Hélène Suffix

Gastaldi Particle

Emmanuelle Suffix

Hélène Angellier-Coussy

Emmanuelle Gastaldi

Angellier-Coussy Particle

Keywords:

In the current context, protein-based materials might be considered as an alternative to the petroleum-based plastics since fully biodegradable and characterized by remarkable functional properties that can be exploited in a wide range of non-food applications. To improve their performances that are often restricted by high water sensitivity and low mechanical properties, a relevant strategy consisted in the development of protein/clay nanocomposite. For this purpose, several examples of protein-based nano-biocomposites were presented with a special attention for the methods used for the incorporation of layered silicates (organically modified or not) into the matrices and the ultimate functional properties exhibited by the resulting materials. In terms of mechanical properties, the addition of nanoclays leads to a significant improvement of material performance with an increase of Young's modulus and tensile strength ranging between 1.5 and 2 times. As regards as barrier properties, the improvement appeared quite moderate in spite of a rather good dispersion of layered silicates that would be expected to result in a tortuous pathway limiting diffusion of gases molecules. Thus, a two-fold reduction in water vapour permeability was obtained, and the same or no effect in the case of permeability toward 0 2 and C0 2 .

Introduction

Among agricultural products, proteins have long and empirically been used as raw materials in a wide range of non-food applications such as adhesives, glues, paints, leather finishes, textile fibers, paper coatings, photographic supports and various moulded plastic items (buttons, door handles, jewellery…). However, in the 1960s the competition with synthetic plastics led the interest for protein-based materials to markedly decrease until a recent renewal of interest for renewable and biodegradable materials, over the last two decades, in response to raising awareness on environmental issues. Owing to their good film-forming properties, several proteins stemming from animal (casein, whey proteins, gelatin, collagen, egg white, keratin and fish myofibrillar proteins) and vegetable (soy proteins, wheat gluten, corn zein and pea protein isolates) sources have received attention for production of films or coatings. Proteins can be defined as natural polymers able to form amorphous three-dimensional structures mainly stabilized by non-covalent interactions. In contrary to most other biodegradable polymers, they are heteropolymers constituted of different amino acids and characterized by specific levels of structure ensuring various interactions and bindings differing in position, type, and energy. Protein-based materials can be prepared using either a solvent process also called casting or a common thermoplastic process such as extrusion or thermomolding. Their formulation often requires the addition of plasticizers to allow their thermo-processability and impart film flexibility. Resulting proteinbased materials are characterized by remarkable functional properties that are highly dependant on structural heterogeneity, especially on interactions between polypeptide chains, thermal sensitivity, and hydrophilic behaviour of proteins.

Protein-based materials are fully biodegradable without toxic products release.

However serious concerns remain regarding their performance, mainly including their high water sensitivity and low mechanical properties.

As largely described in the literature, the incorporation of layered silicates at the nanoscale level into a polymeric matrix often results in significant enhancement of mechanical, barrier and thermal properties. The high affinity of clays to proteins and amino acids bearing neutral, positive, and even negative charges were After a short presentation of the protein structure, which is at the root of the properties of protein-based materials, the second part of the chapter will be devoted to the possible processing routes to prepare protein-based nanobiocomposites. The third part will focus on the structure/properties relationships of proteins (gelatin, milk proteins, soy proteins, corn zein and wheat gluten)/clay nano-biocomposites.

Structure of Proteins

Proteins are heteropolymers constituted of 20 different amino acids and characterized by different levels of structure that permit various interactions and bindings differing in position, type, and energy. The functionalities of proteins-based materials mainly depend on the composition of the amino acids constituting their primary sequence and more precisely on the chemical reactivity of the amino acids lateral groups i.e. polar or hydrophobic, ionised or not, acid or basic, aromatic or aliphatic (Table 12.1). According to the chemical specificity of the amino acids constituting proteins, different types of interactions could be established leading to intra and inter-polypeptidic chain linkages. Among them, the most representative are non-covalent bonds since considered as critical in stabilizing protein threedimensional structure that can be described at four levels i.e., from primary structure to secondary, tertiary and quaternary structure [START_REF] Garrett | Part I: Molecular components of cells[END_REF]. The secondary structure of proteins corresponds to the spatial arrangement of the polypeptide chains, which results from the establishment of periodic hydrogen bindings involving carbonyl and amine groups (CO-NH) of peptide bonds. We distinguish the a-helix and the b-sheets structures. Proteins containing a relatively large proportion of proline and hydroxyproline may be less prone to adopt an ordered structure due to the disruptive effect of these amino acid residues on typical secondary structure. The tertiary structure of proteins results from diverse interactions (hydrogen, hydrophobic, electrostatic, covalent…) between amino acids belonging to the same polypeptide chain but being not neighbours. Finally, the quaternary structure generally results from non-covalent interactions between polypeptide chains. This latter structure is closely related to the functional properties of the final protein-based materials. Whey proteins [START_REF] Braunitzer | Automatische sequenzanalyse eines protein (b lactoglobulin AB)[END_REF][START_REF] Vanaman | The disulfide bonds of bovine a lactalbumin[END_REF] 28.5 12.1). Although the non-covalent bonds are weak chemical forces ranging from 4 to 30 kJ.mol -1 [START_REF] Garrett | Part I: Molecular components of cells[END_REF], they impart stability to the structures generated by their collective action. Van der Waals forces (involving both polar and apolar non-ionised amino acids, 0.4-4 kJ.mol -1 [START_REF] Garrett | Part I: Molecular components of cells[END_REF]) are also implicated but their effect on protein structure is minor. Their highly heterogeneous structure characterized by the absence of repeating units along the polypeptide chains render ineffective Van der Waals interactions that mainly act at short distance with a very low energy input. In aqueous medium proteins generally adopt a conformation in which polar amino acids are located at the interface with solvent whereas the hydrophobic parts of the polypeptide chains tend to avoid solvent by burying within the structure. Depending on the polarity, pH and ionic force of the solvent used, the respective contributions of these interactions can be modified. In pH conditions far from its isoelectric point, proteins are stabilized in the medium since the interactions established between protein and solvent are greater than those formed between amino acids. In addition to non-covalent interactions, covalent disulphide bonds formed by oxidation of two thiol groups also play an important role in the stability of the proteins containing cysteine residues.

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The functional properties of the final materials will depend on the respective contribution of these different kinds of interactions that are closely related to the characteristics of the protein raw material but that can also be modulated by the conditions used to process and store the protein-based materials. Furthermore, this heterogeneous structure offers many opportunities for cross-linking or chemical grafting.

Preparation of Protein-Based Nano-Biocomposites

The formation of a macromolecular network from proteins requires three steps:

first the rupture of low-energy intermolecular bonds that stabilize polymers in the native state, secondly the arrangement and the orientation of polymer chains (shaping), and finally the formation of a three-dimensional network stabilized by new interactions and bonds after the agent that breaks intermolecular bonds is removed [START_REF] Cuq | Proteins as agricultural polymers for packaging production[END_REF]. Due to the high glass transition temperature of proteins (above their temperature of degradation), their formulation often requires the addition of plasticizers to allow their thermo-processability and impart material flexibility.

Besides water molecules that could be considered as the natural plasticizer of most proteins, the most usual plasticizers are polyols (glycerol, sorbitol), mono-, di-, and oligo-saccharides, and fatty acids.

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Several methods have been considered to prepare polymer/layered silicate nanocomposites with an optimal dispersion state of nanofillers [START_REF] Alexandre | Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials[END_REF]. Two of them are compatible with the production of protein-based materials: (1) the intercalation of proteins from solution, which is compatible with the preparation of proteinbased materials via a wet process, commonly called casting, based on the dispersion or solubilisation of proteins in a solvent, and (2) the melt-intercalation technique, which is compatible with the preparation of protein-based materials via a dry, that is, thermo-mechanical process based on the thermoplastic properties of some proteins. These two processes are schematically represented on Fig. 12.1.

Wet Process/Intercalation from Solution

The wet process is suitable for all proteins. The solvents used to prepare protein film-forming solutions are generally based on water and ethanol, and occasionally acetone. Proteins are required to unfold and realign before a new three-dimensional network can be formed, and then stabilized by new inter-and intra-molecular interaction. Depending on the amino acid sequence, dispersing proteins in solution may involve the addition of solvents, disruptive agents, pH adjustment by the addition of acids or bases, or ionic strength control by electrolyte addition [START_REF] Cuq | Proteins as agricultural polymers for packaging production[END_REF]. Then, the functional properties of protein-based films prepared by casting depend on protein concentration in solution, pH, additives, solvent polarity, drying rate and 12.2) for several reasons, including the non-necessity to use plasticizers, the easiness of processing and the possibility to achieve well exfoliated nanocomposite structures due to the capacity of layered silicates to swell in an appropriate solvent (such as water). In some cases, solution intercalation followed by solvent evaporation (often by freeze-drying) is only considered as a preliminary step allowing obtaining good nanoclay dispersion, the resulting dried powders being then shaped using a dry process (Fig. 12.1, Table 12.2).

Dry Process/Melt-Intercalation

Thermoplastic properties of proteins mainly depend on the type and the density of intermolecular interactions. Extrusion requires the formation of a homogeneous protein melt implying processing temperature above the protein softening point. The fact that proteins contain a wide range of inter-molecular interactions reduces chain mobility, increases viscosity and results in a high glass transition temperature often above the decomposition temperature. Furthermore, the heat sensitivity of some proteins that aggregate and cross-link upon heating restricts the window of feasible

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processing conditions. Because chain mobility is a key parameter to achieve successful extrusion, protein/protein interactions can become unfavourable and should thus be controlled during processing. For this purpose, chemical additives, such as sodium sulphite, sodium dodecyl sulphate and urea, known to disrupt covalent and non-covalent bonds are often used as processing aids in combination with plasticizers [START_REF] Verbeek | Extrusion processing and properties of protein-based thermoplastics[END_REF]. Although most of proteins are suitable for the extrusion process, to date, only wheat gluten and soy proteins have led to nano-biocomposites by using a dry process, such as extrusion, roller milling or thermo-molding (Table 12.2).

To prepare nanocomposite materials, layered silicates are mixed with the protein matrix in the molten state under heating and shearing (Fig. 12.1). In the case of proteins containing cysteine residues (such as soy and wheat gluten) the thermal treatment not only allows polymer melting but also induces cross-linking reactions between thiol groups (from rather low temperatures; 70°C in the case of wheat gluten [START_REF] Schofield | The effect of heat on wheat gluten and the involvement of sulfydryl-disulfide interchange reactions[END_REF], for instance). Thus, extrusion conditions (temperature and shearing) should be optimised in order to limit the temperature-induced cross-linking and favour the shear-induced alignment of polymer chains preventing the viscosity increase and allowing processability. When clay layers surface is sufficiently compatible with the protein, polypeptide chains can crawl into the galleries and lead to either an intercalated or exfoliated nanocomposite structure depending on the degree of penetration of the polymer into the clay interlayer. According to Vaia and Giannelis [START_REF] Vaia | Lattice model of polymer melt intercalation in organicallymodified layered silicates[END_REF],

interplay of entropic and energetic factors determines the outcome of polymer intercalation. Exfoliation or complete layer separation depends on the establishment of very favourable interactions between polymer and clay surfaces to overcome the entropy loss associated with polymer confinement and penetration between the clay layers. In the case of clays modified with alkylammonium, the increased conformational freedom of the surfactant chain induced by the clay layers separation might compensate additionally the entropy decrease associated with the polymer chain penetration between the clay. The kinetic of intercalation of the polymer chains between the clay layers also depends on the polymer mobility that is related with the temperature difference between the processing temperature and the polymer glass temperature [START_REF] Vaia | Kinetics of polymer melt intercalation[END_REF]. Although still too little used to prepare protein-based nanocomposites, the polymer melt intercalation is considered as a more efficient and environmentally-benign alternative than the polymer intercalation from solution since requiring no solvent. Moreover, dry processes are all available at the industrial scale and commonly used in plastics processing.

12.4 Structure/Properties Relationships of Protein-Based Nano-Biocomposites

Gelatine-Based Nano-Biocomposites

Gelatine is a protein resulting from partial thermal or chemical hydrolysis of collagen polypeptide chains stemming from animal skin, bones and cartilages.

For commercial production of gelatine, two processes are mainly used differing by gelatine is obtained by an alkaline lime followed by a solubilisation at near neutral pH at 60-90°C [START_REF] Arvanitoyannis | Formation and properties of collagen and gelatin films and coatings[END_REF]. Gelatine was one of the first food protein to be used as raw material not only for food applications but also in the pharmaceutical and photographic film industries due to large availability, low production cost (between 1 and 5 €/kg depending on the animal origin and the targeted application, food or non-food uses) and excellent film forming ability. Gelatine has been successfully used to form, by casting, films that are transparent, flexible, water-resistant and impermeable to oxygen [START_REF] Cuq | Proteins as agricultural polymers for packaging production[END_REF]. Although gelatine can be used as a valuable biopolymer in tissue engineering, its poor mechanical properties, especially in the wet state, and its high water sensitivity, revealed by a high water swelling, restrict its application as a structural biomaterial such as osteosynthetic devices. The development of gelatine/clay nano-biocomposites provided some new opportunities to overcome these limitations and widen end-uses. They are considered as the first group of nano-biocomposites deliberately created from the assembling of a natural polymer and a layered solid. Gelatine/clay nanocomposites can only be prepared using a solvent intercalation based on an ion exchange process. In this manner, the control of pH is crucial because it is required to protonate the amino groups, in order to intercalate positively charged gelatine by replacing the interlayer cation when the pH of the solution is kept below the isoelectric point of the protein [START_REF] Darder | Bio-Nanohybrids based on layered inorganic solids: gelatin nanocomposites[END_REF].

Zheng et al. first prepared gelatine/sodium MMT nano-biocomposites using a solution intercalation method [START_REF] Zheng | Gelatin/montmorillonite hybrid nanocomposite. I. Preparation and properties[END_REF] and MMT contents up to 20 wt%. The work mainly focused on thermal and mechanical properties. XRD results revealed that an intercalated or partially exfoliated structure was achieved. DSC and TGA results indicated that thermal properties of gelatine were significantly improved (the onset thermal decomposition temperature is 40°C higher than that of the pure gelatine for a filler content of 30 wt%), as later confirmed by Martucci et al. [START_REF] Martucci | Nanocomposites based on gelatin and montmorillonite Morphological and thermal studies[END_REF].

The tensile strength and Young's modulus were also improved (Table 12.3), reaching maximum values for MMT content of 17 wt%. The wet mechanical strength was also significantly improved, which was mainly attributed to the nanodispersion of MMT within the gelatine matrix and the barrier effect of MMT sheets to solvent. A significant enhancement of both the Young's modulus and the tensile strength by about a factor 2 without sacrificing toughness was further confirmed by many authors for other sources of gelatine at a low MMT loading (5 wt%) [START_REF] Rao | Gelatin-clay nanocomposites of improved properties[END_REF][START_REF] Bae | Effect of clay content, homogeneization RPM, pH, and ultrasonication on mechanical and barrier properties of fish gelatin/montmorillonite nanocomposite films[END_REF][START_REF] Fernandes | Gelatin-clay bio-nanocomposites: structural and functional properties as advanced materials[END_REF] (Table 12.3).

Concerning barrier properties, several studies showed that the introduction of MMT led to a significant reduction in the water sensibility and oxygen permeability of gelatine due to the creation of an exfoliated nanocomposite structure (Table 12.3). Li et al. first showed that the incorporation of natural MMT led to a significant decrease of both the swelling rate and the maximum solvent uptake [START_REF] Li | Gelatin/Montmorillonite Hybrid nanocomposite. II Swelling behaviour[END_REF]. Then, Bae et al. [START_REF] Bae | Effect of clay content, homogeneization RPM, pH, and ultrasonication on mechanical and barrier properties of fish gelatin/montmorillonite nanocomposite films[END_REF] and Martucci and Ruseckaite [START_REF] Martucci | Biodegradable three-layer film derived from bovine gelatin[END_REF] showed that water vapour permeability could be reduced by about a factor 2 for a 5 wt% MMT filler content. The effect of clay on the oxygen barrier properties is not so pronounced, probably due to an already very low permeability of the neat matrix (Table 12.3).

12 Protein/Clay Nano-biocomposites 9

U N C O R R E C T E D P R O O F 12.4.

Milk Proteins-Based Nano-Biocomposites

Milk has a protein content of about 33 g.L -1 and contains two main types of proteins:

caseins which represent about 80 % of total milk protein and whey proteins (20 % of total) that principally consist of b-lactoglobulin, a-lactalbumin and serum albumin.

Caseins are proteins of major importance in the food industry due to their key role in the cheese making process. Whey proteins are by-products of the cheese or casein

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manufacture that remain in milk serum after pH/rennet-induced casein coagulation.

Increased production of whey because of expanding cheese manufacture has led to identify new uses for whey proteins. This also involves that whey proteins are available at lower price than caseins. The film-forming properties of milk proteins have been indirectly demonstrated by films formed on processing equipment surfaces and at airwater interfaces during heating of non-fat milk [START_REF] Cuq | Proteins as agricultural polymers for packaging production[END_REF]. Caseins and whey proteins have been used to produce transparent, flexible, colourless and odourless edible films and coatings displaying excellent oxygen, aroma and oil barrier properties at low RH, but poor moisture barrier properties as related to their high hydrophilicity and water solubility. To remedy this drawback, several attempts have been made through the incorporation of hydrophobic materials such as lipids in whey and casein film formulation [START_REF] Mchugh | Water vapor permeability properties of edible whey protein-lipid emulsions films[END_REF][START_REF] Krochta | Edible films from casein-lipid emulsions for lightly processed fruits and vegetables[END_REF]. However, phase separation often occurs and limits the efficiency of lipids addition demonstrating the need to develop other strategies to improve water resistance. The creation of a nanocomposite structure via the incorporation of layered silicate fillers in film formulation can be considered as a convenient way to reach this goal. Though, up to date, very few studies have been reported on whey protein/clay nanocomposite materials and not any on casein/clay nanocomposite materials. This is probably due to the high nutritional values and sensory acceptance of casein and whey protein that render them more suitable for edible purposes. In fact, edible applications are not compatible with the nanocomposite route.

Among studies already published on whey protein based nanocomposites, the works of Sothornvit et al. [START_REF] Sothornvit | Effect of nano-clay type on the physical and antimicrobial properties of whey protein isolate/clay composite films[END_REF]20] and Hedenqvist et al. [21] constitute the only available data reporting the effect of clay on the transport properties of whey-based materials. In all studies, nanocomposite films were obtained by the casting process from solutions in which whey protein isolates (WPI) were dispersed in water and heat denatured at temperature above 73°C [START_REF] Perez-Gago | Formation and properties of whey protein films and coatings[END_REF] to allow unfolding of their globular native structure and unmasking of most of the hydrophobic and sulfhydryl groups hidden in the core of the structure. WPI film forming solutions were then mixed with a water suspension of montmorillonites.

In Sothornvit et al. studies, different types of MMT were tested; an unmodified clay (Cloisite Na + ) and two organically modified MMT (Cloisite 30B and Cloisite 20A) with a clay loading of 5 wt% [START_REF] Sothornvit | Effect of nano-clay type on the physical and antimicrobial properties of whey protein isolate/clay composite films[END_REF] or up to 20 wt% [20]. In spite of the lack of morphological and structural analysis provided by the authors, the fact that mechanical properties (tensile strength and elongation at break) of nanocomposites were either unchanged (Cloisite Na + and Cloisite 30B) or decreased (Cloisite 20A) as compared to the neat matrix drops hints that a composite rather than a nanocomposite structure was achieved [START_REF] Sothornvit | Effect of nano-clay type on the physical and antimicrobial properties of whey protein isolate/clay composite films[END_REF]. In the case of nanocomposite films prepared with Cloisite 30B, other results showed a dramatic decrease in tensile properties for clay loadings higher than 5 wt% [20]. The MMT effect on the water vapour permeability (WVP) indicated a small but significant decrease of a 1.4 and 1.6 factor with Cloisite Na + at 5 wt% and Cloisite 30B at 20 wt% respectively, whereas no effect was observed with Cloisite 30B and 20A at 5 wt%. According to the authors, the improvement of the WVP could be directly related to the hydrophilicity of the nanoclays that largely influences their compatibility with the WPI matrix and the level of dispersion.

12 Protein/Clay Nano-biocomposites 11

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edible films by collecting the lipo-proteic skin formed after boiling soya milk [START_REF] Cuq | Proteins as agricultural polymers for packaging production[END_REF].

Soy-based materials can be prepared using either a wet or a dry process (Table 12.1). In the casting process, heating up to 80°C is essential for unfolding proteins and unmasking sulfhydryl groups and hydrophobic side chains that will be involved in the resulting protein network [START_REF] Gennadios | Edible coating and films based on proteins[END_REF]. The concentration of total solids should not exceed 9 %, to avoid gelation and thickening upon heating, and film formation is favoured in slightly basic conditions. SPI-based films alone possess poor flexibility. Therefore, plasticizers are usually used to overcome the brittleness, leading unavoidably to a significant decrease in their tensile strength [START_REF] Wang | Effects of polyhydric alcohols on the mechanical properties of soy protein plastics[END_REF]. Furthermore, they display high water vapour permeability due to the hydrophilic nature of proteins. Several authors succeeded in improving these latter properties by introducing unmodified montmorillonites [START_REF] Rhim | Mechanical and water barrier properties of soy protein and clay mineral composite films[END_REF][START_REF] Chen | Interaction and properties of highly exfoliated soy protein/ montmorillonite nanocomposites[END_REF][START_REF] Kumar | Preparation and characterization of bionanocomposite films based on soy protein isolate and montmorillonite using melt extrusion[END_REF], organically modified montmorillonites (OMMT) [START_REF] Rhim | Mechanical and water barrier properties of soy protein and clay mineral composite films[END_REF][START_REF] Kumar | Effect of type and content of modified montmorillonite on the structure and properties of bio-nanocomposite films based on soy protein isolate and montmorillonite[END_REF] or rectorite [START_REF] Yu | Facile exfoliation of rectorite nanoplatelets in soy protein matrix and reinforced bionanocomposites thereof[END_REF] (Table 12.4).

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Whatever the type of MMT used and the MMT loading (from 5 to 15 wt%), highly exfoliated and intercalated structures have been successfully prepared either via a solution intercalation process in a neutral aqueous media without any special aid [START_REF] Rhim | Mechanical and water barrier properties of soy protein and clay mineral composite films[END_REF][START_REF] Chen | Interaction and properties of highly exfoliated soy protein/ montmorillonite nanocomposites[END_REF][START_REF] Yu | Facile exfoliation of rectorite nanoplatelets in soy protein matrix and reinforced bionanocomposites thereof[END_REF] or via melt extrusion [START_REF] Kumar | Preparation and characterization of bionanocomposite films based on soy protein isolate and montmorillonite using melt extrusion[END_REF][START_REF] Kumar | Effect of type and content of modified montmorillonite on the structure and properties of bio-nanocomposite films based on soy protein isolate and montmorillonite[END_REF]. In fact, in most studies, both the dry and the wet processes were used; the first one with the objective to exfoliate clays in the SPI matrix, and the second to obtain ultimate nanocomposite materials [START_REF] Chen | Interaction and properties of highly exfoliated soy protein/ montmorillonite nanocomposites[END_REF][START_REF] Kumar | Preparation and characterization of bionanocomposite films based on soy protein isolate and montmorillonite using melt extrusion[END_REF][START_REF] Kumar | Effect of type and content of modified montmorillonite on the structure and properties of bio-nanocomposite films based on soy protein isolate and montmorillonite[END_REF][START_REF] Yu | Facile exfoliation of rectorite nanoplatelets in soy protein matrix and reinforced bionanocomposites thereof[END_REF]. According to Chen and Zhang [START_REF] Chen | Interaction and properties of highly exfoliated soy protein/ montmorillonite nanocomposites[END_REF], the high affinity between soy proteins and the MMT layers was due to two types of interactions: hydrogen bonding between the -NH and Si-O groups and electrostatic interactions between the positive-charge-rich domains of soy protein and the negatively charged MMT layers as evidenced by zeta-potential measurements and FTIR analysis. In spite of the negative charge of soy proteins at neutral pH, the heterogeneous distribution of the surface positive charges provided the possibility for soy proteins to intercalate and exfoliate negatively charged MMT layers [START_REF] Chen | Interaction and properties of highly exfoliated soy protein/ montmorillonite nanocomposites[END_REF].

However, structures and properties were shown to strongly depend on the filler content used. An increase in the filler content up to 12 wt% led to a significant increase in both the Young's modulus and the tensile strength (Table 12.4).

It is accompanied by a drop in the deformation at break due to the restricted segmental motion of soy proteins and the establishment of favourable interactions between the clay sheets and the protein chains. For filler content as high as 12-16 wt%, all the mechanical properties were degraded, due to phenomenon of MMT aggregation leading to a phase separation that occurred in the soy protein matrix [START_REF] Chen | Interaction and properties of highly exfoliated soy protein/ montmorillonite nanocomposites[END_REF][START_REF] Yu | Facile exfoliation of rectorite nanoplatelets in soy protein matrix and reinforced bionanocomposites thereof[END_REF].

The introduction of clay led to a small but significant reduction of the water vapour permeability (Table 12.4). However, filler contents higher than 10 wt% are required to get a decrease of about a factor 2. Authors classically attribute this effect to the creation of a tortuous pathway for water vapour to diffuse [START_REF] Kumar | Preparation and characterization of bionanocomposite films based on soy protein isolate and montmorillonite using melt extrusion[END_REF][START_REF] Kumar | Effect of type and content of modified montmorillonite on the structure and properties of bio-nanocomposite films based on soy protein isolate and montmorillonite[END_REF].

Corn Zein-Based Nano-Biocomposites

Zein is the major corn protein fraction (prolamin fraction) commercially produced from corn gluten meal, a by-product of corn wet-milling processing. Corn gluten meal contains about 55-60 % proteins and is available at relatively low price; 600-700 dollar/metric ton depending on the grade (food or not). Zein-based materials can be formed by different processes including casting and thermoplastic processing. Due to its low content in polar amino acids and high amount of apolar amino acids, zein-based films are not water-soluble and exhibit remarkable greaseresistance properties [START_REF] Cuq | Proteins as agricultural polymers for packaging production[END_REF]. However, zein-based materials although less water sensitive than other biopolymers still show high water vapour permeability and low tensile strength when compared with commodity polymers. As it was the case for other proteins, its inherent brittleness may be improved by the use of plasticizers, which, on the other hand, further decrease water vapour and gas barrier properties.

To our knowledge, only one article has been recently published on the development of nano-biocomposite materials to improve the properties of zein-based Author Proof

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films [START_REF] Luecha | Synthesis and properties of corn zein/montmorillonite nanocomposite films[END_REF]. In this study, zein/organically-modified MMT nano-biocomposite films were successfully produced from two different processes, (i) solvent casting and

(ii) blown extrusion involving precipitating zein protein from solution into zein resin and blowing into zein balloon, which resembled the melt intercalation method. Both techniques resulted in partially exfoliated structures, as demonstrated by XRD analysis and TEM observations. The introduction of nanoclays did not affect the translucency and the yellow colour of the films. The thermal stability of zein-based films was significantly improved for films prepared using the blown technique since a loading of only 5 wt% led to a delay of about 10°C of the temperature corresponding to a 20% weight loss. However, the improvement of mechanical and barrier properties of the zein-based films did not evolve linearly with the MMT content. Luecha et al. showed that for the solvent casting technique, critical MMT contents of 5 wt% and 3 wt% resulted in optimal improvement of tensile strength and water vapour barrier properties, respectively [START_REF] Luecha | Synthesis and properties of corn zein/montmorillonite nanocomposite films[END_REF]. The limitation of the blown technique was the restriction to use very low filler contents (below 3 wt%).

Wheat Gluten-Based Nano-Biocomposites

Wheat gluten (WG) has been widely investigated as a protein source because it is annually renewable and readily available at a reasonable cost (from 1 to 1.3 €/kg).

Wheat gluten proteins consist of monomeric gliadins with a molecular weight ranging from 15000 to 85000 and a mixture of glutenin polymers with a molecular weight between about 80000 to several million (Table 12.1). WG proteins can undergo disulphide interchange upon heating, which leads to the formation of a covalent three-dimensional macromolecular network. The reactivity of the WG proteins toward chemical and temperature allows preparing WG-based materials by using two technological approaches: either a solvent-based process or a common thermo-mechanical process based on the thermoplastic properties of WG.

With respect to its unique viscoelastic and film-forming properties, WG is an interesting raw material that can be used as a food packaging material. WG basedfilms present an attractive combination of strength and flexibility [START_REF] Cuq | Study of the temperature effect on the formation of wheat gluten network: influence on mechanical properties and protein solubility[END_REF][START_REF] Angellier-Coussy | Functional properties of thermoformed wheat gluten/montmorillonite materials with respect to formulation and processing conditions[END_REF], a high gas (oxygen and carbon dioxide) permeability in dry condition and a significant gas perm-selectivity at high relative humidity [START_REF] Gontard | Influence of relative humidity and film composition on oxygen and carbon dioxide permeabilities of edible films[END_REF][START_REF] Mujicapaz | Oxygen and carbon dioxide permeability of wheat gluten film: effect of relative humidity and temperature[END_REF][START_REF] Guillaume | Wheat gluten-coated papers for biobased food packaging: structure, surface and transfer properties[END_REF], and good grease [START_REF] Guillaume | Wheat gluten-coated papers for biobased food packaging: structure, surface and transfer properties[END_REF] and aroma barrier properties [START_REF] Chalier | Effect of concentration and relative humidity on the transfer of alkan-2-ones through paper coated with wheat gluten[END_REF] which are key functional properties for food quality preservation. They also are translucent [START_REF] Angellier-Coussy | Functional properties of thermoformed wheat gluten/montmorillonite materials with respect to formulation and processing conditions[END_REF] and can be heat-sealed [START_REF] Cho | Heat-sealing properties of compression-molded wheat gluten films[END_REF].

A downside to the use of WG-based materials is their per se reactivity and thus lower inertia than most conventional petrochemical-based plastics. As mentioned above for soy proteins-based films, WG-based films are sensitive towards water due to the hydrophilic nature of many amino acids constituting the protein chains and to the need for adding hydrophilic plasticizer. As a consequence, their mechanical properties and water vapour barrier properties in high moisture The development of nanocomposite structures has been recently applied to WG-based materials to improve moisture resistance and mechanical properties [START_REF] Olabarrieta | Properties of aged montmorillionite-wheat gluten composite films[END_REF][START_REF] Tunc | Functional properties of wheat gluten/ montmorillonite nanocomposite films processed by casting[END_REF][START_REF] Zhang | Wheat gluten-based natural polymer nanoparticle composites[END_REF][START_REF] Angellier-Coussy | Nanoparticle size aspect ratio and liquid water sorption kinetic in agropolymer-based nanocomposite films[END_REF] as well as to create antimicrobial delivery systems [43,[START_REF] Mascheroni | Anti-microbial effectiveness of relative humidity-controlled carvacrol release from wheat gluten/montmorillonite coated papers[END_REF]. All the studies devoted to WG-based nanocomposites involve the use of both unmodified and organically modified montmorillonites. The properties of these nanocomposite materials are directly related to their structure and, in this respect, the state of dispersion (intercalation/exfoliation) of the inorganic phase seems to be a key parameter [45]. The level of exfoliation highly depends on the nature of the montmorillonites. Almost fully exfoliated structures were obtained in the case of unmodified sodium MMT (Nanofil EXM 757 from Süd-Chemie (from [START_REF] Tunc | Functional properties of wheat gluten/ montmorillonite nanocomposite films processed by casting[END_REF]) or Cloisite Na + from Southern Clay Products, Inc (from [START_REF] Olabarrieta | Properties of aged montmorillionite-wheat gluten composite films[END_REF]). Contrarily, the presence of both stacks on TEM pictures and a d 001 peak characteristic of an intercalated structure on XRD patterns evidenced that organically modified montmorillonites (Cloisite 10A [START_REF] Olabarrieta | Properties of aged montmorillionite-wheat gluten composite films[END_REF] and Cloisite 30B [START_REF] Zhang | Wheat gluten-based natural polymer nanoparticle composites[END_REF]) were not well

U N C O R R E C T E D P R O O F
exfoliated. This highlighted that the affinity between wheat proteins and nanoclays is a key parameter governing the state of dispersion of the nanoclays. The use of unmodified MMT, which are naturally hydrophilic and thus, compatible with proteins seems to be more appropriate.

All studies demonstrated that the introduction of MMT nanoparticles in WG-based materials resulted in significant improvement in tensile strength and Young's modulus with a simultaneous decrease in deformability due to increased interfacial interactions (Table 12.5). An optimal filler content of around 5 wt% was required to achieve the greatest improvement of mechanical properties, due to the possibility of layered silicates to form a tridimensional network (Table 12.5).

In the case of organically modified MMT, the presence of quaternary alkylammonium might act as a plasticizer in the WG matrix, which would play a negative role in achieving higher strength enhancement in the nanocomposites [START_REF] Zhang | Wheat gluten-based natural polymer nanoparticle composites[END_REF].

According to other studies, the moisture sensitivity of WG materials was efficiently reduced when wheat gluten proteins were combined with unmodified MMT [START_REF] Olabarrieta | Properties of aged montmorillionite-wheat gluten composite films[END_REF][START_REF] Tunc | Functional properties of wheat gluten/ montmorillonite nanocomposite films processed by casting[END_REF][START_REF] Angellier-Coussy | Nanoparticle size aspect ratio and liquid water sorption kinetic in agropolymer-based nanocomposite films[END_REF]. For example, the liquid water uptake in cast WG-based nanocomposite films and liquid water diffusivity in thermoformed films were both divided by a factor 2 for respectively 10 wt% and 21.9 wt% of unmodified MMT [START_REF] Tunc | Functional properties of wheat gluten/ montmorillonite nanocomposite films processed by casting[END_REF][START_REF] Angellier-Coussy | Nanoparticle size aspect ratio and liquid water sorption kinetic in agropolymer-based nanocomposite films[END_REF].

A decrease in WVP from 1.6 9 10 -11 to 0.6 9 10 -11 mol.m -1 .s -1 .Pa -1 between the neat matrix and the nanocomposite (from 5 to 10 wt% of MMT) might be related to the establishment of hydrophilic interactions between gluten proteins and nanoclays, which resulted in a lower availability of the hydrophilic sites for water vapour. As regards to O 2 and CO 2 permeability (evaluated at 25°C and 80-90 % RH), no significant effect was observed for WG cast films over the range of studied MMT contents (up to 10 wt%) [START_REF] Tunc | Functional properties of wheat gluten/ montmorillonite nanocomposite films processed by casting[END_REF]. However, increased barrier properties towards 2-heptanone were also obtained for MMT contents C5 wt%. This decrease in permeability was ascribed to a tortuosity effect liable for a significant diffusivity decrease while increasing MMT contents [START_REF] Tunc | Functional properties of wheat gluten/ montmorillonite nanocomposite films processed by casting[END_REF].

Besides the improvement of protein based materials performance, the addition of clays could also be use to control the release of active compound. In this context, an antimicrobial delivery system based on WG as matrix, unmodified MMT as structuring agent and carvacrol as antimicrobial compound have been investigated [START_REF] Mascheroni | Anti-microbial effectiveness of relative humidity-controlled carvacrol release from wheat gluten/montmorillonite coated papers[END_REF]45]. In the presence of 5 wt% MMT, particle size measurements and microscopic observations pointed out large aggregated structures that were supposed to entrap carvacrol molecules. Such structures could be responsible for the increased carvacrol retention as exhibited by both WG film-forming solutions and cast films. Further indications of this assumption were provided by wide angle X-ray analysis that confirmed the establishment of specific interactions between MMT, carvacrol and WG. The effect of MMT on the carvacrol release from wheat gluten coated papers was studied under controlled humidity from 40 to 100 % RH in a second part of this study [START_REF] Mascheroni | Anti-microbial effectiveness of relative humidity-controlled carvacrol release from wheat gluten/montmorillonite coated papers[END_REF]. Results indicated an increase of the carvacrol diffusivities while increasing RH and this RH-induced effect was accentuated in the presence of 5 wt% of MMT. As evidenced by scanning electron microscopy (SEM) and TEM, specific aggregated structures were formed in the presence of carvacrol and 5 wt% MMT creating a preferential pathway for carvacrol diffusion.

U N C O R R E C T E D P R O O F

The antimicrobial efficiency of the MMT-WG-coated papers toward Escherichia coli was assessed and showed that the antimicrobial effect was related to the carvacrol diffusivity. The diffusivity coefficients were utilized to optimize the packaging characteristics required to develop an efficient anti-microbial system and were finally validated against Botrytis cinerea.

Conclusion

In the current context, societal concerns and growing environment awareness have triggered the search for new products and processing that could be considered as an alternative to the existing petroleum-based plastics. In this chapter, several examples of protein-based nano-biocomposites have been presented with a special attention for the methods used for the incorporation of layered silicates (organically modified or not) into the biodegradable matrices and the ultimate functional properties exhibited by these new kind of materials. The majority of the studies are devoted to the influence of filler content on mechanical properties, with very few data concerning other properties such as barrier properties.

One of the advantages of layered silicates as nanofiller is to ensure polymeric material reinforcement in two dimensions without requiring a special process to prepare the materials. Whatever the protein used, unmodified clays which are naturally hydrophilic and thus, compatible with proteins seem to be fully appropriate for the preparation of protein-based nano-biocomposites displaying both a good level of clay dispersion and a low environmental impact. The rather good dispersion of layered silicates leads to the formation of an expected tortuous path limiting the diffusion of small gases molecules, and thus to a significant decrease of permeability towards gases (oxygen, water vapour) and aroma compounds. The addition of nanoclays also leads to a significant improvement of material performance in terms of mechanical properties. The improved properties reported above are generally attained at low silicates contents (&5 wt%) as compared to that of conventional composites. However, although nanoclays are well dispersed within the protein matrix, the resulting improvement of barrier properties is quite moderate and does not exceed a two-fold reduction in the case of water vapour, and the same effect or even no effect in the case of 0 2 and C0 2 . Concerning the effect on mechanical properties, the relative increase in Young's modulus and in tensile strength range between 1.5 and 2 times, with often a dramatic decrease in elongation for filler contents of 5 wt% and 50 %RH.

It seems that it would be more relevant to develop the ''nano-biocomposite'' strategy for other purposes than the improvement of mechanical and barrier properties of protein-based materials. The introduction of nanoclays in proteinbased matrices could provide new additional functionalities such as UV protection or controlled delivery of active agents (antimicrobial compounds, agrochemicals, fertilizers or drugs). These properties are highly demanded in polymers in general, and all the more in the case of bio-sourced polymers with interests in a wide range of applications such as packaging, medicine and agriculture.

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2 H

 2 . Angellier-Coussy and E. Gastaldi U N C O R R E C T E D P R O O F initially noted in soils and already largely used in winemaking for protein precipitation, which hints the possibility to prepare protein-based nanocomposite materials. This strategy has begun to be explored in the field of protein-based materials for gelatin less than ten years ago (first publications in 2002). The development of protein/clay nano-biocomposites has been yet devoted to almost all proteins previously mentioned with the intention of mainly improving mechanical and barrier properties. Protein-based nano-biocomposites are also developed for the controlled release of active agents, for example for antimicrobial applications.

a

  Ionized polar: Glu, Asp, Lys, Arg, His; Non-ionized polar: Gly, Asn, Gln, Ser, Thr, Tyr; Apolar: Ala, Val, Leu, Ileu, Pro, Phe, Met, Trp 4 H. Angellier-Coussy and E. Gastaldi U N C O R R E C T E D P R O O F Amino acids are generally classified in groups depending on their ability to establish interactions via either hydrogen bonds (involving non-ionized polar amino acids, 8-40 kJ.mol -1 [1]), ionic interactions (implying ionized polar amino acids, 42-84 kJ.mol -1 [1]), hydrophobic interactions (between apolar amino acids, 4-12 kJ.mol -1 [1]) or covalent linkages (amino acids involved in disulphide bonds, 330-380 kJ.mol -1 [1]) (Table

Fig. 12 . 1

 121 Fig. 12.1 Schematic representation of the two main technological processes used to prepare protein/clay nano-biocomposite materials

8 H

 8 . Angellier-Coussy and E. Gastaldi U N C O R R E C T E D P R O O F the pH conditions; type A gelatine involves an acid extraction whereas type B

11 a

 11 9 10 -11 [28] c -5 wt% Na ? MMT -6.3 ± 0.9 65 ± 5 4.56 ± 0.15 9 10 -11 -10 wt% Na ? MMT -12.6 ± 0.6 24 ± 5 3.85 ± 0.12 9 10 -11 -15 wt% Na ? MMT -15.6 ± 1.7 18 ± 3 3.34 ± 0.09 9 10 -11 SPI (15 wt% glycerol) -2.3 ± 0.5 11.8 ± 0.4 5.85 ± 0.17 9 10 -11 [29] c -5 wt% Cloisite 30B -15.1 ± 0.9 82 ± 3 4.75 ± 0.08 9 10 -11 -10 wt% Cloisite 30B -16.2 ± 0.8 104 ± 5 4.11 ± 0.12 9 10 -11 -15 wt% Cloisite 30B -18.6 ± 0.3 55 ± 3 3.59 ± 0.14 9 10 -Mechanical properties measured on films stored at 50%RH and WVP according to the ASTM Method E 96-95 b Mechanical properties measured on films stored at 0%RH c mechanical properties measured at 65%RH and WVP between 65 and 100 %RH 12 Protein/Clay Nano-biocomposites 13

  14 H. Angellier-Coussy and E. Gastaldi Layout: T1 Standard SC Book ID: 210696_1_En Book ISBN: 978-1-4471-4101-3 Chapter No.: 12 Date: 10-4-2012 Page: 14/21

12

  Protein/Clay Nano-biocomposites 15 U N C O R R E C T E D P R O O F conditions are relatively poor as compared to synthetic films such as low-density polyethylene.

Table 12 . 1

 121 Characteristics of the main proteins used to prepare protein/clay nano-biocomposites Proteins

	Amino acid ratios

Table 12 .

 12

	Proteins	Solution	Solution intercalation	Melt intercalation	Melt intercalation
		intercalation	followed by solvent	using a thermo-	using a thermo-
		via casting	evaporation, then	mechanical process	mechanical process,
			shaping of the material	(thermoforming or	then shaping via
			using a dry process	extrusion)	casting
			(thermoforming or
			extrusion)
	Gelatine	[10-16]	-	-	-
	Whey	[19-21]	-	-	-
	proteins
	Soy	[26]	[ 27, 30]	-	[ 28, 29]
	proteins
	Corn zein [31]	[ 31]	-	-
	Wheat	[ 39, 40, 41,	[52]	[ 33, 42]	-
	gluten	44]

2 Main proteins used as polymeric materials to develop agromaterials: tested methods to obtain nano-biocomposites

Table 12 .

 12

	P O2 (mol/	W	Ref
	(m.s.Pa)	(%)

3 Mechanical and barrier properties of gelatine/clay nano-biocomposites: Young's modulus (E), tensile strength (TS), deformation at break (e), water vapour permeability (WVP), oxygen permeability (PO2) and maximum water uptake (W) Sample E (GPa) TS (MPa) e (%) WVP (mol/ (m.s.Pa) -30.3 ± 2.4 48 ± 4 17.3 9 10 -12 2.08 9 10 -18 -[13] b -5 wt% Cloisite Na+ -40.9 ± 5.3 38 ± 3 7.6 9 10 -12 9.8 9 10 -19 -

Table 12 .

 12

	Reference

4 Mechanical properties (Young's modulus (E), tensile strength (TS), deformation at break (e)) and water vapour permeability (WVP) properties of soy protein isolate (SPI)/clay nanobiocomposites Sample E (MPa) TS (MPa) e (%) WVP (mol/(m.s.Pa) 2.3 ± 0.5 11.8 ± 0.4 5.85 ± 0.17

Table 12 . 5

 125 Mechanical properties (Young's modulus (E), tensile strength (TS), deformation at break (e)) and water vapour permeability (WVP) properties of wheat gluten/clay nano-

	biocomposites
	Sample	E	TS	e	WVP (mol/	Reference
		(MPa)	(MPa)	(%)	(m.s.Pa)
	WG (10 wt% glycerol)	3.7	1.9	58	1.4 9 10 -11	[40] a, e
	-2.5 wt% Na + MMT	5.6	2.4	55	1.4 9 10 -11
	-5 wt% Na + MMT	10.6	4.7	16	6.6 9 10 -12
	-7.5 wt% Na + MMT	11.4	3.6	15	7.2 9 10 -12
	WG (12 wt% water/15 wt%	65.4	5.8	106 -	[41] b
	glycerol)
	-3 wt% C30B	91.8	9	78	-
	WG (T = 120°C; 37.2 wt%	6.5	1.6	86	-	[33] c
	glycerol)
	-2 wt% Na ? MMT	9.5	1.8	70	-
	-5 wt% Na ? MMT	23.6	2.5	42	-
	WG (T = 80°C; 37.2 wt% glycerol) 1.1	0.31	33	-	[33] c
	-2 wt% Na ? MMT	1.7	0.32	30	-
	-5 wt% Na ? MMT	6.4	0.66	23	-
	WG (no plasticizer)	737	48	-	-	[52] d
	-3 wt% attapulgite	691	50	-	-
	-7 wt% attapulgite	773	59	-	-
	-10 wt% attapulgite	783	57	-	-
	-20 wt% attapulgite	770	49	-	-
	Mechanical properties evaluated at a 70 %RH b 50 %RH c 75 %RH d 53 %RH

e WVP measured between 0 and 100 % RH 16

H. Angellier-Coussy and E. Gastaldi

Protein/Clay Nano-biocomposites

H. Angellier-Coussy and E. Gastaldi Layout: T1 Standard SC Book ID: 210696_1_En Book ISBN: 978-1-4471-4101-3 Chapter No.: 12 Date: 10-4-2012 Page: 18/21

Author Proof

U N C O R R E C T E D P R O O F

In the work reported by Hedenqvist et al. [21] the montmorillonites used were previously intercalated with poly(vinylidene pyrrolidone) which is water soluble and acts as surfactant by easily adsorbing onto the MMT Na + surface. Clay filler loadings tested were up to 4.8 vol% MMT as expressed in inorganic content. This study mainly focused on the effect of clay content on the methanol diffusion through the films in relation to the morphology of the composite and the thermal properties of the polymer matrix. Thermal analysis confirmed that the polymer matrix was influenced by the presence of the nanoparticles as evidenced by a reduction of the glass transition temperature indicating a reduction of the segmental mobility in the nanocomposite. They also noted that the absence of a significant increase in relaxed rubber modulus in the nanocomposites suggests that the reinforcing effect of the filler was limited, due to an incomplete exfoliation of clay nanoparticles, as confirmed by TEM analysis. Finally, a small but significant reduction (by almost two orders of magnitude) in the average methanol diffusivity was obtained for the highest MMT content (4.8vol%). According to these authors, the effect of nanoclays on the penetrant diffusivity was due to a reduction of the segmental mobility that reflects interactions between the filler particles and the polymer matrix and the tortuosity induced by MMT that extends the diffusion pathway of the penetrant molecules.

Beside evaluating the effect of nanoclays on mechanical and transport properties, Sothornvit et al. also studied the antimicrobial properties of WPI films reinforced with Cloisite Na + , Cloisite 30B and Cloisite 20A [START_REF] Sothornvit | Effect of nano-clay type on the physical and antimicrobial properties of whey protein isolate/clay composite films[END_REF]20]. Results obtained showed that WPI/Cloisite 30B nanocomposite films exhibited a significant bacteriostatic effect against Gram-positive bacteria (L. monocytogenes) and no effect on Gram-negative bacteria (E. coli O157:H7) whereas, no antimicrobial activity was observed with WPI films reinforced with Cloisite Na + and Cloisite 20A. According to another study focusing on the antimicrobial activity of nanoclays alone [START_REF] Hong | Antimicrobial activity of organically modified nanoclays[END_REF], this antimicrobial effect was mainly ascribed to the quaternary ammonium group carried by the organically modified clays that might be able to disrupt bacterial cell membranes to cause cell lysis. This hypothesis is supported by the fact that among the different unmodified and organically modified clays studied, the unmodified one was the only clay to display no antimicrobial activity [START_REF] Hong | Antimicrobial activity of organically modified nanoclays[END_REF].

Soy-Based Nano-Biocomposites

Most of the proteins in soybean can be classified as globulin. Protein in the form of meal is one of the typical end products from soybean industrial processing. Protein meal can further be concentrated for the production of commercially available soy protein concentrates and isolates. Because soy protein isolates (SPI) are a highly refined class of soybean proteins that contains more than 90% protein and 18 different amino acids, they became the favourite choice of researchers. The filmforming properties of soy proteins have been traditionally used in Asia to obtain Author Proof


Bernard Kaddour
email: bkaddour@liris.cnrs.frjq@lirmm.fr

Joël Quinqueton

An Aspects oriented approach to dynamically manage applications

The emergence of middleware solutions and new services even on small devices will need adapted distributed management solutions which address these specificities, both in terms of software design and in terms of performance. We propose a management system where these high level and low level management concerns are separated.

The high level management part relies on messages interception mechanisms which, coupled with Aspects Oriented Programming concepts, provides facilities for management applications to dynamically operate, enhance and manage JAVA based applications. The management is transparent for the application which doesn't need to be modified to support management operations as we take advantage of both the JAVA introspection mechanisms and the facilities some Aspects Frameworks offer.

Introduction

Since some years, more and more smart phones or personal digital assistants are widely available and used. These small devices still suffer from some limitations compared with high end fixed terminals such as lower CPU performance or smaller memory size, but even now days they already have enough processing capabilities to host complete high Operating Systems -either Windows or Linux dedicated versions -and they appear more and more as autonomous embedded systems. Meanwhile, industrially accepted middleware solutions appear [START_REF] Ejb | Entreprise JavaBeans Specifications version 2[END_REF] [START_REF] Omg | [END_REF] for new distributed applications. These middleware are currently available for small devices [START_REF] Openccm | [END_REF] and even Multi-Agents Systems based solutions have been introduced in the research area [START_REF] Carabelea | Multi-Agent Platforms on Smart Devices: Dream or Reality ?[END_REF].

Such new distributed application whose components may partially or totally be executed by small devices implies new evolutive and flexible needs for their management to be effective.

Another worth to note point is the large acceptance of the JAVA language, both for J2EE [START_REF] J2ee | [END_REF] or Corba applications and even for Corba frameworks design themselves.

We concentrate to provide solutions widely applicable which use these particularities and we conceptually separate the high and the low parts of the management. The former takes advantage of the large utilization of JAVA for middleware applications and uses Aspect Oriented Programming concepts [START_REF] Kiczales | Aspect oriented Programming[END_REF] to manage applications without invading the underlying middleware. This approach avoids modifications of already existing applications and would ease the translation of introduced mechanisms towards new emerging middleware solutions [START_REF]NET Middleware Services: Introduction[END_REF]. The later introduces monitoring tasks tightly related to the code of the managed JAVA entities.

In this article we concentrate on the high part of the management system and the low-level management part [START_REF] Kaddour | Administration d'applications distribuées par Aspects et expressions programmables[END_REF] is not developed here.

We present in section 2. the principles and main ideas which lead our management system for distributed components. The section 3. discusses how managed objects are obtained and section 4. develops the entities used for management. The section 5. presents how they are layered. We conclude with some future works and perspectives.

Key features of our architecture

Managing disperse heterogonous entities has already been investigated in the network community and standards defined [START_REF]Operations for Version 2 of the Simple Network Management Protocol (SNMPv2)[END_REF] [START_REF]Information Base for Version 2 of the Simple Network Management Protocol (SNMPv2)[END_REF], but their intrinsic centralized and frozen aspects are very limitative [START_REF] Martin-Flatin | A Simple Typology of Distributed Network Management Paradigms[END_REF]. Newer and more application centric solutions have appeared [START_REF]Common Information Model[END_REF][7] [START_REF] Jmx | Java Management extensions[END_REF], but they may not adequately address the evolutive aspect of management: management functions must be easy to place and reuse, easy to remove or to stop and many management functions can simultaneously manage an entity.

The approach consists in a core management system -named kernel of the management system or KMSwhere management applications can be deployed.

The KMS largely relies on the interception of incoming and outgoing messages exchanged by the managed application. It can then spy and filter requests sent or results received by the components of the managed application accordingly to management functions requirements.

Management applications are composed of management activities. Activities are in turn composed of management functions and filters. Management functions are not limited to collect data and can be parts of a complete distributed management application while filters can dynamically be linked with the components they have to interact with when deployed by the KMS. A management application can be required by the KMS or sent by a peer-KMS towards the application to manage. Management application is a nearly autonomous entity containing its management activities. According to the target application and the activities wishes, the KMS when places the filters of the different management activities around the expected components. The KMS can provide local resources or services to management applications.

The management system permits to several management applications to simultaneously operate upon a whole application or only upon its components. For example, one could manage an application with some logging management functions for profiling purposes even if another activity is already managing some of the application's components for synchronization purposes. This simultaneously-multimanaged feature makes the management activities to appear as enhancements of the managed application. Some differences with JMX exists. From the JMX point of view, the proposed management system can be viewed as a set of linked and modified MbeanServer, differently registering parts of activities, while activities appear as autonomous collections of Mbeans inheriting the KMS Interfaces where all the necessary behaviors are implemented.

Requirements and strategies for our management

The introduced architecture deals with entities following the usual client-server model. As we expect to not modify the source code of the managed components, management can be achieved through (i) the external representation of the different elements and the interception of sent and received messages; and (ii) the possibility to directly read and modify some of the variables of the component we manage.

First, the connection with the components we plan to manage as this feature is generally not natively available.

Connection with the managed entities: Interceptors

We distinguish applications originally written in JAVA from the others, in particular from Corba applications. In the case of JAVA applications, we use the introspection possibilities the language offers and the facilities we have to access compiled byte-code. These facilities can be used to create Interceptorswrappers or to modify the application with byte-code injection [2][4].

Some tools [START_REF] Aspectj | The ASpectJ Project[END_REF][2] currently go one step further, permitting to operate upon JAVA code (either at source or byte-code level) with high level concepts and high level languages. Performance impact tends to remain acceptable [START_REF] Aspectwerkz | [END_REF].

These features give the possibility to consider JAVA applications as manageable entities.

For Corba applications, OMG provides some basic introspection facilities services: Portable Interceptors Management application KM [START_REF] Omg | [END_REF]. Interceptors can be used by third parties to spy requests or modify messages exchanged between the middleware and the components. They are often used to enrich Corba with new features, ranging from synchronization to caching [START_REF] Baldoni | CORBA request portable interceptors: analysis and applications[END_REF], but they induce performances penalties. They remain a handy solution as they require only few or not any modification in the original application code. We use them at request and message level (i) for interception purposes and (ii) as a glue with the others parts of the system which are mainly JAVA centric.

The following sections focus on the representation of management activities and on the messages filtering system.

Major components of the system

KMS is implemented as a daemon-process to permit sub-activities exchanges between KMS (currently with sockets). It inspects and registers the entities it is in charge of and interacts with AspectWerkz [START_REF] Aspectwerkz | [END_REF]. It is worth to note that KMS provides JAVA classes and interfaces that management entities mandatory implement or inherit from. That's the way KMS normally use to interact with or force a management entity.

Activities

The original activity concept was first introduced in the Computer Supported Co-operative Work framework. We will continue to use this term although its meaning has deeply been altered.

Main parts of an activity are :

-Roles: We distinguish external roles corresponding to resources that other third parties may provide to the activity from internal roles corresponding to subactivities of the current activity. Both of them are described by means of references towards interfaces, -sub activities: greatest difference with other tools is their expected autonomous abilities as the activity may later ask the KMS to send them, as a whole, over the network, and then deploy them.

constraints and preferences. They are activate by the requests the activity can receive. They can decide to allow, modify, or reject a request. The loading of a particular characters font as a new application conforming to an editor interface is such an example, -internal tools: It's a set of functions embedded in the activity for it's own needs. These tools, usually inactive, can be trigger at any time by the arrival of a new element (via a preference e.g.) or for the activity needs. Particular tools are the incoming or outgoing filters acting upon messages received or emitted by the managed component, -monitoring expressions. These are expressions the activity wants at low-level management for direct code monitoring tasks, -the main body of the activity composed of methods the activity responds to and the set of its private variables. Security policy to apply for each received message or termination of the activity are known methods that every activity must implement or delegate.

set of attributes from which the activity can be designated. E.g. an edition activity can specify an octet string attribute file which is the name of the file it proceeds with.

An activity is encapsulated in a .jar class file. It is the activity responsibility to report to the management system the interface or set of services it can respond to whereas the KMS will soon verify that the different parts of the activity conform to the expected JAVA interfaces.

Interceptors are particular activities as they are special parts of the KMS with high privileges.

An example of activity

To depict how an activity is made up, let us consider an application registered as «Service» and providing the «sub» and «add» operations. This application can be managed by a simple «PositiveAccount» activity (i) to count the number of operation requests received and (ii) to force the «sub» operation to return a positive or nil result. First the activity registers its filters which have to operate around the «Service» managed application. Inc_Count will count the number of «add» or «sub» requests the application will receive while a Postive instance will check and maybe modify the value returned by «sub» to be positive or nil. Finally the management activity registers itself as a «Count» activity. It can in turn be suspend or remove (and even itself be partially managed), its filters moved or stopped by the KMS.

PositiveAccount

Contexts

When an activity A is deployed by the KMS, the KMS firstly ensures by introspections that the different parts of the activity conform to the expected ones: outFilter class inherits from the KMS_outFilter class, Activity from KMS_Activity, etc. Lastly, KMS starts the init() method in a dedicated thread.

The activity then creates its own objects and asks the KMS to register objects which interact with other parts of the system. Such objects are (i) filters that the KMS will connect to managed objects of the managed application and (ii) constraints & preferences which may help the activity to customize the environment.

Finally, the activity can use KMS services (such as communication services to send its sub-activities over the network towards another KMS) or require extra services provided by other activities.

Major differences between filters and constraints & preferences is the kind of tasks they are concerned with. Constraints and Preferences (C&P) are used by the activity to express its desires and restrictions. Once registered by the KMS on behalf of the activity, the KMS ensures that before providing a service (or an external activity) to A, it will first check it against the constraints and then against the preferences registered by the activity A. It is up to the KMS to make as many necessary retries before it sends to A the resulting service. This C&P mechanism gives to the management system some capability to adapt environment resources with activities wishes.

The set of tools, constraints, preferences, services provided to others represent the environment (named context) created by the deployed activity.

In turn, an activity must be deployed in an already existing context.

Two abstract contexts are introduced:

site-context corresponds to the operating system abstract activity. It is the main owner of the local resources (hardware, communication links, etc) and it have higher privileges. This context can offer services to its sub-contexts or acts as delegate for them. For example, a sub-context can delegate its security to the site-context security policies.

user-context corresponds to a user environment in which will be (by default) deployed the activities that the user will later execute.

Deployment of management activities creates contexts that are then organized as a tree. Each context has a father-context and may have child-contexts. Child-contexts can be either contexts tied to subactivities or contexts created by another activity explicitly deployed in the current context.

Constraints and Preferences (C&P) deal with contexts messages (or actions) received by the context such as requests for the creation or the insertion of new contexts.

In particular, before being deployed in a context C, an activity will have to conform to the constraints imposed by any context surrounding C. Starting from constraints of the site-context to constraints of C.

This mechanism gives to the management system the capability to adapt activities with the environment wishes explained by means of C&P.

On the other hand, Filters deal with sent or received messages (methods calls and return values) by the managed application. This is obtained transparently (from the managed object point of view) by using Aspect oriented programming concepts and tools.

The management of the methods & the filtering

Aspect oriented programming

This paradigm mainly due to [START_REF] Kiczales | Aspect oriented Programming[END_REF] has for objectives to capture some singularities that aren't actually properly take in charge by the object oriented model (OO). Worth to note that Aspect oriented programming (AOP) doesn't tend to replace OO model but rather tends to improve and to ease the software development work.

In practice, AOP complements OO programming by allowing to dynamically modify and improve the static OO model with the inclusion of some new needed code required to fulfill some new expected requirements.

AOP has introduced some concepts for addition or modification of existent code, mostly:

-Join Points. These are well-defined points in the flow of a program. Methods call or return, exception handler entry point and even field set or get operations are Join points examples.

-Points cut. They are mainly used to identify Join points from different classes.

-Advices which define both the code of the aspect and, combined with join points, when it will have to be executed. Usual advices are before advices as we expect the new aspect's code to be executed before the join point, and after advices as we expect the new aspect's code to be executed after the join point.

AOP frameworks are now available [START_REF] Aspectj | The ASpectJ Project[END_REF] [START_REF] Aspectwerkz | [END_REF]. We prefer AspectWerkz (AW) [START_REF] Aspectwerkz | [END_REF] as it provides a so-called online-mode to operate upon already compiled pieces of code (.class) and, most interesting, have a useful API providing all of the requirements needed to dynamically manipulate aspects. Finally, AW (from WebLogic/BEA) intends in a mid-future to fully interact with Jboss and a bringing together of AW and AspectJ is on the way.

We note that classes are AOP main concerns, objects being at glance out of concern. That's to bypass this limitation and to have the possibility to select and operate upon classes instances that naming attributes have been introduced as part of our activities.

Layout of the filters

When launched under AW using: aspectwerkz -Dkms.xml -cp kms app.jar any method m() of app.jar will be executed after the before aspects known by kms have been executed and the result of m() will be delivered after the after aspects known by kms have been executed.

Filters registered by the KMS on behalf of an activity are inserted in the in-filters or in the out-filters chain according to the activity's request and to the interface they inherited from. From the managed object point of view, each filter then appears as a before aspect if placed in the in-filters chain or as an after aspect if placed in the out-filters chain. Many filters can simultaneously be present in a chain, these filters may have been required by one or several different activities. Filters placed by the KMS have highest priority, follow activities filters, then sub-activities filters then sub-sub-activities filters etc. Connected to AW, KMS is informed of the methods calls and returns of each object. For a method call, depending on the caller and on the call parameters, KMS selects in the in-filters the list of filters to apply. These filters are then sequentially executed using AW API (mainly through the method Object joinPoint.proceedMod(), a modify version of the original proceed() one). If desired, parameters can be altered by using the AW API. Registered filters to select and apply for a method return value follow the same approach.

To avoid incoherence in these chains, the possibility of one or several different activities managing an object -e.g. a logging activity initiated by a user A and a synchronization activity required by a second user B -depends of the application and of the KMS's choices. Moreover, only filters registered with enhanced privileges or filters registered by the KMS for its own needs can modify or stop a message. Left part of Fig. 2 depicts a managed object wrapped in a JVM/AW execution environment. A dash line represents the execution path taken by an m() request. First KMS in-filters are applied, then the activity's in-filters before m() is effectively called. The returned result have to pass through activity's outfilters and finally the KMS out-filters before being sent back to the requestor.

Future work and conclusion

The purpose of this work is to investigate the problem of management for distributed applications and provide means to operate in a distributive and dynamic manner. We suggest to use Aspects concepts as a possible solution to allow an evolutive and decentralized high level management scheme without We plan to define virtual MIBs for the KMS which will allow usual SNMP tools to interact with it and provide new facilities to manage software or even hardware component, but some problems remain as our interfaces directly compete with the SNMP OBECT-TYPE macro.

Figure 1 .

 1 Figure 1. Architecture overview.

Figure 1 .

 1 Figure 1. depicts the expected scenario.A management application can be required by the KMS or sent by a peer-KMS towards the application to manage. Management application is a nearly autonomous entity containing its management activities. According to the target application and the activities wishes, the KMS when places the filters of the different management activities around the expected components. The KMS can provide local resources or services to management applications.

Figure 2 .

 2 Figure 2. Filters and messages paths.

  modifying neither the underlying middleware nor the managed objects.


M Yap

San Min

P Maurine

M Bastian
email: magali.bastian@infineon.com

M Robert

A Novel Dummy Bitline Driver for Read Margin Improvement in an eSRAM

Keywords: dummy bitline driver, low power, self-timed memory, SRAM, statistical design

Aggressive scaling of transistors is often accompanied by an increase in variability of its intrinsic parameters. In this paper, we point out the importance of considering sensitivity performances due to process variations during SRAM design. We propose a novel dummy bitline driver, an essential component in a self timed memory, which is less sensitive to process variations. A statistical sizing method of this dummy bitline driver is introduced so as to improve the read timing margin, while ensuring a high timing yield. The memory considered is a 256kb SRAM design in 90nm technology node.

Introduction

Technology fabrications have led to the realization of system on chip whereby functional blocks coexist, like embedded memories which can occupy up to 80% of the chip's area. Hence, the overall performances and the fabrication yield of the chips rely heavily on memory's yield. Simultaneously with the rapid increase of memory blocks within the chips, technology evolution is accompanied by an increase of variability effects owing to process variations, which appear during the manufacturing steps.

Generally, process variability can be classified into 2 distinct groups of manufacturing processes namely: global and local variations. Global variations originate from numerous factors: non uniform chemical mechanical polishing [START_REF] Unsal | Impact of Parameter Variations on Circuits and Microarchitecture[END_REF], lens aberrations [START_REF] Orshansky | Characterization of Spatial Intrafield gate CD variability, its impact on Circuit performance, and Spatial Mask-level Correction[END_REF] and non-uniformity of temperature [START_REF] Unsal | Impact of Parameter Variations on Circuits and Microarchitecture[END_REF], whereas local variations stem from a variety of factors like random dopant fluctuations [START_REF] Mukhopadhyay | Modeling of Failure Probability and Statistical Design of SRAM Array for Yield Enhancement in Nanoscaled CMOS[END_REF] and line edge roughness [START_REF] Croon | Line Edge Roughness: Characterization, Modeling and Impact on Device Behavior[END_REF]. In fact transistor scaling has exacerbated the impact of local and global variations, affecting performances of integrated circuits like maximum operation frequency and static power consumption.

To handle the impact of process variations in circuit design, corner based methodology is performed by characterizing the circuit across process corners. However, the increase of variability in manufacturing process results in an underestimation of performances in the operating frequency of an integrated circuit. This can therefore impact on the convergence of the design flow. In this paper, we highlight the importance of considering process variations in the design of an SRAM. We propose a novel dummy bitline driver which tracks the discharge time of the bitline in a read operation and triggers the sense amplifier at the right time. This structure is less sensitive to process variation. A statistical sizing method of the driver is also introduced to improve the read timing margin while guaranteeing a high timing yield.

The paper is organized as follows. Section 2 introduces a simple way of computing the required read timing margin without being too pessimistic and of calculating the probability of fulfilling this constraint. Section 3 presents a new structure, dubbed Dummy Bitline Driver (DBD), having its timing performances less sensitive to manufacturing processes compared to a more classic DBD [START_REF] Amrutur | A Replica Technique for Wordline and Sense Control in Low Power SRAMs[END_REF]. In section 4, we will introduce a statistical sizing method of the DBD which is independent of the process corner. Section 5 compares the results obtained between the proposed and the classic structures.

Modelling Approach

Conventionally, the characterization of a circuit involves performing several simulations across best and worst case corners to verify whether its performances and timing constraints are met under all conditions. For example, the worst case delay is defined by considering that principal parameters p i of transistors have their values at ±m i ⋅σ pi (m i ∈Ν) around their mean values µ pi (σ pi represents the standard deviation of the statistical distribution of parameter p i ). The set up of such a simple approach, through a proper choice of m i values, allows the worst case to be defined at n⋅σ D , with σ D being the standard deviation of the delay distribution.

Failing to account for local variations across process corners is not a serious problem as far as simple data paths are considered. However, this issue is far more complex for data path with racing conditions. This approach incurs optimistic and pessimistic estimations of worst and best case methods. In this section, we will introduce a way of computing the required read timing margin without being too pessimistic or optimistic. Consider the signal races during a read operation between signals A and B issued from the same control block. Signal A activates a selected memory cell (denoted by CC in Fig. 1) which discharges bitline BL, whereas signal B triggers the sense amplifier during the discharge process of BL (Fig. 1). Let us also assume that the signal A should arrive at most 0 ps after signal B for a proper read operation of a selected SRAM cell. Let Let us now evaluate the probability of meeting a timing constraint. Assuming that all distributions are normal, the mean value and the standard deviation of distribution D are given by:

ρ ⋅ σ ⋅ σ ⋅ - σ + σ = σ - = B A B A D A B D µ µ µ 2 2 2 (1)
Using the Galton approximation, with the hypothesis that µ D >0, the probability P V of satisfying the timing constraint for all values of ρ is computed as follows:

                    σ ⋅ π ⋅ - - + ⋅ = 2 2 2 1 1 2 1 D D V µ exp P (2)
As the sensitivities of delays to process variations V A =σ A /µ A and V B =σ B /µ B are known and found to be relatively constant over a wide range of µ A and µ B values (±20%), the value µ B and subsequently that of the read timing margin µ D Yield (Appendix A.1) can be computed as follows to guarantee a proper read operation defined at n⋅σ:

A Yield D µ a c b b a µ -           + ⋅ - ⋅ - = 1 1 2 (3) 2 2 2 2 2 2 1 A A B A A B n µ c V n b V n a σ ⋅ - = ⋅ - = µ - ρ ⋅ σ ⋅ ⋅ =

Dummy Bitline Driver with Reduced Variance

In a more specific context, involved in the design of advanced technologies, the corner method seems no longer enough to satisfy the timing constraints without the use of an increasing timing margins caused by an increase of local variations. This fact brings up a question: Is it possible to maintain, or even reduce the design timing margins through design?

To do so, we have defined a Dummy Bitline Driver (DBD) structure (Fig. 3a) which is less sensitive to process variations compared to a more classic structure (Fig. 3b). Indeed, the DBD is an essential component of a self-timed SRAM. In the absence of an internal clock signal, the DBD coupled with the dummy bit line acts as a metronome to fire the sense amplifier at the appropriate time during a read operation. It guarantees, as shown in Fig. 1, the proper triggering of the appropriate sense amplifier when the potential difference of the input signals between BL and BLB of the sense amplifier has reached the required level (10% of VDD).

The topology of the proposed DBD has been realized such that the discharge characteristics of dummy bitline (Fig. 1) being discharged by the DBD match those of bitline being discharged by an SRAM cell represented in Fig. 3c. As shown in Fig. 3a, transistors PD and PGi (i=1 to 4) of the proposed DBD are akin to transistors PDcci (i=1, 2) and PGcci (i=1, 2) of the SRAM cell. Moreover, logic gates g1 and g2 will mimic the signal WEN which controls pass gate PGcci. The transistor Pr is used for precharging dummy bitline, connected to pin 'out', at Vdd before any read operation. Transistor N1 sets node Z to 0 V at the beginning of a read cycle operation. When the internal signal WLSDUM is at '1' during a read mode, inputs pins Iadji (i=1 to 4) are activated by hardcoding them individually at Vdd. Hence, they can be used to adjust the discharge current of the DBD with respect to the actual supplied voltage of the memory. In doing so, the read timing margin can be adapted to the supply voltage applied.

It should be noted that the reference DBD has also the same functionalities as the proposed DBD. The main difference lies in the use of stacked transistors for representing pass gate and pull down transistors of the SRAM cell. This condition causes the sensitivity of the read current flowing through PGi and PDi (i=1..4) to be less representative of the read current flowing through PGcc1 and PDcc1 in the 6T SRAM cell.

timing margin ∆µ D /µ Dref lying between 14.5% to 25.2%.

Conclusion

Due to the pessimism of corner analysis method, we have proposed a simple way of computing the required read timing margin and of calculating the probability of meeting this constraint. A statistical optimization method has also been developed to ensure a predefined timing yield. The developed design approach has been particularly introduced to optimize the critical path of the SRAM memory, in which the dummy bitline driver has been replaced by a more robust structure to manufacturing process variations. Results have demonstrated that the use of the optimization method and the proposed dummy bitline driver improves significantly the reduction in the design timing margins, while ensuring a given timing yield.

Figure 1 :

 1 Figure 1: Signal race between paths A and B in an SRAM

  µ A , µ B and σ A, σ B be the mean values and the standard deviations of the propagation delay distributions of signals A and B. Let µ D and σ D represent the mean and the standard deviation values of the path delay difference D (read timing margin) between A and B.

Figure 3 :

 3 Figure 3: (a) Proposed DBD (b) Reference DBD (c) 6T SRAM cell

Table 1 :

 1 Variability reduction

	Iadji Iadji	Vdd (v) Vdd (v)	T (˚c) T (˚c)	µ A (ps) µ A (ps)	V A (%) V A (%)	µ B ref (ps) µ B ref (ps)	V B ref (%) V B ref (%)	µ B prop (ps) µ B prop (ps)	V B prop (%) V B prop (%)	∆V B / V B ref (%) ∆V B / V B ref (%)
	1 1	1.00 1.00	-40 -40	1919 1919	9.0 9.0	2412 2412	10.8 10.8	2303 2303	9.8 9.8	9.3 9.3
			125 125	2209 2209	7.7 7.7	2497 2497	9.1 9.1	2453 2453	6.8 6.8	24.7 24.7
	1, 2 1, 2	1.08 1.08	-40 -40	1534 1534	7.5 7.5	1793 1793	8.5 8.5	1734 1734	7.5 7.5	11.7 11.7
			125 125	1827 1827	6.7 6.7	2029 2029	7.7 7.7	1998 1998	6.0 6.0	22.6 22.6
	1, 2, 3 1, 2, 3	1.20 1.20	-40 -40	1181 1181	6.0 6.0	1306 1306	6.9 6.9	1277 1277	5.9 5.9	13.7 13.7
			125 125	1448 1448	5.7 5.7	1585 1585	6.5 6.5	1565 1565	5.2 5.2	19.8 19.8
	1, 2, 3, 4 1, 2, 3, 4	1.32 1.32	-40 -40	963 963	5.2 5.2	1043 1043	5.4 5.4	1023 1023	5.1 5.1	5.8 5.8
			125 125	1200 1200	5.0 5.0	1315 1315	5.5 5.5	1289 1289	4.8 4.8	13.3 13.3

Table 2 :

 2 Reduction of read timing margin

	Iadji Iadji	Vdd (v) Vdd (v)	T (˚c) T (˚c)	µ A (ps) µ A (ps)	µ d ref (ps) µ d ref (ps)	P V P V	ref (%) ref (%)	µ d prop (ps) µ d prop (ps)	P V P V	prop (%) prop (%)	∆µ d / µ d ref (%) ∆µ d / µ d ref (%)
	1 1	1.00 1.00	-40 -40	1919 1919	493 493		99.99 99.99	384 384		100.00 100.00	22.2 22.2
			125 125	2209 2209	288 288		99.81 99.81	244 244		99.97 99.97	15.4 15.4
	1, 2 1, 2	1.08 1.08	-40 -40	1534 1534	259 259		99.99 99.99	201 201		99.99 99.99	22.5 22.5
			125 125	1827 1827	201 201		99.86 99.86	170 170		99.95 99.95	15.5 15.5
	1, 2, 3 1, 2, 3	1.20 1.20	-40 -40	1181 1181	125 125		99.94 99.94	96 96		99.88 99.88	23.2 23.2
			125 125	1448 1448	137 137		99.91 99.91	117 117		99.96 99.96	14.5 14.5
	1, 2, 3, 4 1, 2, 3, 4	1.32 1.32	-40 -40	963 963	80 80		99.97 99.97	60 60		99.67 99.67	25.2 25.2
			125 125	1200 1200	115 115		99.99 99.99	89 89		99.97 99.97	22.7 22.7

Statistical Sizing Method

In order to perform comparisons between the reference and proposed DBDs under constant timing yield, we have developed a sizing methodology.

Step 1 (identification of most critical condition): Starting from an initial solution, the first step involves identifying the voltage and temperature (V, T) Crit conditions having the poorest timing yield. To identify the critical condition, transient simulations of the timing performances of critical paths A and B in the memory are done under different temperature and voltage conditions covering this whole range to obtain µ A and µ B . The critical condition corresponds to the highest numerical value of the following expression:

Step 2 (variability estimation): The second step requires the estimation of the variability of paths A and B involved in the signal races. To do so, Monte Carlo simulations of the critical path are performed at the critical conditions (V, T) Crit found in step 1. Once these statistical simulations are performed and the values of µ A , µ B , σ A , σ B and ρ are obtained, the value of the required timing margin µ D Yield corresponding to a timing yield is computed using (3).

Step 3 (sizing for a given timing yield): The third step consists in sizing the DBD at a typical process and under (V, T) Crit to obtain the computed µ D Yield .

Step 4 (first verification step of the timing yield):

Once the above sizing procedure is over, the first verification step consists in performing Monte Carlo simulations on the critical path at (V,T) Crit to obtain µ A , µ B , σ A , σ B and ρ values. The constraint of the timing yield is then evaluated using (2). If the computed value fulfills the predefined constraint, we proceed with the second verification step. Otherwise, we reiterate step 3 with the new values of µ A , µ B , σ A , σ B and ρ.

Step 5 (second verification step of the timing yield): It implies verifying that the constraint of the timing yield satisfies all temperature and supply voltage conditions. This is done through Monte Carlo simulations in order to estimate the values of µ A , µ B , σ A , σ B and ρ for different values of V and T. Once the statistical simulation has been done, the timing yield is processed. If the values obtained for the various (V, T) couples are greater than the predefined constraint at (V, T) Crit , the verification step is over. However, if the constraint is not satisfied, step 1 should be repeated with the new sizing obtained.

Performance Comparisons

To perform performance comparisons, both reference and proposed DBDs have been placed in the critical path of a 256kb SRAM memory. The model card, used in Hspice simulations, is the bsim4.3.0 which takes into account local and global variations. The sizing methodology developed in section four has been applied to pass gate transistors PG1 to PG4 and pull down transistors PD, PD1 to PD4 (Fig. 3a and 3b) at four operating voltages considered i.e. 1.0V, 1.08V, 1.2V and 1.32V. The timing yield had been set at 99.87% i.e. n=3 and the correlation value ρ considered was equal to 0.9. At each operating voltage, the appropriate adjustments of pins Iadji were performed.

Once the statistical sizing method has been done, we performed 2000 Monte Carlo runs in order to obtain the mean values (µ A and µ B ) and standard deviation values (σ A and σ B ) of the characteristic delays of the signal races of paths A and B over the whole voltage and temperature ranges considered. The results obtained were used to compute in table 1 the reduction in the delay variance (∆V B ) of path B between proposed (prop) and reference (ref) DBDs and in table 2, the probability P V (2) of meeting the timing constraint, the read timing margin (1) of the reference µ dref and proposed µ dprop DBDs and subsequently the reduction in read timing margin ∆µ D between proposed and reference DBDs.

Table 1 shows the reduction in variability obtained.

The first column Iadji corresponds to the respective branches of transistors selected with respect to supply voltage. For instance Iadji=1, 2 means that branches Iadj1 and Iadj2 are selected at Vdd= 1.08V. The reduction in variability (∆VB/V Bref ) is quite important, lying between 5.8% and 24.7%. This reduction has been achieved by using pass gate transistors PGi in the proposed DBD which is 2 to 3 times the size of the PGi used in the reference DBD.

In table 2, we can see that the values of the probability P V of fulfilling the read timing constraint have been computed. As expected, the values of P V are very close to the required 99.87% (3σ) for both the reference and proposed structures. Simultaneously, we observe a reduction in the read

Appendix

A.1) Estimation of design margin µ D at n.σ D Suppose that we want to have a read margin µ D at n.σ D , such that:

. n (A.1) As we have seen previously in section 2, µ D and D can be defined using [START_REF] Unsal | Impact of Parameter Variations on Circuits and Microarchitecture[END_REF]. Expression (A.1) can therefore be represented by the following equation: As the delay of signal B should be greater than that of signal A, delay µ B in (A.2) becomes:

Once the delay of µ B is computed, the required design margin µ D Yield in (1) is given by:

The probability P V of fulfilling a timing constraint is given by (2). In fact, since (V, T) crit represents the condition showing the highest probability of the occurrence of a timing constraint violation P V should be minimum at this condition. Thus, P V is minimum if