-polymer coupling in a second step. -Alkyne-P(MOx) was synthesized with very good yields (96%) in acetonitrile at 80 °C using propargyl tosylate as initiator. Polymerization was ended with a saturated solution of potassium hydroxyde in methanol (Scheme 1). The conversion was limited to 50% to avoid any transfer and terminating reactions (supporting information, Figures S1 andS2). 36 Two P(MOx) with different molecular weights were prepared. After purification by precipitation in diethyl ether, size exclusion chromatography in water/methanol mixture permitted to determine the experimental molar mass values at 1000 and 2200 g.mol -1 , corresponding to degrees of polymerization equal to 11 and 25, respectively, whereas the dispersity (Đ) was the same (1.2) in both cases (supporting information, Table S1).

On the other hand, ω-Br-P(t-BA) were obtained by ATRP in the presence of ethyl 2-bromoisobutyrate, Cu(I)Br, and PMDETA, as previously reported by Monge et al. [37][38] Polymerizations were conducted in bulk at 40 or 50 °C and stopped after 70% conversion avoiding termination and transfer reactions (supporting information, Figure S3). A good agreement between experimental and theoretical molecular weights was obtained and dispersities were quite narrow (supporting information, Table S2). Three molecular weights of the ω-Br-P(t-BA) building blocks were prepared: 2500, 3900 and 14500 g.mol -1 with a dispersity inferior to 1.3. Then, in a post-polymerization reaction, ω-Br-P(t-BA) was converted into ω-N 3 -P(t-BA) using trimethylsilyl azide, tetrabutylammonium fluoride and potassium fluoride as phase transfer reagent in THF at room temperature.

Both hydrophilic and hydrophobic building blocks were characterized by [START_REF] Gohy | Block Copolymers II[END_REF] H NMR spectroscopy in deuterated chloroform attributing notably the alkyne proton of -N 3 -P(MOx) at δ = 2.4 ppm (supporting information, Figure S2) as well as the -CH-Br of ω-Br-P(t-BA) at δ = 4.15 ppm, overlapping with the methylene group brought by the ethyl-2-bromoisobutyrate initiator (supporting information, Figure S4). The post-polymerization modification of ω-Br-P(t-BA) into ω-N 3 -P(t-BA) was characterized by the appearance of the signal corresponding to the proton bore by the carbon in  of the azido function at δ = 3.7 ppm.

Scheme 1.

Reactional pathway for the synthesis of functionalized P(MOx) and P(t-BA) homopolymers, and resulting amphiphilic copolymers by CuAAC polymer-polymer coupling.

The matrix-assisted laser desorption ionization time-of-flight (MALDI Tof) mass spectrometry was used to confirm the nature of -alkyne-P(MOx) and ω-N 3 -P(t-BA) end-groups. This study permitted the attribution of all distributions encountered in each spectrum. The abbreviation was given to all populations taking into account the head and end groups of P(MOx) and P(t-BA). In both cases, the interval between two consecutive peaks of the same family corresponded to the molecular weight of the N-acetylethylenimine (85.05 Da) or the tert-butyl acrylate (128.05 Da) units. In the alkyne-P(MOx) spectrum, a multimodal distribution displayed six populations and the main series was centered at 2306.40 Da (n = 26, MK + type) corresponding to the targeted structure with a potassium cation (structure 1:

) (Figure 1 and Table 1). More accurately, the two main populations ( and structure 4: ) were attributed to the expected product with a propargylic end group, coming from the initiator and a hydroxyl group due to the terminating agent, and only differed from the associated cation (Scheme 2). Three other less abundant series exhibited alkyne head group with various end groups (structure 2:

, structure 3: and structure 5:

). Structure 2 was terminated by an oxazolinium end group which revealed the persistence of few propagating chains whereas structure 3 exhibited a -NHCH 2 CH 2 OH end group as previously described by Schubert et al. 40 in the case of the polymerization of the 2-ethyl-2-oxazoline initiated by propargyl tosylate. The P(MOx) end chain of the structure 5 was attributed to an unsatured oxazoline cycle resulting from transfer reaction between propagating chain and monomer. 41 Finally, a very small distribution (structure 6:

) corresponded to a P(MOx) with a proton as head group and an oxazolinium specie as end group. This structure resulted from the partial initiation with paratoluenesulfonic acid (H + ), which was a by-product coming from the hydrolysis of the tosylate initiator. 42 A second explanation of the H-head group of polyoxazoline appearance should correspond to the transfer reaction between propagating chain and monomer which yields to the structure 5 and an oxazoline specie bearing H-N bond. The modified oxazoline results from the nucleophilic attack of a MOx unit on the proton coming from the elimination on the propagating specie. Thus, the attack of another MOx unit on the modified oxazoline generates propagating polyoxazoline chain bearing proton head group. To conclude, the MALDI-Tof experiment on -alkyne-P(MOx) demonstrated that the latter was terminated by an alkyne function in all cases except for not abundant structure 6.

Table 1. Analytical data derived from MALDI-Tof spectroscopy for the characterization of -alkyne poly(2-methyl-2-oxazoline) (M n,th = 2600 g.mol -1 ). The MALDI-Tof spectrum of ω-N 3 -P(t-BA) was also deeply investigated. It was composed of three different distributions as illustrated in Figure 2 and Table 2. The main population (structure 1) was attributed to the targeted structure (structure 1:

Assignment

), corresponding to a poly(tert-butyl acrylate) with an ethyl propionate head group coming from the initiator and an azide end group. Two other minor distributions were detected. The first one was attributed to a P(t-BA) with an azide end group and an acrylic acid head group (structure 2:

) whereas the other one corresponded to the structure 3 ( ), in which the bromide atom was substituted by an hydroxyl group. From MALDI-Tof spectroscopy, we can conclude that functionalization of both PMOx and PtBA homopolymers by propargylic and azido groups, respectively, is almost quantitative.

Polymer-polymer coupling by CuAAC chemistry

The polymer-polymer coupling between -alkyne-P(MOx) and ω-N 3 -P(t-BA) homopolymers was carried out using the copper-catalyzed azide-alkyne Huisgen 1,3-dipolar cycloaddition (CuAAC) as displayed in Scheme 1. In the typical coupling protocol, both blocks were dissolved in DMF in the presence of copper (I) bromide (Cu(I)Br) and PMDETA at 50 °C under nitrogen during one week. The difficulty of this approach was to couple macromolecular segments with antagonist properties, which was not favored.

To explore potential scope of the coupling reaction and to obtain amphiphilic copolymers with various lengths of hydrophilic and hydrophobic blocks, P(MOx) blocks of 1000 and 2200 g.mol -1 and hydrophobic P(t-BA) blocks of 2500, 3900 and 14500 g.mol -1 were combined. Coupling reaction was successful when low molar mass P(MOx) reacted with low molecular weight P(t-BA) (DPn equal to 19 and 30) (Table 3). On the other hand, CuAAC did not occur for higher P(t-BA) molecular weight (DPn equal to 113) as no shift of the size exclusion chromatography trace was observed. This result was explained by the high hydrophilic character of P(MOx) which was unfavorable for coupling with a hydrophobic block. , 70 °C, PMMA standards).

1 H NMR in deuterated chloroform showed the signals corresponding to the proton of the triazole ring and to the proton of the tert-butyl acrylate moiety linked to the nitrogen atom of the triazole at 8 and 4.2 ppm, respectively (Figure 3). We also noticed the presence of signals corresponding to both P(MOx) and P(t-BA) moieties, indicating that the coupling was probably effective. To confirm that point, amphiphilic copolymers synthesized were also characterized by 2D diffusion-ordered NMR spectroscopy (DOSY) analysis which produced two-dimensional correlation maps showing chemical shifts and diffusion coefficient on the horizontal and vertical axes, respectively. 43 This powerful technique allowed discriminating macromolecules by its diffusion coefficient e.g. precursor polymers and resulting copolymers. 44 The 2D DOSY NMR spectra of the P(MOx) 25 , P(t-BA) 30 homopolymers and the resulting P(MOx) 25 -b-P(t-BA) 30 were achieved in DMF-d 7 (Figure 4). The diffusion coefficient values of -alkyne-P(MOx) and of the ω-N 3 -P(t-BA) were measured (blue and red dotted line, respectively) and proved to be different (-9.61 and -9.4 m 2 .s -1 , respectively). On the other hand, P(MOx) and P(t-BA) signals had the same diffusion coefficient value (-9.9 m 2 .s -1 ) for the P(MOx) 25 -b-P(t-BA) 30 amphiphilic copolymer, thereby demonstrating the effective coupling between the P(MOx) and P(t-BA) blocks. There was also no signal of uncoupled homopolymers which confirmed that copolymer was pure. Additional proof for the obtaining of P(MOx) n -b-P(t-BA) m block copolymers was given by the normalized size exclusion chromatography (SEC) traces of the individual precursor polymers as well as the resulting P(MOx) 25 -b-P(t-BA) 30 block copolymer (Figure 5). The same graphs were realized for all P(MOx) n -b-P(t-BA) m copolymers (supporting information, Figures S5-S7). The shift of the SEC trace for the block copolymers to a lower elution time in comparison with the initial homopolymers clearly indicated the effectiveness of the coupling process via CuAAC in the case of low P(MOx) and P(t-BA) molecular weights, and copolymers were obtained with low dispersities (Ð between 1.1-1.3) (Table 3). In conclusion, from all the results obtained, we concluded that pure P(MOx) n -b-P(t-BA) m amphiphilic block copolymers were isolated. As a consequence, their self-assembly in water was studied.

Self-assembling of P(MOx) n -b-P(t-BA) m amphiphilic copolymers

The ability of block copolymers to self-assemble in selective solvents allowed various well-defined nanostructures with tunable sizes and morphologies such as micelles, for instance. Solutions of P(MOx) nb-P(t-BA) m (0.3 mg.mL -1 ) were prepared in DMF which was a good solvent for both P(MOx) and P(t-BA) blocks, dialyzed against MilliQ water, and finally filtered through a membrane with a nominal pore size of 0.45 μm. [START_REF] Gohy | Block Copolymers II[END_REF]45 Thus, in water, we expected copolymers to form micelles composed of P(t-BA) micellar core surrounded by a P(MOx) corona. The morphology of polymer aggregates was first studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). All samples were characterized in bulk. P(MOx) 11 -b-P(t-BA) 30 was chosen to illustrate the self-assembling of these copolymers as shown in Figure 6 (see also supporting information, Figure S8). AFM measurements were carried out in air using silicon wafers treated with RCA in the tapping mode whereas TEM micrographs were obtained by casting of a dilute micellar solution on a carbon-coated copper grid. The TEM and AFM pictures were in agreement with the formation of spherical micelles. Radii determined by TEM (Table 4) were relatively high due to the tip convolution effects and/or the flattening of the object on the surface. [46][47] The copolymer mixtures were then analyzed by both fluorescence spectroscopy and static light scattering (SLS) in order to determine the critical micellar concentration (CMC) of P(MOx) n -b-P(t-BA) m nanoparticles. In the first technique, pyrene was chosen as the fluorescent probe since it preferentially partitions into hydrophobic microenvironments with a concurrent change in the fluorescence intensities. 48 From the plot of fluorescence intensity versus copolymer concentration (Figure 7a and supporting information, Figure S9), an abrupt decrease in I 1 /I 3 value was detected during increasing copolymer concentrations, indicating the formation of micelles and the transfer of pyrene into the hydrophobic core of the micelles. The critical micellar concentration (CMC) was found around 35 mg.L -1 , except for the P(MOx) 25 -b-P(t-BA) 19 which showed a CMC at 11 mg.L -1 (Table 5). This value was a little bit surprising and from all the results we assumed that the CMC depended on the hydrophilic block length only when hydrophobic block was short. The CMC was also determined by static light scattering (SLS), tracking the intensity as a function of concentration (Figure 7b and supporting information Figures S9-11). For lower concentration in copolymer, a very weak scattered intensity corresponding to unimers was detected whereas the dramatically increase of intensity indicated the formation of micellar structures. The CMC determined by fluorescence spectroscopy were closed from the literature 49 and from those determined by SLS. Once again, the lowest CMC value was obtained for the P(MOx) 25 -b-P(t-BA) 19 amphiphilic copolymer, confirming the fluorescence measurements. Dynamic light scattering (DLS) experiments were performed on copolymer dissolved in water (C = 50 mg.mL -1 ) to get more details on the size distribution and to better determine the hydrodynamic radius (R H ) of the micelles (Figure 8 and supporting information, Figure S11). From the Stokes-Einstein equation, R H was calculated from the DLS correlation function using CONTIN algorithm. For all P(MOx) nb-P(t-BA) m copolymers, the results confirmed the obtaining of monodisperse micelles, showing a polydispersity index around 0.11-0.21 (Table 5). The R H of the micelles ranged from 40 to 60 nm for P(MOx) 11 -b-P(t-BA) m and P(MOx) 25 -b-P(t-BA) m , respectively. We concluded that the hydrodynamic radius was dependant of the hydrophilic poly(oxazoline) block.

CONCLUSIONS

In conclusion, this work reported the synthesis of P(MOx) n -b-P(t-BA) m amphiphilic diblock copolymers by copper catalyzed azide-alkyne cycloaddition reaction. Such kind of synthesis was to date only rarely reported in the literature due to the antagonist properties of the combined homopolymers. -Alkynepolyoxazoline and ω-N 3 -P(t-BA) were first prepared and characterized by MALDI-Tof which proved the efficiency of CROP and ATRP to functionalize the end groups of P(t-BA) and P(MOx) by azide and alkyne end groups, respectively. Then, coupling of homopolymers was achieved varying molecular weights of both blocks and proved to be efficient for relatively low molecular weight derivatives. Resulting copolymers were successfully characterized by DOSY NMR and SEC, showing the interest of Huisgen's cycloaddition in polymer field. Finally, self-organization of P(MOx) n -b-P(t-BA) m copolymers in water was studied. As polymer-polymer coupling was successful, allowing the obtaining of pure amphiphilic copolymers, we managed to form very well-defined micelles in water, which were fully characterize by fluorescence spectroscopy, SLS, DLS, and microscopy.

Figure 1 .

 1 Figure 1. MALDI-Tof spectrum of the -alkyne-poly(2-methyl-2-oxazoline) (M n,th = 2600 g.mol -1 ) (HCCA matrix with CF 3 COOK salt).

Scheme 2 .

 2 Scheme 2. Chemical structures of poly(2-methyl-2-oxazoline) chains found by MALDI-Tof analysis (polymerization and side reactions).

Figure 2 .

 2 Figure 2. MALDI-Tof spectrum of the ω-N 3 -poly(tert-butyl acrylate) (M n,th = 2200 g.mol -1 ) (2,3dihydrobenzoic acid matrix with NaI).

Figure 3 . 1 H

 31 Figure 3. 1 H NMR spectrum of P(MOx) 25 -b-P(t-BA) 19 in deuterated chloroform.

Figure 4 .

 4 Figure 4. 2D DOSY NMR in DMF-d 7 of -alkyne-P(MOx) 25 (blue dotted line), ω-N 3 -P(t-BA) 30 (red dotted line), and P(MOx) 25 -b-P(t-BA) 30 copolymer.

Figure 5 .

 5 Figure 5. SEC profiles of -alkyne-P(MOx) 11 (blue line), ω-N 3 -P(t-BA) 19 (red line) and resulting P(MOx) 11b-P(t-BA) 19 diblock copolymer (green line) (eluent: DMF, 0.8 mL.min -1 , 70 °C, standards: PMMA).

Figure 6 .

 6 Figure 6. AFM (left) and TEM (right) images of micelles (P(MOx) 11 -b-P(t-BA) 30 ).

Figure 7 .

 7 Figure 7. CMC a) by fluorescence spectroscopy ([P(MOx) 11 -b-P(t-BA) 30 ] varied from 1x10 -3 to 0.2 mg.mL -1 ; [pyrene]= 9x10 -7 M); b) by static light scattering measurements ([P(MOx) 11 -b-P(t-BA) 30 ] varied from 1x10 - 3 to 0.2 mg.mL -1 ).

Figure 8 .

 8 Figure 8. Autocorrelation function measured by DLS at scattering angle equal to  = 173°, at 25 °C and distribution of the hydrodynamic radius calculated by CONTIN algorithm for 50 mg.L -1 for P(MOx) 11 -b-P(t-BA) 30 .

36 1 H

 1 NMR (300 MHz, CDCl 3 ) δ (ppm): 4.2 (m, N-CH 2 -CCH), 3.5-3.1 (m, CH 2 P(MOx)), 2.4 (s, CH 2 -CCH), 2.3-2.0 (m, CH 3 P(MOx)).

RM formula RM calc RM exp M p , cal M p,exp Calculation

	C 3 OH 4 K	95.1	95.05	2221.35 2221.31	n*85.05+RM
	C 7 NOH 10	124	124.15 2250.25 2250.44	(n-1)*85.05+RM
	C 5 ONH 9 K	138.1	138.15 2264.35 2264.42	(n-1)*85.05+RM
	C 3 OH 5	57	57.1	2268.3	2268.40	n*85.05+RM
	C 7 H 9 NOK	162.1	163.05 2288.35 2289.33	(n-1)*85.05+RM
	C 4 H 8 NO	86	86	2297.3	2297.30	(n-1)*85.05+RM

RM: residual mass.

Table 2 .

 2 Analytical data derived from MALDI-Tof spectroscopy for the characterization of ω-N 3 -poly(tertbutyl acrylate) (M n,th = 2200 g.mol -1 ).

	O			O						O
	EtO	N 3 m	Na +	EtO	m-1	N 3	Na +	EtO	m-1 OH	Na +
	O	O			O	O	O	OH			O	O
	1				2						3
	Assignment		RM formula	RM cal			RM exp		M p , cal	M p,exp	Calculation
			C 6 O 2 N 3 H 11 Na	180.19 180.45 1716.79	1717.05	m*128.05+RM
			C 9 O 4 N 3 H 15 Na	252.26 251.48 1788.86	1788.08	(m-1)*128.05+RM
			C 6 O

3 H 12 Na 155.17 153.48 1819.82 1818.13 m*128.05+RM RM: residual mass.

Table 3 .

 3 Characterization data for P(MOx) n -b-P(t-BA) m amphiphilic diblock copolymers.

		M n,PMOx	a	M n, P(t-BA)	a	M n, PMOx-b-P(t-BA)	a	Ð a
	copolymer
		g.mol -1	g.mol -1		g.mol -1
	P(MOx) 11 -b-P(t-BA) 19	1000		2500		3400		1.1
	P(MOx) 11 -b-P(t-BA) 30	1000		3900		4500		1.3
	P(MOx) 25 -b-P(t-BA) 19	2200		2500		5600		1.1
	P(MOx) 25 -b-P(t-BA) 30	2200		3900		6600		1.1
	P(MOx) 11 -b-P(t-BA) 113	1000		14500		-		-
	P(MOx) 25 -b-P(t-BA) 113	2200		14500		-		-

a Calculated by size exclusion chromatography in DMF (0.8 mL.min -1

Table 4 .

 4 Radius and height of the micelles determined by Atomic Force Microscopy (AFM).

	Copolymer	R (nm)	H (nm)
	P(MOx) 11 -b-P(t-BA) 19	50	20
	P(MOx) 11 -b-P(t-BA) 30	120	75
	P(MOx) 25 -b-P(t-BA) 19	85	35
	P(MOx) 25 -b-P(t-BA) 30	150	60

Table 5 .

 5 Characterization of P(MOx) n -b-P(t-BA) m in water by fluorescence spectroscopy and scattering

	light experiments.
	Copolymer	CMC fluo (mg.L -1 )	CMC SLS (mg.L -1 )	(nm) a R H	Dispersity a
	P(MOx) 11 -b-P(t-BA) 19	35	36	40	0.21
	P(MOx) 11 -b-P(t-BA) 30	34	38	45	0.21
	P(MOx) 25 -b-P(t-BA) 19	11	19	55	0.11
	P(MOx) 25 -b-P(t-BA) 30	36	45	60	0.18

a : determined by Dynamic Light Scattering (DLS) from CONTIN method.

ACKNOWLEDGEMENTS. The authors would like to thanks the French Ministry of Education and Research for a grant (B.G.), and Dr. Romain Parret and Prof. Jean-Marie Devoisselle for technical assistance in AFM and fluorescence spectroscopy, respectively.


R Ponche
email: remi.ponche@irccyn.ec-nantes.fr

J Y Hascoet

O Kerbrat
email: olvier.kerbrat@irccyn.ec-nantes.fr

P Mognol

A new global approach to design for additive manufacturing

Nowadays, due to rapid prototyping processes improvements, a functional part can be built directly through additive manufacturing. It is now accepted that these new processes can increase productivity while enabling a mass and cost reduction and an increase of the parts functionality. However, in order to achieve this, new design methods have to be developed to take into account the specificities of these processes, with the Design For Additive Manufacturing (DFAM) concept. In this context, a methodology to obtain a suitable design of parts built through additive manufacturing is proposed ; both design requirements and manufacturing constraints are taken into account.

Introduction

Recent progress has permitted transition from Rapid Prototyping to Additive Manufacturing (AM). Indeed, today, with this kind of manufacturing process, not only prototypes can be produced but also real functional parts in current materials including metals, polymers and ceramics [START_REF] Mognol | A novel approach to produce functionally graded materials for additive manufacturing[END_REF]. Because AM for production eliminates the need of tooling and can generate free forms, many of the current restrictions of design for manufacturing and assembly are no longer valid [START_REF] Pessard | Complex cast parts with rapid tooling : rapid manufacturing point of view[END_REF]. However, whatever the technology used [START_REF] Wohlers | Wohlers report : Additive Manufacturing State of the Industry[END_REF], as in all the manufacturing processes, the AM ones have characteristics and specificities of their own which may have an impact on the manufactured parts quality. In order to utilize in the best way the AM possibilities in terms of design and to ensure the quality of the produced parts, a global numerical chain which allows to move from functional specifications of a part to its manufacturing, must be defined (figure 1). The purpose of this numerical chain [START_REF] Bonnard | A new digital chain for additive manufacturing processes[END_REF] is to reach a global process control from knowledge of process obtained from experimentations, measurements and simulations. Among the prerequisite to achieving such numerical chain, a Design For Manufacturing (DFM) [START_REF] Gupta | Automated manufacturability analysis : a survey[END_REF] approach is required which allows the taking into account of the AM processes capabilities and limits directly from the design stage.

Design for additive manufacturing

Several works have been carried out concerning the classical DFM approach [START_REF] Shukor | Manufacturability analysis system : issues and future trends[END_REF] for AM. Concerning manufacturability estimation, manufacturing cost and time have been analysed [START_REF] Alexander | Part orientation and build cost determination in layered manufacturing[END_REF][START_REF] Ruffo | Cost estimation for rapid manufacturing laser sintering production for low to medium volumes[END_REF] according to the manufacturing sequence. Similarly, the relationship between parts surfaces quality and manufacturing sequence has been studied [START_REF] Arni | Manufacturability Analysis of Flatness Tolerances in Solid Freeform Fabrication[END_REF][START_REF] Ancäču | The computation of Pareto-optimal set in multicriterial optimization of rapid prototyping processes[END_REF]. From these different specific works, [START_REF] Kerbrat | Manufacturability analysis to combine additive and subtractive processes[END_REF] proposed a methodology to map parts in relation to its manufacturability. Based only on geometrical analysis, these studies are limited because they do not take into account the physical phenomena which occur during the manufacturing process [START_REF] He | Transport phenomena during direct metal deposition[END_REF][START_REF] Wen | Modeling of coaxial powder flow for the laser direct deposition process[END_REF] which may have an impact on parts quality [START_REF] Alimardani | A 3D dynamic numerical approach for temperature and thermal stress distributions in multilayer laser solid freeform fabrication process[END_REF] and therefore on their manufacturing time and cost. Concerning manufacturability improvement, there have been very few studies reported on AM processes. General build guidelines have been established [START_REF] Filippi | The Design Guidelines (DGLs), a knowledge-based system for industrial design developed accordingly to ISO-GPS (Geometrical Product Specifications) concepts[END_REF][START_REF] Teitelbaum | Proposed build guidelines for use in fused deposition modeling to reduce build time and material volume[END_REF] and a methodology proposed by [START_REF] Sambu | Geometric Tailoring : A Design for Manufacturing Method for Rapid Prototyping and Rapid Tooling[END_REF] enables modification of some non-critical geometric features. Moreover, a method for providing mesostructures within a part so as to achieve improved functional requirements part has been established [START_REF] Rosen | Computer-aided design for additive manufacturing of cellular Structures[END_REF]. Here again, the purpose is to minimize fabrication cost and time without a real awareness of the process planning, which do not allow to guarantee the parts quality expected. In addition to these limitations, classical DFM approaches may restrict the new perspectives of design opened up by the AM processes.

Partial approach versus global approach

Indeed, all these works enable to determine and to improve the manufacturability of a part from its CAD model for a given AM process. Because they start from an initial geometry (given by the initial CAD model), these analysis can be qualified as "partial approaches" (figure 2). In this case, it is difficult to determine the real optimized characteristics for a given AM process while fulfilling original functional specifications. Indeed, the initial CAD model was thought to be manufactured by an initial manufacturing process often very different from an AM process (for example machining which is the most often used), moreover the proposed modifications are local and the result is never far from the initial design. The CAD model that is obtained is thus never really designed for the AM process that is chosen. On the contrary, a "global approach" (figure 3) starts directly from both the chosen manufacturing process characteristics and the functional specifications of the parts to design. Designers can thus determine the geometry which optimizes the use of the chosen AM process characteristics while meeting the functional specifications. The purpose is not to limit geometry by an initial idea of the part shapes but to define it only from the manufacturing process and the functional specifications. This new way of thinking is in opposite with traditional Design For Manufacturing (DFM) methodologies. However, capitalization of the entire knowledge about the manufacturing processes is needed. A beginning of a global approach, based on topology optimization with manufacturing constraints, has been applied to casting process ( [START_REF] Harzheim | A review of optimization of cast parts using topology optimization part II[END_REF]).

Scope of the paper

Although the global approach seems very interesting, no existing work has been carried out in this way in AM. Because these processes are quite young, still little known and very different from the other manufacturing processes, the psychological inertia phenomena may prevent the designer from utilizing in the best way all their capabilities ( [START_REF] Hague | Unlocking the Design Potential of Rapid Manufacturing[END_REF]). Moreover, most of the AM processes are based on a layer by layer manufacturing where the material is locally merged thanks to a local moving energy source (usually a laser or an electron bean) which follows a programmed manufacturing trajectory. It entails that the characteristics of the manufactured volumes, in terms of microstructure [START_REF] Costa | Rapid tooling by laser powder deposition : Process simulation using finite element analysis[END_REF], geometry [START_REF] Alimardani | A 3D dynamic numerical approach for temperature and thermal stress distributions in multilayer laser solid freeform fabrication process[END_REF] and manufacturing time [START_REF] Ancäču | The computation of Pareto-optimal set in multicriterial optimization of rapid prototyping processes[END_REF] depend first on the manufacturing direction and secondly on the manufacturing trajectories. In addition to the consideration of the processes' characteristics, the choice of the Manufacturing Direction (M D) and the Manufacturing Trajectories (M T ) according to functional specifications is thus the key of a global DFAM which would facilitate designers to explore new design spaces. That is why a new methodology which starts directly from both the functional specifications and the process characteristics is proposed in this paper.

Firstly, in section two, the required data is presented. Then in section three, the different steps of the methodology, based on the choice of M D, are explained. The methodology has been applied on a part manufactured by a Powder Based Metal Deposition (PBMD) process [START_REF] Ruan | A Review of Layer Based Manufacturing Processes for Metals[END_REF] ; this constitutes the fourth section.

Functional specifications

The global purpose is to propose a structured approach which would help the designer to integrate the knowledge of the chosen AM process in his design to meet the functional specifications. The functional specifications can be detailed as :

-Functional surfaces (F S) : type, dimensions and position -Dimensional and geometrical specifications, linked to the F S -Mechanical requirements, they depend on the chosen material characteristics -"Empty volumes" : dimensions and position. They correspond to the volumes which must not contain material, due to the assembly constraints of the designed part into the system to which it belongs.

Context

The study context is an influential factor. Because it can be translated into a concrete objective in terms of mass, cost or manufacturing time, it has thus to be taken into account too.

Manufacturing characteristics

Manufacturing characteristics are linked to one another and they cannot be seen separately, a global view which draws upon all the knowledge and experience of the community is consequently needed. The main characteristics of the manufacturing machines which must be taken into account are :

-Kinematics -Maximal and minimal dimensions -Capability in terms of accuracy -Required accessibility But also physical phenomena involved in the manufacturing process, which are decisive in terms of final properties of parts and which are linked to the manufacturing sequence.

Finishing process characteristics

Similarly, if specifications (geometrical and dimensional) cannot be directly reached by the AM process that is chosen, a finishing process is needed. Because it can influence the final geometry, in particular in terms of overthickness and required accessibility, it has to be taken into account.

The proposed design methodology

From this data, a structured methodology can help designers in taking into account the manufacturing constraints while suggesting him an appropriate design for AM. The methodology is presented in figure 4. It is divided into three main steps which enable to include gradually manufacturing knowledge in the shapes and the volumes of the parts to be designed. The first step is a global analysis which allows to delimit the design problem in terms of geometrical dimensions in relation to the dimensional characteristics of the AM process. The second one allows to fulfil the dimensional and geometrical specifications in relation to the AM process capability and the finishing process characteristics. Finally, the third step allows to fulfil the physical and assembly requirements in relation to the capability of the AM process

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Step 1 : Analysis

The F S are obtained from the functional analysis of the product and are given by the designer. The first methodology step enables to find out if all the surfaces can be merged with one another by the chosen AM process into a single part. A first geometrical analysis is carried out ; it takes into account the maximal and minimal dimensions which can be obtained by the chosen manufacturing machine. If the dimensions are not suitable, the product has to be modified or divided into different parts by the designer and the functional specifications of these new parts are then studied.

Step 2 :Determination of the functional volumes

The Functional Volumes (F V ) are defined directly from the F S on to which a thickness is added. Indeed, only the tolerances in the normal direction of the surfaces are significant in terms of functionality. The others are initially ignored. The thickness, denoted E, depends on :

-Dimensional accuracy of the AM process denoted a -Tolerances linked to the F S, denoted p For each F S, there are two different possibilities : firstly, if a finishing step is not needed (p ≥ a), thickness is determined from equation 1 which ensures the functional minimal thickness and from equation 2 which ensures the compatibility with the AM process.

E ≥ t + a 2 (1)
E = n.d -(n -1).d.α ( 2
)
where t is the minimal thickness corresponding to the local mechanical requirements ; d is the minimal dimension that can be obtained by the chosen AM process ; α is the overlap between two adjacent paths ; and n is a positive integer. Equation 1 is illustrated in figure 5(a). If a finishing step is needed (p < a), then the thickness is determined from equations 2, 3 and 4 (figure 5(a)).

E ≥ t + a 2 + e min ( 3
)
E ≤ t + a 2 + e max ( 4
)
Where e min and e max are the minimal and maximal overthicknesses which depend on the finishing process and the surfaces geometry. There can be different values of the parameters d, α and a (according to the M D).

Step 3 : Determination of the linking volumes

The purpose of this step is to merge the F V to define the volumes of the part while taking into account at best both design requirements and manufacturing constraints. In the case of additive manufacturing, volumes are usually obtained layer by layer. It involves that their geometries strongly depend on the M D. Indeed, the choices of the M D have a direct influence on material quantity (need of supports), build time [START_REF] Singh | Multi-Direction Slicing for Layered Manufacturing[END_REF] and mechanical properties [START_REF] Caulfield | Dependence of mechanical properties of polyamide components on build parameters in the SLS process[END_REF]. Moreover, the M T defining the energy source path during the process, have a strong impact on the physical phenomena that occur during the manufacturing process and therefore on the final part quality [START_REF] Foroozmehr | Effect of path planning on the laser powder deposition process : thermal and structural evaluation[END_REF]. That is why, the determination of the M D and of the M T is at the center of the methodology.

The Linking Volume (LV ) definition is divided into four steps. The first step is to determine the most critical M D, which can be characterized by the shape or the number of F V which can be manufactured in the same way. It is carried out according to the study context and the capability of the process given by its kinematics and the physical phenomena involved. The second step is, in the chosen M D, to merge the selected F V which can be manufactured from the same substrate. It is carried out according to the empty volumes, which must not be, in the M D, between two F V manufactured from a same substrate. In the third step, the substrates and supports shapes are determined according to :

-The selected F V -The other F V -The kinematics of the process -The accessibility required by the process -The mechanical requirements -The empty volumes -The study context -The physical phenomena that occur during the process The latter being strongly linked to the type and the shape of the M T that are used, it is essential to select suitable M T among all the possibilities to control them and to guaranty the expected geometrical quality. A classification of the different M T that are possible has been done (figure 6) to describe and parametrize each one of them. The last step is to check if all the volumes are merged. If it is not the case, the first three steps are repeated while taking into account the F S that have not been yet analyzed and the substrates obtained previously and so on until all the F V are merged. In the end, the process is complete and an appropriate design for the chosen AM process is obtained. Indeed, the AM process specifications are taken into account step by step in parallel with the functional requirements. this ensures that the most possible process-related knowledge is taken into account to obtain the final shapes of the studied part.

Example

The proposed methodology has been applied to a case of a robot hinge in stainless steel (figure 7). The study input data are detailed then the three steps of the DFAM methodology are illustrate in this section.

Input data

Functional specifications

The case is composed of twenty F S : four bearing holders (hollow cylinders) and sixteen flat surfaces, which are shown with their nominal dimensions in black in figure 8. To enable the assembly of the case with the other parts of the robot, some empty volumes are defined. They are represented by the transparent volumes.

The functional analysis of the robot has enabled to determine the geometrical and dimensional specifications linked with each surfaces. An extract is shown in figure 9. The mechanical requirements are translated into a final minimal thickness t for each surface. It is 5 mm for the hollow cylinders and 3 mm for the based planes.

Context

Because of the robotic context, the global objective is to minimize the mass of the studied part.

Manufacturing characteristics

The additive manufacturing process

The AM process chosen to manufacture this case is the CLAD process (figure 10). It is a PBMD process, based on the 3D layer by layer deposition of laser melted powders. Its main characteristics are presented in table 1.

In this example, the assumptions that parameters d, a and α are equal in all the directions is made.

The constraints due to the physical phenomena linked with the process (in particular the thermal phenomena) are considered in ways : first, the substrates thickness must be at least equal to the thicknesses of the volumes that it enables to manufacture. Secondly, because the discontinuities in the M T may generate an unwanted variation of the process parameters and also a gap between the designed and the manufactured part, the geometry will be chosen to limit them.

The finishing process

The finishing process that is chosen is high speed milling ; in view the specification related to the F S, this choice involves a minimal and a maximal overthickness of 0.5 mm and 1 mm respectively.

Step 1 : Analysis

According to the geometrical analysis of the functional surfaces (figures 8 and 9) and to the maximal and minimal dimensions imposed by the manufacturing process (table 1), all the dimensions are compatible and all the F S can be merged in only one part.

Step 2 : determination of the functional volumes

The thicknesses E are determined from the different parameters of equations 2 and 3 ; For each F S the results are given in table 2. All the functional volumes are shown in figure 11(a

Step 3 : determination of the linking volumes

The (M D) are determined, due to the robotic context, to minimize the mass of the part and therefore the support structures quantity. A geometrical analysis of the F V enables to determine two M D : Z 1 and Z 2 (figure 11(a)). Z 1 enables to manufacture heighten F V (in dark grey) and Z 2 enables to obtain the two last (in light grey). If one or other of the M D is favoured (which means that it is analysed in first), the final part geometry could be not the same. Both cases will be therefore analysed and the geometry which represented the best way of satisfying the study context will be selected. In case 1, Z 1 is favoured whereas in case 2, Z 2 is favoured.

Case 1 : when Z 1 is favoured

Linking volumes in Z 1 To minimize supports in the empty volumes shown in figure 8 and thus to minimize the finishing operations, all the selected F V linked to Z 1 must not be manufactured together but from two different substrates. Their positions are determined (figure 11(b)) to minimize supports, time of finishing and to guarantee accessibility for the powder feed nozzle.

The selected F V geometry is locally modified in relation to the value of e max and e min to minimize the discontinuity of the M T . In particular here, in the case of the raster discrete paths, it involves to define a radius R (figure 12) which value is given by equation 5.

R =

(2)

(2) -1

.(e maxe min ) (

Then the geometry of the substrates is defined according to the F V position and their thickness (in black in figure 11(c)). Because all F V are not merged, a second M D is analysed. Linking volumes in Z 2 As previously, because of the empty volumes, the two cylinders linked to Z 2 cannot be manufactured together. It involves two substrates (figure 13(a)). Moreover, the space between these F V and those already analysed (those linked to Z 1) ) being lower than the required one by the powder feed nozzle of the CLAD machine (table 1), the accessibility requirements is not satisfied and the M D Z 2 cannot be used, the process is repeated one more time.

Linking volumes in Z 3 A third M D Z 3 is determined from the previous substrates (linked to Z 1 ) and F V (figure 13(b)). The substrate and supports linked to Z 3 are determined, as previously, due to the context, to minimize the material quantity and to avoid, much as possible the empty volumes. The substrate dimensions are given by the volumes that it merges. The supports geometry are simply defined by the orthogonal projection of the volumes linked to Z 3 onto the substrate (figure 13(c)). Finally, all the F V are merged, the design process is then complete and the final blank part is obtained. From the F V in figure 11(a), the same reasoning is applied, starting with the analysed of Z 2 .

Linking volumes in Z 2 In the same way as above, because of the empty volumes, the two cylinders linked to Z 2 cannot be manufactured together. Two substrates whose geometry is shown in black in figure 14(a) are therefore defined.

Linking volumes in Z 1 Similarly, all the F V linked to Z 1 cannot be manufactured together. First, as in the case where Z 1 was favoured, to minimize supports in the empty volumes, a minimum of two separated substrates is needed. Moreover, because of the manufacturing of the cylinders linked to Z 2 , the accessibility required by the powder feed nozzle involves that each one of these two substrates have to be subdivided once again into tree separated substrates. In the same way as previously, the geometry of the F V linked to Z 1 is locally modified in relation to the value of e max and e min and then the substrates geometry is defined (in black in the figure 14(b)).

New linking volumes in Z 2 All the F V being not merged, an other M D should be thus analysed. Because of the accessibility requirement, all the substrates previously obtained cannot be merged to one another following only one M D. Z 2 , being already selected and allowing to merge again several volumes of the part, is re-analysed. The result is shown in figure 14(c).

Linking volumes in Z 3 Because all the F V are still not merged, the design process is repeated once again. All the previous substrates (defined in Z 2 and Z 1 ) can be merged into a third M D : Z 3 . The substrate position and geometry is determined then the supports are defined by the orthogonal projection of the volumes linked to Z 3 onto the substrate (figure 14(d)).

Finally, all the F V are merged, the design process is thus complete and the final blank part is obtained.

Final result

Thanks to the proposed methodology, each shape of the part has been designed in order to utilize the CLADő process characteristics and capabilities to fulfil the study functional specifications while taking into account its general context. In the initial stage, from the geometrical analysis of the functional surfaces and according to the objective of minimizing the final part mass, two manufacturing directions have been selected (Z 1 and Z 2 ). Because, the favouring of the one or other may impact on the the final part geometry, the two cases have been studied. The final results, obtained after the considering of the finishing operation which enables to meet the required specifications (geometrical and dimensional) and to remove the material in the empty volumes, are shown in figure 15. A FEM analysis has been done to simulate the mechanical behaviour of the proposed designs under the robot normal condition of use. Because, the results are quite close to each other compared with the material limits, they do not really enable to make a choice between both proposed solutions. However, as it is shown in table 3, the favouring of Z 2 over Z 1 results in a lower mass of the blank part (around 19%) and of the finished part (around 9%). It allows therefore a lower manufacturing cost and to meet better the global objective given by the robotic context of the study. This solutions is thus finally selected.

Conclusion

This paper described the initial stage of a promising research project which deals with a global DFAM approach. A new methodology is proposed to obtain an appropriate design for AM processes. Contrary to the classical DFM approaches, to prevent the psychological inertia phenomena which may limit the design innovation and to best utilize the AM processes capabilities, the proposed methodology starts directly from both functional specifications and AM processes characteristics. The required data for a such global DFAM approach has been presented. Then the three steps of the methodology allowing to reach to take into account all of them has been detailed and illustrated by a case study taken from the robotic field.

Further research will be conducted to optimize the methodology in particular regarding the local optimization of the shapes and the internal structures of the linking volumes in terms of functionality as it is done for example by [START_REF] De | Virtual topological optimisation of scaffolds for rapid prototyping[END_REF]. In parallel, new criteria of choice for manufacturing trajectories will be developed to always more adapt the local geometry regarding to the physical phenomena which occur during the manufacturing process.

Figure 1 -

 1 Figure 1 -Global numerical chain concept.

Figure 2 -

 2 Figure 2 -Partial approach.

Figure 3 -

 3 Figure 3 -Global approach.

Figure 4 -

 4 Figure 4 -The proposed DFAM methodology.

Figure 5 -

 5 Figure 5 -Definitions of the thickness : (a) without finishing(b) with finishing.

123452674893AB782CD67E89DF 123456 Figure 6 -

 1234566 Figure 6 -Classification of the Manufacturing Trajectories (M T ).

Figure 7 -

 7 Figure 7 -Global view of the studied system.

Figure 8 -

 8 Figure 8 -The F S and the empty volumes of the part to design.

Figure 9 -

 9 Figure 9 -An extract of the specifications taken from the functional analysis of the case.

  Figure 10 -Clad process.

1 2 (

 12 b) Position of the substrates.

1 2 (

 12 c) The LV geometry in Z 1 .

Figure 11 -

 11 Figure 11 -Definition of the LV in Z 1 when Z 1 is favoured.

Figure 12 -

 12 Figure 12 -Local geometrical modifications to avoid the M T discontinuities.

1 2 (

 12 a) Position of the substrates in Z 2 .

1 2 (

 12 b) Position of the substrate in Z 3 .

1 2 (

 12 c) Geometry of the substrate and the supports in Z 3 .

Figure 13 -

 13 Figure 13 -Definition of the LV in Z 2 then in Z 3 when Z 1 is favoured.

4. 5 . 2

 52 Case 2 : when Z 2 is favoured

1 2 (

 12 a) The LV geometry in Z 2 .

1 2 (

 12 b) The LV geometry in Z 1 .

1 2 (

 12 c) The LV geometry in Z 2 after its reanalysed.

1 2 (

 12 d) The LV geometry in Z 3 .

Figure 14 -

 14 Figure 14 -Definition of the different LV when Z 2 is favoured.

(a) When Z 1

 1 is favoured. (b) When Z 2 is favoured.

Figure 15 -

 15 Figure 15 -The final geometry.

Table 1 -

 1 CLAD process characteristics.

Table 2 -

 2 ). Definition of the F S thicknesses.

	Surface type	t	a	e min	e max	d	E
		(mm) (mm) (mm) (mm) (mm) (mm)
	Cylinder	5	0.2	0.5	1	0.8	5.9
	Flat	3	0.2	0.5	1	0.8	3.6

Table 3 -

 3 Characteristics of the two prosed solutions.

Proposed requirements for a global DFAMIn this section, requirements for a global DFAM are presented. They constitute the required data for the proposed methodology.


Tobias Isenberg
email: isenberg@cs.rug.nl

Mark Hancock
email: mark.hancock@uwaterloo.ca

Gestures vs. Postures: 'Gestural' Touch Interaction in 3D Environments

Keywords: Gestures, (quasi-)postures, direct-touch interaction, direct manipulation, 3D interaction, scientific visualization

We revisit the concept of a 'gesture' and discuss it, in particular, in the context of 3D interaction. We argue that one should distinguish between postures, quasi-postures, and gestures and that the first two should be (and are currently) preferred in 3D interfaces for directly-manipulative interaction with the environment in user-maintained modes. In scientific visualization we saw use cases of quasi-postures, suggesting that they are one way to re-use the same input space in order to facilitate several means of exploration.

Introduction

Multi-touch interaction has become one of the most prevalent metaphors for interacting with a variety of different devices including smart phones, tablet computers, tabletops, and wall displays. It also has been applied to numerous different domains, including gaming, museum exhibits, psychotherapy [START_REF] Hancock | Supporting Sandtray Therapy on an Interactive Tabletop[END_REF], 3D scene design [START_REF] Kin | Eden: A Professional Multitouch Tool for Constructing Virtual Organic Environments[END_REF], and interactive scientific visualization. Not only in this latter domain (our own background and area of interest) but also more generally in most touch-interactive environments we observed a frequent naming of (multi-)touch 'gestures' as one of the reasons that makes direct-touch interaction intuitive and natural. The term 'gesture' and its use in the context of touch interaction, however, is often not clearly defined or well understood which can lead to confusion when discussing direct-touch interaction techniques. We found this confusion, in particular, to be relevant in 3D interaction due to its focus on the manipulation of objects or data in 3D space from the 2D touch surface (Fig. 1). In this paper we thus attempt a definition of terms related to 'gestures' to guide the further discussion of their role, properties, and applicability in 3D interaction. We discuss gestures,1 related terms, and interaction properties in the context of general touch-based Figure 1: In touch-driven scientific visualization settings, the input provided on the 2D touch-sensitive display has to be mapped to 3D manipulations. Moreover, multiple different exploration techniques such as data navigation, data manipulation, data selection, cutting plane placement and manipulation, data probing, seed particle placement, etc. have to be accessible from within the same input space and researchers need to be able to quickly and intuitively switch between these different interaction techniques [START_REF] Isenberg | Position Paper: Touch Interaction in Scientific Visualization[END_REF].

interaction with 3D environments and, specifically, environments for scientific visualization to facilitate the creation of well-designed 3D data manipulation techniques.

'Gestural' Interaction

In touch interfaces we face the challenge that the vocabulary of the primary means of input is limited: touch surfaces typically register touch down, touch move, and touch release events. In this respect they resemble mouse-based interfaces where a similar limitation of a few fundamental event types exists. In both cases, thus, we need to specify additional modes to clearly state what effect a given interaction event should cause and, ultimately, to be able to create flexible interfaces. While mouse-based interfaces can rely on, in particular, a number of mouse or keyboard buttons to specify such modes, such buttons are not readily available in touch-based interaction. Therefore, interface designers typically rely on 'gestures' to specify interaction modalities. 'Gestural' interaction is often employed in touch-based (and related) interfaces. For example, researchers have established gesture catalogs (e. g., [START_REF] Villamor | Touch Gesture Reference Guide[END_REF]) that support a variety of applications, have discussed object-dependent gestures (e. g., [START_REF] Nijboer | Exploring Frame Gestures for Fluid Freehand Sketching[END_REF]), or distinguish between the shape or size of the touching finger/hand/object (e. g., [START_REF] Isenberg | Interactive Exploratory Visualization of 2D Vector Fields[END_REF][START_REF] Wu | Multi-Finger and Whole Hand Gestural Interaction Techniques for Multi-User Tabletop Displays[END_REF]). Others have extracted intuitive gestures through demonstration [START_REF] Cohé | tBox: A 3D Transformation Widget Designed for Touch-Screens[END_REF][START_REF] Frisch | Investigating Multi-Touch and Pen Gestures for Diagram Editing on Interactive Surfaces[END_REF][START_REF] North | Understanding Multitouch Manipulation for Surface Computing[END_REF][START_REF] Wobbrock | User-Defined Gestures for Surface Computing[END_REF], have examined cooperative gestures performed by groups of people [START_REF] Morris | Cooperative Gestures: Multi-User Gestural Interactions for Co-Located Groupware[END_REF], or have focused on multi-modal interaction with gestures (e. g., [START_REF] Tse | Multimodal Multiplayer Tabletop Gaming[END_REF]). However, it seems that most authors employ an implicit notion of 'gestures.' A notable exception are Baudel and Beaudouin-Lafon [START_REF] Baudel | Charade: Remote Control of Objects Using Free-Hand Gestures[END_REF] who state that gestures have a start position, a dynamic phase, and an end position. The start and end positions do not only refer to positions of a hand in space but also include the notion of finger/wrist configurations and their concept of a 'gesture' clearly contains the notion of a dynamic motion. This notion is seen in contrast to "steady positions" that other systems recognize which are considered to be less powerful than gestures. Interestingly, this gesture concept is thought to specify (a series of) discrete lexical entities and the use of gestures as commands, i. e., the discrete change into different statuses or modes. A related concept is discussed by Wu et al. [START_REF] Wu | Gesture Registration, Relaxation, and Reuse for Multi-Point Direct-Touch Surfaces[END_REF] who emphasize the design principles of registration and relaxation. Gesture registration relies on a posture to start the recognition, as a "functional transition of a tool from one interaction style to another," similar to Baudel and Beaudouin-Lafon's [START_REF] Baudel | Charade: Remote Control of Objects Using Free-Hand Gestures[END_REF] notion of gestures as commands. The second concept, the relaxation phase, relaxes the posture requirements to allow people to perform a gesture more freely. It also emphasizes that motion is an integral part of the gesture, a notion also emphasized by others [START_REF] Cleveringa | Assisting Gesture Interaction on Multi-Touch Screens[END_REF]. However, Wu et al. also mention a distinction between continuous and discrete gestures, probably referring to their postures being part of the 'gesture' concept. In fact, they hint at "a more sophisticated" gesture definition which "considers both the hand posture and dynamic actions that occur immediately after the posture is recognized, within a predefined time window."

We base our own definitions on a similar notion and discuss why such extended postures can be advantageous. Wobbrock et al. [START_REF] Wobbrock | User-Defined Gestures for Surface Computing[END_REF] explore the 'gesture' concept by describing a taxonomy which classifies gestures according to their form, their nature, their binding, and their flow: The form describes whether a hand pose is static or dynamic both in its location and appearance. The nature describes whether a gesture visually depicts a symbol, acts physically on objects, indicates a metaphor, or has no related connection to the object it references. A gesture's binding refers to a gestures location with respect to object or world features. Finally, the flow can be continuous or discrete, i. e. the invoked response can occur during or after the gesture. This taxonomy adds several important aspects of gestures. The form dimension both emphasizes the notion of a configuration or posture as the basis of a gesture as well as the temporal aspect-gestures are defined as an action that takes a given time during which the finger(s)/hand(s) can be moved. An aspect added by Wobbrock et al. [START_REF] Wobbrock | User-Defined Gestures for Surface Computing[END_REF] is the dependence of gestures on a frame of referencea specific object or the world in general. Another important aspect added in Wobbrock et al.'s [START_REF] Wobbrock | User-Defined Gestures for Surface Computing[END_REF] taxonomy is that gestures can already have specified an action (mode) while they are still being performed-leaving the continuing motion as a means to provide further input. One other observation by Wobbrock et al. [START_REF] Wobbrock | User-Defined Gestures for Surface Computing[END_REF] that is important for our discussion is that people, when proposing gestures for certain actions, think about the gestures using metaphors such as the computer desktop and/or program windows. This notion, one could speculate, may have been prompted by their experiment's design which used two-dimensional "blocks" that were manipulated by "commands" on a two-dimensional surface. This design places much emphasis on abstract and discrete events that are common in the computer desktop such as opening, activating, and closing windows, undo/redo, or minimize/maximize. In fact, more than 70% of Wobbrock et al.'s [START_REF] Wobbrock | User-Defined Gestures for Surface Computing[END_REF] commands can be seen as being such discrete tasks. In the context of interacting with 3D objects or spaces, however, we tend to focus, e. g., on the continuous direct manipulation of objects. Thus, we now take a closer look at 'gestural' interaction with 3D environments and the 'gesture' concept.

Postures vs. Gestures

To start this discussion let us first precisely define what we understand a 'gesture' to be. Based on the notions by Baudel and Beaudouin-Lafon [START_REF] Baudel | Charade: Remote Control of Objects Using Free-Hand Gestures[END_REF], Wu et al. [START_REF] Wu | Gesture Registration, Relaxation, and Reuse for Multi-Point Direct-Touch Surfaces[END_REF], and Wobbrock et al. [START_REF] Wobbrock | User-Defined Gestures for Surface Computing[END_REF], we consider a touch gesture to be a way to invoke manipulations in a direct-touch environment that is started by touching the surface in a welldefined initial configuration and that is continued for some time in a well-defined motion pattern (incl. the null motion) during which the configuration may change.

This definition captures the previously mentioned notions of configurations and a dynamic phase [START_REF] Baudel | Charade: Remote Control of Objects Using Free-Hand Gestures[END_REF][START_REF] Wobbrock | User-Defined Gestures for Surface Computing[END_REF] as well as gesture registration and relaxation [START_REF] Wu | Gesture Registration, Relaxation, and Reuse for Multi-Point Direct-Touch Surfaces[END_REF]. It is well suited to specify commands such as described by Wobbrock et al. [START_REF] Wobbrock | User-Defined Gestures for Surface Computing[END_REF] for 2D settings. In most 3D environments for which touch interfaces have been developed so far, however, the interaction focuses on the change of the camera view or the direct manipulation of objects. For example, in a 3D visualization of a brain the main operations are translations, re-orientations, resizing, or the translation of cutting planes. This concentration on the directly-manipulative interaction brings with it an exploratory nature and the need for mental focus on the object manipulation: one primarily thinks about the object/data/scene that one is manipulating while the interaction to enable this manipulation has to become second nature. Therefore, requiring people to remember and employ well-defined motion patterns (which also delay the invoking of the intended manipulations) can be prohibitive to an intuitive interaction. Moreover, the gesture as in the definition can only specify discrete manipulations due to its reliance on a well-defined motion pattern. Therefore, we need to ask if the well-defined motion pattern is essential to specify the intended manipulation. Instead, we could relax this constraint which would free up a potential motion for the specification of input parameters, to support a continuous and direct interaction with 3D objects or the 3D space. We thus define a touch posture to be a way to invoke manipulations in a direct-touch environment that is characterized by touching the surface in a well-defined initial configuration whose effect can be parametrized by a subsequent dynamic action.

The essential difference between gestures and postures is whether the dynamic action (e. g., a motion or a change of the initial configuration) characterizes the manipulation or not. The notion of a posture has the advantage for 3D interaction that directly-manipulative interactions can be specified while the posture is active. With gestures, in contrast, a direct manipulation of the system or objects has to be specified after the gesture has been fully and uniquely recognized. Both definitions thus differ in the type of modes that they encourage. Postures tend to specify user-maintained modes since they are only active as long as (a part of) the posture/configuration is maintained (often the dominant hand that specifies the motion). Gestures often specify system-maintained modes (or status changes) because a motion pattern for specifying parameters can only be started after the gesture is completed, while the system maintains the interaction mode. Consequently, gestures are also more prone to modal errors [START_REF] Buxton | Chunking and Phrasing and the Design of Human-Computer Dialogues[END_REF][START_REF] Sellen | The Prevention of Mode Errors Through Sensory Feedback[END_REF].

For both definitions it is important to clarify the term configuration. We understand this concept relatively broadly to include the number of fingers (single-or bi-manually), their (relative) positions, the touching size or shape, touch IDs, and even marker properties such as 2D barcodes. In this notion we include the touched element on the surface such as a button or a screen region, as specified by Wobbrock et al.'s [START_REF] Wobbrock | User-Defined Gestures for Surface Computing[END_REF] binding category. Thus it is possible to use the same configurations on different screen regions/ bindings to define different postures or gestures. For both gestures and postures, however, the configurations have to be learned by users, but a configuration's binding can also serve as a reminder of a gesture's/posture's function. In contrast, a gesture's motion pattern has to be learned unless it resembles a shape associated with the action (e. g., scribble to delete)-the motion in postures directly affects object parameters whose change is typically visible.

As noticed before by Wu et al. [START_REF] Wu | Gesture Registration, Relaxation, and Reuse for Multi-Point Direct-Touch Surfaces[END_REF], sometimes it is not only the static configuration that characterizes a posture but in some cases people also use "dynamic actions that occur immediately after the posture is recognized." We thus extend the posture concept to define a quasi-posture as a a posture whose initial configuration is augmented with a brief initial dynamic action but where this action's continuation is also used to parameterize the effect.

The brief initial dynamic action can be captured in the form of a few samples of an initial motion such as the initial direction of motion, which it typically directly tied to the effect that is intended with the quasi-posture. This connection and the briefness makes quasi-postures different from gestures whose motion paths are typically complex and not tied to the gesture's effect. Quasi-postures are similar to postures in that the configurations have to be learned and in that they can be used to define user-controlled interaction modes due to the briefness of the dynamic action.

'Gestural' 3D Direct-Touch Interaction

The previous discussion of the 'gesture' concept arose from our work with 3D environments and, specifically, from touch interaction with scientific visualizations. Next, we explore the three different 'gesture' notions by first analyzing a number of general 3D touch interaction techniques and later discussing examples from SciVis where a continuous and direct exploration is essential. We specifically examine which concept is used primarily and for what purpose. Many of the published 3D touch interaction techniques do not use gestures but, in fact, rely entirely on finger configurations to specify the interaction modality so that they should be seen as postures. Typically, their configurations consist of one or a few contact points that touch the objects or widget elements to be manipulated, without requiring motion patterns (Fig. 2). This is true for 3D RST interaction [START_REF] Reisman | A Screen-Space Formulation for 2D and 3D Direct Manipulation[END_REF] (and, in fact, also the common 2D pinching technique [START_REF] Hancock | Rotation and Translation Mechanisms for Tabletop Interaction[END_REF]), shallow-depth interaction [START_REF] Hancock | Shallow-Depth 3D Interaction: Design and Evaluation of One-, Two-and Three-Touch Techniques[END_REF], sticky tools [START_REF] Hancock | Sticky Tools: Full 6DOF Force-Based Interaction for Multi-Touch Tables[END_REF][START_REF] Hancock | Supporting Sandtray Therapy on an Interactive Tabletop[END_REF], surface physics [START_REF] Wilson | Simulating Grasping Behavior on an Imaging Interactive Surface[END_REF][START_REF] Wilson | Bringing Physics to the Surface[END_REF], DabR [START_REF] Edelmann | The DabR -A Multitouch System for Intuitive 3D Scene Navigation[END_REF], and z-positioning [START_REF] Martinet | 3D Positioning Techniques for Multi-Touch Displays[END_REF].

General 3D Interaction Techniques

A further group of techniques also largely relies on (quasi-) postures but adds one or two gestures for very specific purposes. For example, Benko and Feiner's [START_REF] Benko | Balloon Selection: A Multi-Finger Technique for Accurate Low-Fatigue 3D Selection[END_REF] balloon selection uses postural interaction for the entire 3D placement process and only uses a tap gesture for the final selection action. Similarly, the tBox manipulation [START_REF] Cohé | tBox: A 3D Transformation Widget Designed for Touch-Screens[END_REF] relies almost exclusively on postures and one quasi-posture that distinguishes between two-finger RST (both fingers moving) and pivoted rotation (one finger is moving; the static finger is the pivot point). The only gestures in tBox are a doubletap for (un-)selecting objects and a double-tap on one of the widget's edges to invoke translation in the plane as opposed to along the initially selected second axis. Toucheo's [START_REF] Hachet | Toucheo: Multitouch and Stereo Combined in a Seamless Workspace[END_REF] widget also mostly employs postures, its only gesture is a double-tap to raise objects to a pre-defined level.

Most of Eden's [START_REF] Kin | Eden: A Professional Multitouch Tool for Constructing Virtual Organic Environments[END_REF] 3D manipulation 'gestures' are also postures according to our definition. The only exceptions are the one-finger taps (null motion gestures in zero time) to place a pivot point for camera rotation or to add new objects to the scene. However, adding objects to the scene can also be seen as a posture without a continuing direct manipulation which could potentially be enhanced by interpreting a subsequent motion as a translation on the surface of the model (seemingly not done by Kin et al. [START_REF] Kin | Eden: A Professional Multitouch Tool for Constructing Virtual Organic Environments[END_REF]).

A nice example of postural interaction with true volumetric displays in which the postures are not only defined on the display surface but in 3D space was described by Grossman et al. [START_REF] Grossman | Multi-Finger Gestural Interaction with 3D Volumetric Displays[END_REF]. Similar to Eden [START_REF] Kin | Eden: A Professional Multitouch Tool for Constructing Virtual Organic Environments[END_REF], the vast majority of 'gestures' are postures, the only exceptions being tapping and a trigger gesture for certain selection events.

We found one example that employs gestures to a larger degree: Navidget [START_REF] Hachet | Navidget for Easy 3D Camera Positioning from 2D Inputs[END_REF]. While its selection interaction has both gestural (completing a circle) and postural (drawing a unique selection region) aspects, the widget itself is invoked with a tap-and-hold gesture. Gestures are also used to control the widget including touching the peripheral ribbon (to switch between back and front views) and moving horizontally/vertically (to exit the resize mode). However, the interaction also has postural (directly-manipulative) aspects in how the view direction is continually controlled.

Touch Interaction with 3D Scientific Visualization

In addition to the issue of mapping 2D input to 3D manipulations, touch-based interaction with scientific visualization (SciVis) adds several further challenges such as the integration of multiple interaction techniques in the same input space as well as precise control [START_REF] Isenberg | Position Paper: Touch Interaction in Scientific Visualization[END_REF]. We examine several tools w.r.t. their use of gestures, postures, and quasi-postures to understand how they employ these concepts and if it differs from the basic 3D techniques discussed above. [START_REF] Fu | Multi-Touch Techniques for Exploring Large-Scale 3D Astrophysical Simulations[END_REF] astronomy visualization system tackles the interaction with spatial and scale space. They use postures to specify interaction modes based on numbers of contact points and, for two simultaneous contact points, their rel-ative position w.r.t. each other. Tapping is the only gesture that is used-for selecting items (stars). A related approach used for astronomical data is FI3D [START_REF] Yu | FI3D: Direct-Touch Interaction for the Exploration of 3D Scientific Visualization Spaces[END_REF] which also provides means to interact with the space-rather than with single objects. Here, one-/two-finger postures and their location w.r.t. a widget are used to specify modes, some of these being quasi-postures that differentiate according to the direction of movement w.r.t. the widget frame (Fig. 3). Sultanum et al. [START_REF] Sultanum | Touching the Depths: Introducing Tabletop Interaction to Reservoir Engineering[END_REF][START_REF] Sultanum | Techniques for Interactive Reservoir Visualization on Tabletops[END_REF] described two systems for exploring geology. In their initial system [START_REF] Sultanum | Touching the Depths: Introducing Tabletop Interaction to Reservoir Engineering[END_REF] they use one-finger orbiting, two-finger RST, and axis-aligned cutting with two fingers on the data volume's corners which are all postures. The interaction in an extended system [START_REF] Sultanum | Techniques for Interactive Reservoir Visualization on Tabletops[END_REF] are also all postural (including the use of tangibles). Interesting is the distinction between the four-finger splitting and the four-finger peeling which is done using a quasi-posture that looks at the (initial) relative directions of the points in a cluster.

Fu et al.'s

The systems mentioned so far use regular displays for depicting the 3D data. Two more systems, however, rely on stereoscopic displays. Butkiewicz and Ware's [START_REF] Butkiewicz | Exploratory Analysis of Ocean Flow Models with Stereoscopic Multi-Touch[END_REF] oceanographic visualization combines touch sensing with a depth camera for touch identification and can distinguish two-handed and single-handed-multi-finger postures for 3D positioning. Slice WIM [START_REF] Coffey | Interactive Slice WIM: Navigating and Interrogating Volume Datasets Using a Multi-Surface, Multi-Touch VR Interface[END_REF][START_REF] Coffey | Slice WIM: A Multi-Surface, Multi-Touch Interface for Overview+Detail Exploration of Volume Datasets in Virtual Reality[END_REF], in contrast, relies on touch interaction on a projected surface in its stereoscopic environment and uses postures which are specified w.r.t. a 2D widget.

In summary, in the scientific visualization systems we examined we found only postures and no gestures being used for directly-manipulative interaction-the only exception being tap gestures for selection. However, it is interesting that two of the six systems also used quasi-postures. The need for using quasi-postures in addition to regular postures may arise due to SciVis' requirement to combine multiple interaction techniques in the same input space and the need for data analysis incl. filtering or clustering.

Implications for Future Interactive Systems

As we have seen in our analysis of both general 3D directtouch interaction techniques as well as touch-based scientific visualization systems, postures have already been used rather than gestures for directly-manipulative control in most cases. This may be due to the inherent benefit of user-maintained modes as opposed to system-maintained modes. However, the real challenge-in particular for scientific visualization-is how to combine and integrate many of these interaction techniques such that they can be used within the same 2D input space [START_REF] Isenberg | Position Paper: Touch Interaction in Scientific Visualization[END_REF]. In fact, many of the postures we surveyed for specifying 3D interactions conflict with each other by nature, so posture-based systems have to be designed such that those conflicts are avoided.

Quasi-postures are one way to extend the interaction vocabulary without requiring new (and potentially conflicting) configurations, and these should thus be explored to a larger extent. However, we will not be able to cover all necessary interactions with intuitive single-handed quasipostures, so additional means need to be explored. One option is to use postural bi-manual interaction in which a 'button' is held down with one hand as part of the configuration to specify the main interaction type, while the other (dominant) hand performs a precise posture followed by the direct manipulation. In fact, the button only needs to be held down until the parameter specification has started because at that point the manipulation causes the visualization to change so that visual feedback is provided. This use of visual feedback can also be developed a step further for one-handed quasi-postures, so that separate modes of interaction could be specified using something akin to a FlowMenu [START_REF] Guimbretiére | FlowMenu: Combining Command, Text, and Data Entry[END_REF] and continued using a posture. For example, an n-directional slider widget could be used to initiate n different modes of interaction. At n = 4 (Fig. 4), this widget would allow different interaction when starting the gesture in the up, down, left, and right directions, and once the slider's end is reached the posture would begin.

Figure 5: Person interacting with a particle simulation from astronomy using the FI3D widget [START_REF] Yu | FI3D: Direct-Touch Interaction for the Exploration of 3D Scientific Visualization Spaces[END_REF], performing a directlymanipulative interaction. Image from [START_REF] Yu | FI3D: Direct-Touch Interaction for the Exploration of 3D Scientific Visualization Spaces[END_REF], © IEEE Computer Society 2010, used with permission.

Figure 6: Person interacting with a visualization system for fluid mechanics [START_REF] Klein | A Design Study of Direct-Touch Interaction for Exploratory 3D Scientific Visualization[END_REF] in which several different postural and quasipostural interaction techniques are integrated and can be accessed from the same input space. This integration facilitates the exploration of complex 3D data.

In addition to the challenge of creating an integrated set of interaction techniques, more work is also needed to understand cases where the distinction between postures and gestures is not entirely clear. One such case is singletapping: it can be a posture to define a location [START_REF] Kin | Eden: A Professional Multitouch Tool for Constructing Virtual Organic Environments[END_REF] or a gesture to define an action [START_REF] Benko | Balloon Selection: A Multi-Finger Technique for Accurate Low-Fatigue 3D Selection[END_REF]. One question is, for example, if such an interaction can only be either a posture or a gesture in an integrated interactive system, or if there are also intuitive ways to use both concepts simultaneously.

Conclusion

In this paper we argued that using the term 'gesture' in a generalizing way to describe direct-touch interaction can be problematic, in particular (but not only) in the context of 3D interaction. The reason for this is that 'gestures' can be and often are confused with real gestures that require motion patterns that have to be learned, that are typically used to specify status changes or system-controlled modes, and that thus can be prone to modal errors caused by these modes. Instead, (quasi-)postures are often better suited (and typically used) for directly-manipulative interaction in user-controlled modes, in particular, in 3D environments because here the motion that follows a posture can directly control the parameters one is adjusting (e. g., Fig. 5).

Our survey of direct-touch 3D interactions and touch-based SciVis tools showed that postures are indeed overwhelmingly used for direct manipulation-gestures, instead, are used for discrete tasks, e. g., selections. We argued, however, that more work is needed to be able to control the many different parameters in a scientific visualization and, thus, we need to find ways that allow us to integrate different posture-based control within the same 2D input space. We are currently in the process of exploring such possibilities for specific scientific domains such as flow visualiza-tion [START_REF] Klein | A Design Study of Direct-Touch Interaction for Exploratory 3D Scientific Visualization[END_REF] and hope that this work (Fig. 6) as well as our present position paper on 'gestural' interaction foster research into direct-touch interaction with 3D scientific data.

Figure 2 :

 2 Figure 2: Example for posturecontrolled direct manipulation of 3D objects: sticky tools [17, 18].The one-, two-, or three-finger configurations uniquely determine the interaction modalities, and the following motion directly parametrizes the object manipulation. Images from[START_REF] Hancock | Supporting Sandtray Therapy on an Interactive Tabletop[END_REF], © ACM 2010, used with permission.

Figure 3 :

 3 Figure 3: Example from FI3Dfor quasi-posture-controlled interaction[START_REF] Yu | FI3D: Direct-Touch Interaction for the Exploration of 3D Scientific Visualization Spaces[END_REF]: depending on the initial motion along or perpendicular to the widget frame, a rotation around the z-axis or a trackball rotation is initiated, respectively. Image from[START_REF] Yu | FI3D: Direct-Touch Interaction for the Exploration of 3D Scientific Visualization Spaces[END_REF], © IEEE Computer Society 2010, used with permission.

Figure 4 :

 4 Figure 4: This example shows a low-fidelity prototype of how a directional slider could be used to provide visual feedback for a quasi-posture to enable several modes of interaction.

To distinguish the different meanings, we use 'gesture' for the traditional (general) concept of interaction specification, while gesture refers to more narrow definition given later in this paper. If gesture is used without quotation marks and in normal roman type, then the classification is either not clear or we discuss related work which does not make this distinction.

Acknowledgments

We thank Björn Hartmann, Petra Isenberg, and Frédéric Vernier for inspiring discussions on the topic.