Properties of Protein-based Materials

The macroscopic properties of the protein-based, 3D macromolecular networks partially depend on system stabilising interactions. The water solubility of protein materials depends on the nature and density of intermolecular interactions. Materials are soluble in water when the energy of the interprotein bonds is lower than the energy of the interactions that could be established between water and polar groups not involved in the network. The presence of physical nodes (i.e., chain entanglements), covalent intermolecular bonds and/or a high interaction density is sufficient to produce films that are completely or partially insoluble in water [77]. For example, the presence of intermolecular covalent bonds in wheat gluten- or keratin-based materials makes them insoluble. 

The mechanical properties of protein-based materials are substantially lower than those of standard synthetic materials, such as polyvinylidene chloride (PVDC) or polyester (Table 11.11). The mechanical properties of protein-based materials were measured and modelled as a function of film characteristics [74, 131, 132]. For stronger materials (e.g., based on wheat gluten, corn gluten and myofibrillar proteins, critical deformation (DC) = 0.7 mm) and elastic modulus (K = 510 N/m) values are slightly lower than those of reference materials such as LDPE (DC = 2.3 mm, K = 135 N/m), cellulose (DC = 3.3 mm, K = 350 N/m) or even PVC films. The mechanical properties of corn gluten-based material are close to those of PVC. 

Films References Tensile Strength (MPa) Elongation (%) Film thickness (pm) Temperature ( C) RH (%) 

Synthetic materials (O) -1- thermoplastic polyurethane elastomer (Dow Chemical), -2- PVC, -3- PVC plasticised with di-2-ethylhexylphthalate, 

The barrier properties of protein materials depend on the nature and density of the macromolecular network, and more particularly on the proportion and distribution of nonpolar amino acids relative to polar amino acids [11,27], The protein composition and structural organisation of the network enables some chemical groups to remain free, which means that they are sites of potential interactions with permeating molecules. Generally for protein-based materials, most free hydrophilic groups are able to interact with water vapour and permit water transfer phenomena, to the detriment of hydrophobic gas transfers (e.g., nitrogen and O2). 

The macroscopic properties of the protein-based, three-dimensional macromolecular networks partially depend on system-stabilising interactions. The water solubility of protein materials depends on the nature and density of intermolecular interactions. Materials are soluble in water when the energy of inter-protein bonds is lower than the energy of 

The mechanical properties of protein-based materials can partly be related to the distribution and intensity of inter- and intra-molecular interactions that take place in primary and spatial structures. The cohesion of protein materials mainly depends on the distribution and intensity of intra- and inter-protein interactions, as well as interactions with other components. For example, in soy-based materials, hydrophobic interactions 

PVDC poly(vinylidene chi LDPE low density polyetl RH relative humidity (%) oride) rylene HDPE higfj density poi X film thickness (pm) T temperature (°C) yethyle me  

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The aroma barrier properties of protein-based materials seem especially interesting for blocking nonpolar compound permeation. However, it is hard to determine the relationship between the physico-chemical properties of aroma compounds and their... 

The macroscopic properties of protein-based materials partially depend on protein-network stabilizing interactions. The presence of "physical bonds" (i.e. folded chains), covalent intermolecular bonds and/or a high interaction density... 

The mechanical properties of protein-based materials have been studied (Table II) and modeled (9,31-33). 

The mechanical properties of protein-based materials closely depend on the plasticizer content, temperature and ambient relative humidity (16,34,35). At constant temperature and composition, an increase in relative humidity leads to a major change in the material properties, with a sharp drop in mechanical strength and a concomitant sharp rise in distortion. These modifications occur when the Tg of the material is surpassed (Figure 1). These variations can be reduced by implementing crosslinking treatments (physical or chemical) or using high cellulose or mineral loads (22). 

Although the amino acid sequence as well as the secondary structure of fibroin differs from those of spidroin, the fibers spun from these proteins, that is, silkworm silk and spider silk have comparable mechanical properties. These may be attributed to the structural characteristics, both at the molecular and filament level. The superior mechanical properties of silk-based materials, such as films, coatings, scaffolds, and fibers produced using reconstituted or recombinant silk proteins, are determined by their condensed structures. 

Protein-based polymers have the potential to surpass the polyesters and other polymers because they can be directly produced in microorganisms and plants by recombinant DNA technology resulting in the capacity for diverse and precisely controlled composition and sequence. This is not possible with any other polymer, and it increases range of properties and the numbers of applications. Remarkably, with the proper design of composition, protein-based materials can be thermoplastics, melting at temperatures as much as 100°C below their decomposition temperatures. Therefore, they can be molded, extruded, or drawn into shapes as desired. Aspects of protein-based materials as plastics is also considered below. 

There is analogy in the development of protein-based materials. Bioelastics, Inc., the general partner to Bioelastics Research Ltd. (BRL), has been working for about 15 years to arrive at a killer app that could launch the protein-based material industry. More specifically, BRL has been developing the scientific and intellectual property foundation that would pave the way for the extraordinary materials capacity of protein-based polymers to result in successful commercial applications. 

The initial preparation of protein-based polymers utilized solution and solid phase peptide chemistry. This made possible the preparation of more than 1,000 polymer compositions. As discussed in Chapter 5, these compositions were studied for determination of their basic properties, for the development of the set of phenomenological axioms for protein engineering and function, and for the demonstration of the basic mechanism that underlies function. In short, it is the chemical synthesis that has allowed development of much of the basic science and the demonstration of the potential of protein-based materials in a timely manner. Mostly because of the historical relevance, but also because of the unique contributions of chemical synthesis to arriving at satisfactory purification of microbially prepared protein-based polymers, a brief description of the chemical synthesis of protein-based polymers is given below. 

ProLastin polymers are a family of protein-based materials w hose resorption rate in vivo can be controlled by adjusting the sequence and not just the composition of the polymer (Cappello et al, 1995). These adjustments can be made so as to cause little change in the formulation characteristics of the materials, their physical forms, or their mechanical properties. They have good mechanical integrity with no need for chemical crosslinking. They degrade by enzymatic proteolysis and are presumed to resorb by surface erosion. Their breakdowm products are peptides or amino acids w hich are electroneutral at physiological pH and cause no undue inflammation or tissue response. 

In order to be able to describe and predict changes in the physical-chemical properties of proteins during dry processing according to temperature and RH, it is essential to construct the state diagram relative to the water (or plasticiser) content [3,174]. Figure 11.7 shows the different steps involved in the formation of protein-based materials using the dry process [13,136,152,175] ... 

The mechanical properties of protein-based films can be markedly improved by adding fibres (i.e., composite materials). Mechanical properties are always highly dependent on the temperature and RH of the protein material (Figure 11.9). This modification, (i.e., sharp increase in deformation at break and decrease in mechanical strength), occurs suddenly when the material crosses the Tg range [174]. 

The gas barrier properties (O2, CO2 and ethylene) of protein-based materials are highly attractive since they are minimal under low RH conditions. Oxygen permeability (around 1 amol/m/s/Pa) is comparable to ethylene vinyl alcohol (EVOH) properties (0.2 amol/m/s/Pa) and much lower than the properties of LDPE (1,000 amol/m/s/Pa) [61] (Table 11.12). The O2 permeability of protein films is about 10-fold higher that EVOH-based films, mainly due to the high plasticiser content of protein-based films. 

Physical or chemical modifications of protein-based materials (films, materials, resins, adhesives, etc.) have also been proposed to enhance their water resistance and limit the impact of relative humidity on their properties or to modify their functionality (e.g. strengthen the surfactant effect) (6,11-13). 

Plasticizers are generally required for the formation of protein-based materials (] 1,14-18). These agents modify the raw material formation conditions and the functional properties of these protein-based materials (i.e. a decrease in resistance, rigidity and barrier properties and an increase in flexibility and maximal elongation of the materials). Polyols (e.g. glycerol and sorbitol), amines (e.g. tri-ethanolamine) and organic acids (e.g. lactic acid) are the most common plasticizers for such applications. Completely or partially water insoluble amphipolar plasticizers such as short-chain fatty acids (e.g. octanoic acid) can be used since some protein chain domains are markedly apolar. 

It is essential to determine the phase equilibrium patterns (Figure 1) of protein-based materials according to the moisture (or plasticizer) contents in order to be able to control the material formation conditions and predict variations in the properties of the end products under different usage conditions (temperature and relative humidity) 6,10,19JO),... 

While the barrier properties of synthetic materials remain quite stable at high relative humidity, the gas-barrier properties of material proteins (as for all properties of hydrocolloid-based materials) are highly relative humidity- and temperature-dependent. The O2 and CO2 permeabilities are about 1000-fold... 

The measurement of bulk modulus of macroscopic fibers provided the fundamental knowledge to understand the physical properties of protein based structures, such as wool, tendon, bone, connective tissues, silk, etc. in relation to their chemical composition and eventually opened a variety of ways to improve their advantageous properties or mimic them in partially or totally synthetic materials [18]. 

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