Bio-based materials to deliver and to supply natural antioxidants: intrinsic active materials for food packaging and biomedical applications

The main focus of my Ph.D. research is to study a new way to supply natural antioxidant molecules within a specific substrate using biocompatible polymers, the latter being either synthetic or natural. Collins Dictionary defines the word “antioxidant” as: any substance that inhibits oxidation, as a substance that inhibits oxidative deterioration of gasoline, rubbers, plastics, soaps, etc. an enzyme or other organic substance, as vitamin E or beta carotene, that is capable of counteracting the damaging effects of oxidation in animal tissues and food. The first definition is closer to the field of polymers used for the production of goods that are commonly used. If the antioxidant protects the material during its processing (that usually occurs at high temperatures), it is identified as secondary antioxidant: on the other side, an antioxidant used to prevent goods degradation (i.e. weathering) is defined as primary antioxidant. The common practice consists in a proper combination of primary and secondary antioxidant in order to obtain a final material that is able to be processed and to resist for the entire life of the products. The choice of the proper antioxidant depends on the polymer and therefore on the technological field of use and must be done according to three important parameters: The temperature required for the process (i.e. melt extrusion, mold injection etc.): the higher the processing temperature, the higher must be the degradation temperature of the antioxidant. The time required for the transformation: the kinetic of decomposition must be considered as a crucial parameter because, even if the antioxidants are stable in a range of temperatures and can prevent degradation, a prolonged thermal stress, occurring at temperature close to the decomposition temperature, can promote the reaction and consume all the antioxidant leaving the polymeric material without protection against oxidation. The external stress exposure during goods life: UV light (indoor or outdoor applications), thermal stress (oxidation kinetic is dramatically dependent to temperature), moisture (antioxidants can be soluble in water therefore continuous washings can promote migration of antioxidant leaving polymer without protection). Radical attack is the most common route of polymeric material degradation; active radicals can be generated by thermal stress and ultraviolet irradiation and in both cases an active radical can attack the polymeric chain modifying the structure and changing material properties. The role of antioxidant is to interrupt the cycle preventing the degradation of the material. The second definition, more familiar with biochemical field, defines another kind of antioxidant, or better, another kind of role that antioxidant should have in a process. Oxygen radicals can attack, with the same mechanism shown in figure 1, a lot of biological substrate leading to degradation. If the attack occurs in non-living animal substrate (i.e. food) the degradation leads a low-quality or even a non-comestible product. If the degradation occurs at a cellular level, an “oxidative stress” is present: this can promote a lot of diseases, and even lead to cancer in living tissues. The use of active substances, i.e. of substances that can have active functions beyond the inert passive containment and protection of the product to preserve packaged food is a novel approach used in packaging that lies beneath the field of the so-called “active packaging”. Food companies, and also the academic world, have a great interest in this field. Economists have estimated that active packaging is a business worth 2.8$ billion 2014 that will reach 4.0$ billion in 2019 only in the US. In the academic field, the interest around active packaging is steadily growing: indeed, the key words “active packaging” give almost 15000 results related to papers and patents (SciFinder® - 2016) with a trend that dramatically increases in the latest 10 years. Up to 1996 the total amount of publications in the field was 1886 whether in 2016 year only, there are more than 1000 publications. The framework here presented is the background of the present research, active packaging is defined as the field related to “packaging having active functions beyond the inert passive containment and protection of the product”: it is one of the most promising novel strategies in the field of food packaging. Some active packaging solutions are already present in the market, and examples are: devices for moisture control, oxygen scavengers, CO2 emitter, antibacterial coating and also radical scavengers. Active packaging solutions rely on two main techniques, one involving the use of external devices put in the package (i.e. silica small bags to absorb moisture or iron sachets as oxygen scavengers) – this solution is not appealing for consumers in food packaging – the other involving the use of additives compounded with the polymers used for packaging. One of the most relevant issue of the latter is related to the migration of active substances into food over time that causes food contamination and alteration. The PhD project is dedicated to the development of a new approach for active packaging, capable of potentially solving (or at least dramatically limit) the problems evidenced: such approach relies on the “in situ” synthesis of intrinsically active polymers – antioxidant being the preferred ones – with the potential of becoming materials used to produce films for packaging having intrinsic active features. Another issue related to packaging (and food packaging in particular) is the huge environmental impact (waste food packed with standard PE, PP or PET is disposed as general waste, thus raising two ethical issues, one related to food waste and the other related to health concerns related to traditional plastics end-life in the last years the use of biopolymers as packaging materials has rapidly grown, even if they are still a niche market. The PhD project was therefore focused on the synthesis of intrinsically active biopolymers, to combine both the environmental sustainability of such biopolymers and the new approach related to active packaging solutions: some natural antioxidant molecules (or their simple derivatives) were chosen according to their potential reaction in “in situ” polymerization of lactide to give poly lactic acid (PLA). Lactide reacts giving PLA via Ring Opening Polymerization (ROP) that can be initiated by an aliphatic alcoholic moiety whereas aromatic ones do not react with lactide. Benzyl alcohols bearing a phenolic moiety can act as polymer initiator leaving unreacted the phenol groups, responsible for antioxidant features. The first step was to synthesize intrinsic antioxidant polymers using bulk polymerization of lactide. Different antioxidant compounds were found as possible active molecule, namely: Tyrosol (Tyr), Vanillyl Alcohol (VA), Methyl Ascorbic Acid (AA), Pyridoxine (Pyr) and dihydroxy benzyl alcohol (DBA). The criterion of choice was based on the presence of at least an aliphatic alcoholic moiety allowing potential reaction with lactide. The second criterion used was the resistance at high temperatures, since PLA bulk polymerization involves the use of high temperatures of about 190°C. The study of thermal properties (performed via TGA) revealed that only VA and Tyr have the required long-term thermal stability at 190°C. Thanks to these two criteria, VA and Tyr were chosen as the molecules that could be added in the feed used for the bulk polymerization of lactide. Given the presence of a phenolic and of a –CH2-OH moiety in both molecules, they could act as bi-functional comonomer in the polymerization. Nevertheless, the phenolic moiety is too acid to form a stable bond with lactide: this means that VA and Tyr act as mono-functional initiators in the ring opening polymerization (ROP) of lactide. They were therefore used in low concentration in the polymerization feed (0,1% mol/mol on lactide) to allow having polymers with high molecular weight, necessary to obtain films via solution casting. To evaluate “in vitro” antioxidant features of such polymers, DPPH assay were used to assess the antioxidant power of the new polymers. Despite VA and Tyr were used in low concentration, PLA bearing VA resulted having 8.3% of radical scavenging (whereas pure PLA has no radical scavenging activity, as expected), where the scavenging ability of pure VA is 94%. Besides, the relatively high molecular weight (comparable to the one of Natureworks PLA Ingeo® 4043D, commonly used in food packaging) of this polymer allowed the production of films via solution casting: this indicates that the polymer, in a future, could be used also for film production via melt extrusion. The research project was then devoted to the research of further improvements to the antioxidant power of PLA+VA and to find a way to widen possible application fields of this new class of intrinsically antioxidant polymers. Polymerizations were performed with increasing VA content (up to 0.2%); this strategy revealed some problems: 1) The antioxidant molecule acts as chain initiator therefore as its concentration increase, the average degree of polymerization (DPn) decreases, jeopardizing the possibility to process the polymers (i.e. to obtain homogeneous films via solution casting). 2) The Vanillyl alcohol has an intrinsic high reactivity due to its antioxidant moiety that leads to unexpected side reactions. Regarding point 1, the theoretical approach relies on the studies about the degree of polymerization (DPn). DPn at full conversion (i.e. 100%) can be determined with the following equation: DPn=n_monomer/((n_initiator×f) ) 1) nmonomer are the moles of monomer, ninitiatior are the moles of initiator and f is the number of functionalities of active molecules able to react with the monomer. Equation 1 shows that the increase of chain initiator concentration decreases DPn value. For instance, a 0,1% mol/mol concentration leads to a polymer with maximum DPn = 1.000 (real DPn is lower) while 0.5% mol/mol decreases maximum DPn to 200. Since melt viscosity strongly depends upon molecular weight, and therefore DPn, low DPn leads to low melt viscosity, that doesn’t allow for melt extrusion of films: DPn of industrial PLA used for film production is higher than 600. This consideration led us to choose a maximum concentration of VA lower than 0.2% mol/mol. Regarding point 2, VA has an intrinsic high reactivity. If its concentration is higher than 0,5 mol%, at 190°C and in the presence of a catalyst, side reactions occur. Due to what described about the above points 1 and 2, the experiments were conducted on PLA containing 0,1% mol/mol of VA. The very promising DPPH results on this sample allowed to perform shelf life test with a real industrial food matrix: an industrial salami was chosen due to its high fat content that makes it a critical food in terms of shelf life requirements. The test required a long preparation time since, to perform reliable tests, a minimum of 160 salami slices must be tested: PLA films were used as interlayer between two salami slices. For each packaging of two salami slices, a film of about 100cm2 is required. It means that for a reliable shelf life test at least 8000cm2 of active PLA (the surface of 12 A4 sheets) was required; solvent casting deposition permits to produce only one A4 sheet of PLA film every 15h, therefore the production of all polymer sheets for the test took more than 180h. The results obtained were very good: PLA+VA enhances the stability of food matrix decreasing the oxidation kinetic: the degradation begins after 15d whereas, in non-active packaged salami, the degradation starts after few days. DPPH value does not decrease during the test (i.e. no migration of VA occurs over time) Therefore, at the end of the work, a new class of polymeric materials with intrinsic antioxidant properties was developed, tested with “in vitro” assay and also with “in vivo” shelf life test. CONCLUSION During the Ph.D project, several issues related to intrinsically antioxidant polymers were addressed: The possibility to obtain intrinsic antioxidant environmentally friendly polymers via ROP of lactide was verified: antioxidant molecule such as Tyr and VA can react with lactide using the same approach used for industrial process of PLA synthesis. The possibility to increase the quantity of VA in lactide bulk ROP was tested: the maximum quantity of VA should be below 0,2% mol/mol and therefore no real change in antioxidant feature could probably be achieved in comparison to the polymers with 0,1% of VA. The polymers obtained were tested in-vitro confirming that the antioxidant moiety (phenolic moiety) still remain active after the polymerization leading to an “active” material. Afterwards, the PLA containing VA was tested in-vivo using a standard procedure for shelf-life studies: the in-vitro results were confirmed by these assays, since the material reduces the oxidation rate of packaged food enhancing the shelf-life of the products. The second part of the Ph.D. project was devoted to the use of natural antioxidants for biomedical application. Oxidative stress, as was previously mentioned, is one of the main causes of a series of degenerative diseases (like cancer, heart failure, infections), including those pathologies affecting the Central Nervous System like Parkinson’s disease, Alzheimer’s disease, and also depression. The presence of free radical, as reactive oxygen species (ROS) in mitochondrial ambient, can disturb the normal redox cellular equilibrium leading to damage the cellular ambient, including lipids, protein and also DNA; however, reactive oxygen species can be beneficial as they are used by the immune system as a way to attack and kill pathogens. Short-term oxidative stress may also be important in prevention of aging by induction of a process named mitohormesis, an adaption of human body of stress caused by the presence of endogenous ROS. Not only the endogenous ROS uncontrolled production can cause diseases but also an external trauma, for instance a fracture of bone or muscle sprain, can modify the cellular redox equilibrium leading to an exceeded production of radical. The administration of antioxidant in the damaged area can restore the normal redox equilibrium enhancing the healing process. A possible way to prevent, or at list to reduce, the effect of oxidative stress is the consumption of antioxidants, that are abundant in vegetable and fruits; the most famous are Vitamin C, Vitamin E, natural flavonoids, carotenoids. Even if a well-balanced diet can provide many nutrients, sometimes it could not be enough for many causes (for instance exceeded stress due to the life style, high intensity training, unhealthy habits, smoke etc.): to complete the daily supply of antioxidants with supplements can be a good solution. Antioxidant supplements represent a 23$ billion/year industry (n.d.r. 2015) that continue to spread its market. One of the relevant issue of antioxidant supplement, and also for antioxidant molecules present in foods, is related to the “real” availability of the active molecule inside the human body. In other terms, the effects of antioxidant are not only dependent on their concentration in food or supplements but especially on the so-called “bioavailability” and “bioaccessibility”. Bioavailability is defined as the proportion of an antioxidant that is digested, absorbed, and utilized in normal metabolism; measurement of bioavailability heavily relies upon estimates of amounts of antioxidant absorbed. On the other hand, bioaccessibility is a commonly used term defined as the amount of an ingested nutrient that is available for absorption in the gut after digestion. In these terms, the bioavailability strictly depends on the bioaccessibility. Bioaccessibility rapidly decreases over time: in fact, the concentration of antioxidant dramatically decreases 2h after digestion: therefore, in the human body the concentration of antioxidants rapidly increases after the assumption of food and supplements and, with the same velocity, decreases. A system that is able to protect and to release an antioxidant under specific conditions could be a solution to overcome these problems. Scientists have developed a lot of systems for controlled release of substances: dendrimers, nano and micro particles, brush polymer, hierarchical scaffolds etc., Even if the structure and therefore the production methods are different, the key concept is almost the same: the active principle molecule is entrapped in a system which can create weak reversible interactions and, with a specific variation of pH or solvent solubility (the active principle should be more soluble in water than in the macromolecular chains of the scaffolds), the release occurs. A controlled release has great advantages in comparison to “standard” method: The release occurs in the target substrate reducing dramatically the loss of activity and preventing the eventually side effects. The active principle concentration can be kept constant obtaining better results in terms of performance and efficacy. The concentration of active substances in supplements could be decreased reducing the costs of production. Two different release systems were investigated during this Ph.D. thesis: High porous scaffolds Release system for ingestion Biomedical scaffolds are defined as a “solid framework able to hold cells or tissue together” therefore they required specific features: thermal stability (Tg > human body’s Temperature), resistance to standard sterilization procedure (i.e. thermal treatment, UV irradiation, cryo-treatment etc.), biocompatibility (reduced inflammatory response due to chemical structure of material in contact with tissues), and maybe bioresorbable properties tuned for specific scopes (decomposition time, due to enzymatic and chemical attack has to be tailored on the time required for medical treatment). A lot of materials can be used for scaffolds production, for instance metals and ceramic materials present a lot of advantages in terms of resistance to sterilization treatment and resistance to human body ambient; however, they promote inflammatory process and cannot be absorbed by human body requiring a surgery at the end of treatment. Polymer-based scaffolds can overcome these problems; polymers can be tailored for specific purpose avoiding problem related to inflammatory process and bioresorbable property that can be finely tuned (for instance the ambient in gastrointestinal tract is widely different than ambient presents in broken bone). Differently, polymers present a less stable structure (i.e. low thermal stability in comparison to metals or ceramics) that leads to a low resistance to sterilization treatments. A lot of polymers with high biocompatibility for this purpose were developed: polyacrilates (PA), polyurethanes (PU), polyvinylpyrrolidone (PVP), polyglicolide (PLGA),poly (lactic acid) (PLA) etc., Another possibility is the use of natural polymers, extracted from natural sources, for the formation of scaffolds, the most common being collagen and chitosan (from animal sources) and pectin and alginate (from vegetable sources). Synthetic polymers have the great advantage that they can be synthetized with the same features and controlling impurities: the reproducibility of the release in terms of kinetic and released quantities is higher than in other systems. On the other hand, natural polymers are completely biocompatible with human body, in particular with the digestive system, even if they have an intrinsically variability in their structure (ex. the quantity of methyl ester in pectin or the molecular weight or the percentage of acetylation in chitosan). During the Ph.D. project, different polymeric systems were studied for the delivery of antioxidants in human body. Two different approaches were pursued: The first approach consists in the use of a standard industrial poly (lactic acid) for the delivery of a modified natural antioxidant: oligotyrosol (oligoTyr). The second approach considers the use of natural polymers (i.e. pectin and alginate) for the delivery of Ellagic Acid. Oligotyrosol was obtained by horseradish peroxidase coupling of Tyrosol, a natural phenol obtained from olive and also green tea. Tyrosol does not present exceptional properties, having a radical scavenging power of 24% whether oligoTyr has a radical scavenging power of 48% (these values were obtained via DPPH analysis in standard conditions). The increased antioxidant power of oligoTyr leads to obtain new properties, since oligoTyr promote the ossification process in human osteoblast cells. The healing process of broken bone takes at least 8 weeks and a continuous administering of oligoTyr could promote the healing process. To have a continuous administering of oligoTyr, two approaches can be used: periodical targeted injection in the damaged zone. Insertion of a scaffold loaded with oligoTyr that is able to release the active principle and that, at the end of the process, can spontaneously degrade. The second option presents two great advantages: first of all, a patient can avoid continuous annoying injections and moreover a constant rate of active substance release avoids toxicity issues due to high concentrations. From this regard, PLA seems a good candidate for this purpose: it is biocompatible and can be digested via enzymatic attack of proteinase K. The first step of the work was dedicated to the study of a reliable procedure to obtain a hierarchical scaffold (high porous scaffolds promote the adhesion of bone cells and, increasing the surface area, can promote the release). Two different methods were used: •Method A. PLA was dissolved in THF in a 100 mL glass flask. Methanol was added at room temperature under mechanical stirring up to 95/5 v/v THF/methanol ratio. The solution was frozen by immersion of the flask into liquid nitrogen and was then poured in warm water. The solid PLA scaffolds that separated were recovered after removal of the solvent by filtration. •Method B. PLA was dispersed in 1,4-dioxane in a 100 mL glass flask at room temperature and taken under mechanical stirring overnight. The resulting homogeneous solution was frozen by immersion of the flask into liquid nitrogen and the solvent was removed by sublimation at room temperature under vacuum (2.5 × 10-3 Bar). Both methods were used for the preparation of scaffolds loaded with Gallic Acid (GA), used as a model molecule, Tyrosol (Tyr) and oligoTyrosol (oligoTyr). SEM images of samples prepared with the two methods showed that different structures can be obtained, revealing differences in superficial area and in pores dimensions. The first method leads to obtain scaffolds with very small pores (minimum measured dimension = 300nm) whereas scaffolds prepared with the method B have larger pores (minimum measured dimension = 2µm); method B also allows to obtain scaffolds with a smooth surface. The first important result achieved is that a method that allows to control the morphology of the scaffold that can be tailored for each application was developed. Once a method to control the morphology was obtained, another important parameter was to verify how the active molecules affect the formation of crystalline domains and of the amorphous phase. Gallic Acid was chosen for this purpose. It was also verified if the concentration of GA decreases during the scaffold preparation: UV quantitative analyses revealed a decrease of GA concentration due to migration in water promoted by melted THF during the solvent elimination phase. On the other hand, method B intrinsically prevents the leakage of substances, therefore this method was used for the preparation of scaffolds loaded with Tyr and oligoTyr. The following step was the preparation of scaffolds loaded with oligoTyr and Tyr: given their antioxidant features, Tyr and OligoTyr are able to reduce oxidative stress and promote osteoblastic cell growth. The aim was to obtain a scaffold able to constantly release oligoTyr in a period of about 2 months. In collaboration with the group of Prof. A. Napolitano from University of Naples Federico II, the release kinetics of oligoTyr and Tyr in phosphate buffer were investigated, in order to simulate human body condition. The group of Professor B. Burlando of University of Piemonte Orientale studied the biological effect of pure oligoTyr, Tyr and PLA and also verified the effect of osteoblastic cells growth, expressed as alkaline phosphatase (in particular ALP) activities. It was verified that PLA, Tyr and oligoTyr are not toxic for human osteosarcoma cells and Tyr and also oligoTyr promote cells growth. Besides the use of a synthetic polymer as bulk material for the production of scaffolds, natural polymers were also tested for the vehiculation of antioxidant molecules. Pectin and Alginate were chosen due to their easy availability and processability. Both polymers are water soluble polysaccharides that can be extracted from plants; in particular, alginates are widely extracted from brown seaweeds whereas pectin is refined from citrus peel but also from apple, apricot and carrots. Both polymers present carboxylic groups along the polysaccharides chain which can be used to coordinate metals; the coordination of bivalent ion leads to obtain a gel-like structure. Such structure permits to use the alginate and pectin as a material for the encapsulation of an active principle. Even if both materials are able to form gels, the different macromolecular structure between pectin and alginate (for each repeating unit alginate has a carboxylic moiety while the concentration of carboxylic moiety in pectin depends on the amount of galacturonic acid and on the percentage of its deacetylation) leads to different mechanical properties of the gel. Pectin-Ca(II) gel is reversible and stiffer than the gel obtained with Ca(II) coordinated to Alginate. These differences could be used to obtain different materials able to use different substrates. The first part of the work was devoted to the study of the material that can be obtained through calcium complexation with pectin: the materials obtained were characterized in terms of rheological properties, analyzing the gel obtained with different concentration of pectin in water. Also the rheological properties of the alginate gels were assessed; these analyses were performed in order to assess which are the best materials (in terms of rheological properties and gel stability) that should be used for the encapsulation of active molecules. The further step was to individuate an active molecule with the potential to be exploited in the future also in scale-up processes; one of the most appealing substances for that purpose was Ellagic Acid, an aromatic polyphenol present in many fruits especially berries and pomegranate. One of the great advantages of using such polymers is the possibility to work using water as a solvent, therefore the antioxidants that will be encapsulated should be soluble in water. Ellagic acid is almost insoluble in water (10 µg/ml) therefore a method to enhance such solubility was studied: the easiest way is to obtain a salt. Strong bases, such as NaOH or ammonia lead to deprotonation of ellagic acid and promote the decomposition via quinon formation. Weak and medium bases, such as substituted amine, are able to deprotonate the ellagic acid avoiding the degradation. The weak bases chosen for this purpose is the L-lysine: an essential amino acid, bearing an extra amine group, is the best possible solution due to its intrinsic biocompatibility and low cost. The use of L-lysine allowed to increase the solubility of ellagic acid up to 400.000 times obtaining a solubility of 40mg/ml. The last step was to encapsulate the ellagic acid-lysine salt in pectin and alginate gels and to study the kinetic of the release. The kinetics were studied in different conditions modifying the environment (water and phosphate buffer at pH 7,4). The results were very good: both pectin and alginate can control the release of ellagic acid-lysine salt. Moreover, modifying the production process, the release can be tailored: the release can be accelerated or decelerated reaching well-defined concentrations of ellagic acid. It is possible to obtain very low release rate (5% of loaded feed in 72h) or, modifying the environment and the structure, to reach very high release rate (25% of loaded feed in 2h). CONCLUSION During the Ph.D. project, several issues were addressed: High porous scaffolds based on synthetic industrial polymer (PLA) able to release active substances were produced; the structure of scaffolds can be tailored modifying the method of preparation. Two natural polymers, pectin and alginate, were used for the production of scaffolds with elevated biocompatibility able to control the release of an active principle. The release can be modulated modifying the composition of scaffolds.