Purified oxidized oligosaccharide and applications thereof

Abstract

The invention is directed to a method for preparing a purified oxidized oligosaccharide product comprising the steps of contacting an oligosaccharide and an oxidizing agent comprising an oxoanion such as a perhalogenate, in a solvent, to obtain a crude oxidized oligosaccharide product comprising oxidized oligosaccharide and contaminants; contacting the crude oxidized oligosaccharide product and a contaminant-lean ion exchange resin composition comprising a combination of an anion exchange resin and a cation exchange resin to obtain a contaminant-rich ion exchange resin composition; and separating the oxidized oligosaccharide from the contaminant-rich ion exchange resin composition to obtain a purified oxidized oligosaccharide product.

Description

[0001]25 P136927PC00 Title: Purified oxidized oligosaccharide and applications thereof TECHNICAL FIELD The invention is in the field of oligosaccharides such as cyclodextrins. In particular, the present invention relates to a method for preparing purifying an oligosaccharide and to uses thereof in hydrogels. BACKGROUND Oxidized oligosaccharides, and oxidized β-cyclodextrin in particular, are valuable building blocks for medical preparations. For instance, T.T.H. Thi et al., in RSC Adv., 2017, 7, 34053-34062 describes dual-functional hydrogels with high adhesiveness and hydrophobic drug delivery. Oxidized oligosaccharides may be produced by periodate oxidation of the corresponding oligosaccharide, e.g. β-cyclodextrin (β-CD), by T.T.H. Thi et al., mentioned above. The periodate oxidation is a common procedure for the chemical modification of oligosaccharides. However, the purification is generally conducted by dialysis, which is time consuming and not suitable for large scale reactions. Further, when using dialysis as purification method, the reaction needs to be quenched to guarantee that no further oxidation will occur. Quenching is typically carried out with glycerol and / or ethylene glycol and increases the level of iodate impurities and the formation of formaldehyde. This means that the reagent used for the quenching step needs to be removed as well during purification. Another drawback of time-consuming dialysis is that it requires large volumes of water and come with the risk of other contamination. For example, WO2020 / 249695A1 describes the oxidation of β-cyclodextrin using sodium periodate, after which the reaction was terminated by the addition of ethylene glycol. The oxidized β- cyclodextrin was purified by dialysis. Purification of oxidized oligosaccharides may also be performed by addition of calcium chloride followed by filtration and dialysis.Lou et al., Carbohydrate Polymers, 2020, 231, 115678 describe the oxidation of β-cyclodextrin using sodium periodate followed by successive addition of ethylene glycol and calcium chloride to stop the reaction and react with iodate. The precipitate of calcium iodate was removed by filtration and the obtained solution was dialyzed for 4h. A similar method is described by Bergal et al., Carbohydrate Research 2023, 533, 108936. Purification of oxidized oligosaccharides may also be performed with an ion exchange resin. When using an ion exchange resin, the periodate is exchanged with the counter-ion of the resin used, e.g. acetate ions. Examples on periodate oxidized dextran, wherein purification is performed with an ion exchange resin may be found in Domb AJ et al., Journal of Polymer Science Part A: Polymer Chemistry 1996, 34 (7), 1229-1236. This paper describes the oxidation of dextran with potassium periodate followed by removal of excess periodate with Dowex-1 in the acetate form. In Sokolsky-Papkov et al., Biomacromolecules 2006, 7, 5, 1529– 1535, dextran is oxidized using potassium periodate and purified with a Dowex-1 anion-exchange chromatography (acetate form, pH = 7). The purified solution was then dialyzed and freeze-dried. M. Kobayashi et al., Agricultural and Biological Chemistry, 1988, 52(11), 2695-2702 describes how dialdehyde derivative of cyclodextrin (dial-CD) was prepared by oxidation with sodium metaperiodate and purified by column chromatography on a Sephadex G-15 column. In US5624567A a process is disclosed for removing iodine / iodide from aqueous solutions. This process employs a strong-base anion exchange resin. However, when using a strong-base ion exchange resin, degradation of the oxidized oligosaccharide product may occur due to the very basic pH. If the removal of the periodate is combined with dialysis, then the potential degradation may be further worsened due to prolonged exposure to harsh conditions. Methods for purifying polysaccharides by removing undesirable flavors, odors and / or colors are described in EP0554818. These methods are however based on stream-stripping, which is incompatible with many oligosaccharides and do not address the above-mentioned drawbacks. There is therefore a need to provide a method to produce and purify oxidized oligosacharides quickly and efficiently. Ideally such a method applies to oligosaccharides – and cyclodextrins in particular – that may, for instance, be obtained from polysaccharides such as chondroitin sulphate, alginate, starch, pectin, dextran, cellulose, and hyaluronic acid. Purification of oxidized oligosaccharides that are obtained from periodate oxidation is particularly important when the oxidized oligosaccharides is to be used in (bio)medical applications such as use in combination with gelatin functionalized with hydroxyphenylpropionic acid (or caffeic acid, dihydrocaffeic acid, etc.) in dynamic hydrogels with improved viscoelasticity for cell / organoids culture. However, periodate oxidation has the drawback of generating iodate, which may in addition to the remaining non-reacted excess of periodate, be problematic. Removal of iodate is crucial because residuals can potentially form radicals and other unwanted byproducts. This can lead to contamination of the oxidized oligosaccharides, reproducibility problems (residual iodate can interfere with analytical methods for quantification of degree of oxidation), or undesirable property changes like discoloration (e.g. yellowing) of oligosaccharides during storage or at elevated temperatures, resulting in decreased biocompatibility of the product. SUMMARY The present inventors surprisingly found that oxidized oligosaccharides can very effectively and efficiently be purified by a combination of anion exchange and cation exchange resins. This combination allows the use of a strongly basic anion exchange resin, without unduly exposing the oxidized oligosaccharides to low pH and concomitant degradation. Favorably, this combination of resins makes to use of quenching reagents unnecessary, and results in highly purified oligosaccharides having lower degrees of contamination available by conventional purification methods. The inventors further found that these highly purified oligosaccharides can favorably be used in a variety of biological and medical applications. Accordingly, in a first aspect, the present invention is directed to a method for preparing a purified and oxidized oligosaccharide product comprising the following steps: i) contacting an oligosaccharide and an oxidizing agent comprising an oxoanion such as a perhalogenate in a solvent to obtain a crude oxidized oligosaccharide product comprising oxidized oligosaccharide and contaminants; ii) contacting the crude oxidized oligosaccharide product and a contaminant-lean ion exchange resin composition comprising a combination of an anion exchange resin and a cation exchange resin to obtain a contaminant-rich ion exchange resin composition; iii) separating the oxidized oligosaccharide from the contaminant- rich ion exchange resin composition to obtain a purified oxidized oligosaccharide product. In a second aspect, the invention is directed to a purified oxidized oligosaccharide product comprising less than 5 wt%, preferably less than 2 wt% oxoanions, based on the total weight of the purified oxidized oligosaccharide product. In a third aspect, the invention is directed to products containing the purified oxidized oligosaccharide product, such as hydrogels prepared from protein- based polymers. DETAILED DESCRIPTION In a first aspect, the present invention is directed to a method for preparing a purified and oxidized oligosaccharide product comprising the following steps: i) contacting, in a solvent, an oligosaccharide and an oxidizing agent comprising an oxoanion such as a perhalogenate to obtain a crude oxidized oligosaccharide product comprising oxidized oligosaccharide and contaminants; ii) contacting the crude oxidized oligosaccharide product and a contaminant-lean ion exchange resin composition comprising a combination of an anion exchange resin and a cation exchange resin to obtain a contaminant-rich ion exchange resin composition; iii) separating the oxidized oligosaccharide from the contaminant- rich ion exchange resin composition to obtain a purified oxidized oligosaccharide product (herein also referred to as the oxidized oligosaccharide product). Oligosaccharides are carbohydrate chains containing 3-30, preferably 3- 20, more preferably 3-10 monosaccharides (sugar units). For instance, cyclodextrins are a family of cyclic oligosaccharides, consisting of a macrocyclic ring of glucose subunits joined by α-1,4 glycosidic bonds. Examples of cyclodextrins include alpha, beta and gamma cyclodextrins. Cyclodextrins may be produced from starch or other polysaccharides by enzymatic conversion. The method of the present invention is particularly suitable for β-cyclodextrin (β-CD, having 7 glucose subunits). The method may be applied on a mixture of oligosaccharides, or – preferably – on a single oligosaccharide. The oligosaccharide in accordance with the present invention may also me a dimer or cluster of cyclodextrins. Dimers are structures where two cyclodextrin molecules are linked together. This linkage can occur through covalent bonds, hydrogen bonds, or other types of chemical interactions. The formation of dimers can enhance certain properties of the cyclodextrins, such as their ability to form inclusion complexes with guest molecules. Clusters are larger assemblies composed of multiple cyclodextrin units. Clusters can form through similar interactions as dimers but involve more than two molecules. Clusters can exhibit unique properties due to the collective behavior of the cyclodextrin units, potentially enhancing their application in fields like drug delivery, where they might form more stable or selective complexes with target molecules. Step i) is carried out in a solvent. The solvent preferably comprises water, such as only water or water in combination with a cosolvent such as dimethylsulfoxide (DMSO). Typically water is used. In a typical embodiment, the method includes a process of dissolving the oligosaccharide in the solvent and subsequently adding the oxidation agent. The concentration of the oligosaccharide in the solvent may vary widely, provided the oligosaccharide is soluble at the selected reaction temperature. Preferably, the oligosaccharide is dissolved in water or in a water / co-solvent mixture. As co-solvent, for instance dimethylsulfoxide (DMSO) may be used. The oligosaccharide may be dissolved in a concentration in the range of 0.5 to 40 wt%, preferably 2-10 wt%, more preferably about 5 wt%, based on the weight of the solvent. To dissolve the oligosaccharide at higher concentrations, elevated temperatures may be employed. The oxidizing agent that is used to oxidize the oligosaccharide comprises an oxoanion. The term oxoanion herein refers to an anionic species comprising one or more atoms of oxygen chemically bonded to a central atom or atoms, wherein the anionic species inherently possesses the chemical property of being able to undergo a reduction reaction by gaining electrons, thereby facilitating the oxidation of oligosaccharide in the process. Oxidation agents based on oxoanions are known to be able to oxidize hydroxyl groups. Perhalides and in particular periodates are known to efficiently carry out such reactions and are therefore preferred (see for instance Bergal and Andac, Carbohydrate Research 533 (2023) 108936, and references cited therein). However, reaction conditions could be selected such that other oxidation agents may also oxidize the oligosaccharide to the desired extend. Accordingly, the oxoanion may be selected from the group consisting of fluorine- based salt including hypofluorite (FO−), chlorine-based salts including hypochlorite (ClO−), chlorite (ClO2−), chlorate (ClO3−), perchlorate (ClO4−), bromine-based salts including bromate (BrO3−), perbromate (BrO4−), iodine-based salts including hypoiodite (IO−), iodite (IO2−), iodate (IO3−), periodates such as metaperiodate (IO4−), mesoperiodate (IO53−) and orthoperiodate (IO65−), astatine-based salts including astatate (AtO3−), perastatate (AtO4−), manganate-based salts such as permanganate (MnO4-), and combinations thereof. Still, perchlorate (ClO4−), perbromate (BrO4−), periodates such as metaperiodate (IO4−), mesoperiodate (IO53−) and orthoperiodate (IO65−), and combinations thereof are particularly preferred, of which periodate, and especially metaperiodate (IO4–) as even more preferred. Most preferably, the oxidation agent comprises sodium metaperiodate (NaIO4). The oxidation may be performed at conventional oxidation conditions, with a temperature in the range of 0 to 55 °C or even higher. For instance, it is known to perform periodate oxidation of cellulose at temperatures up to 85 °C, if the oxidation time is sufficiently short and in particular in the presence of metal salts such as lithium chloride acting as activators. See for instance Sirvio et al., Carbohydrate Polymers 2011, 83(3),1293-1297. By oxidizing the oligosaccharide, the hydroxyl groups thereof are converted into aldehyde groups. Oxidation therefore changes the aldehyde content of the oligosaccharide. Any aldehyde content is in principle permissible. However, the preferred range of oxidation is between 15-22%. The degree of oxidation can be expressed also as content of aldehyde in mmol / g. In this case, if use is made of β- CD, it this corresponds to 2.8 mmol / g to 4.1 mmol / g. The molar ratio of oxidizing agent versus oligosaccharide may vary from 0.5:1 to 8:1, more preferably from 2:1 to 3.5:1. Step ii) involves the use of the ion exchange resin composition to purify the oxidized oligosaccharide. Before this purification is carried out, this oxidized oligosaccharide is referred to as the crude oxidized oligosaccharide because it still contains considerable amount of contaminants such as any unreacted oxoanions and reaction residues thereof like the reduced forms of the oxoanions (which may itself also be oxoanions, albeit in a lower oxidation state than the oxoanion in the oxidation agent). In a typical embodiment, the crude oxidized oligosaccharide product in step ii) is dispersed and / or dissolved in a solvent that is the same type of solvent as in step i). In a preferred embodiment, the solvent used in step i) is continued in step ii), i.e. is not removed in-between steps i) and ii). The ion exchange resin composition comprises a combination of an anion exchange resin and a cation exchange resin. An ion exchange resin is a resin that acts as a medium for ion exchange. It is an insoluble matrix (or support structure) normally in the form of small (0.25–1.43 mm radius) microbeads, usually white or yellowish, fabricated from an organic polymer substrate. The beads are typically porous, providing a large surface area on and inside them where the trapping of ions occurs along with the accompanying release of other ions, and thus the process is called ion exchange. Anion exchange resins may be either strongly or weakly basic. Typical anion-exchange resins are based on crosslinked polystyrene. Preferably, strongly basic anion resins are used for the present invention as these effectively remove the contaminants. The anion exchange resin is preferably in OH- form with a Total Exchange Capacity ≥ 1.1 eq / L. It may also be in an acetate form or bicarbonate form. Various suitable anion exchange resins include Purolite™ A400 OH, Purolite™ A500 OH, Diaion™ NSA100. For instance, Amberlite™ FPA22 or Amberlite™ IRA-402 may be used. Cation exchange resins may be either strongly or weakly acidic. Typical cation-exchange resins are based on crosslinked acrylates. Preferably, weakly acidic cation resins are used for the present invention. The cation exchange resin is preferably in H+form. Various suitable cation exchange resins include DOWEX™ HCR-S / S, Purolite™ C100E, Purolite™ C100H. For instance, Amberlite™ CG50, Amberlite™ FPC88 H or Amberlite™ IRC-120 may be used. The anion exchange resin is part of the ion exchange resin composition and as such the ion exchange resin composition is capable of exchanging the anion it is carrying for at least part of the contaminants of in the crude oxidized oligosaccharide product. The oxidation agent and its reaction residues (i.e. the contaminants) in the crude oxidized oligosaccharide product are anionic and can be exchanged for the anion that the anion exchange resin. Before this exchange, the ion exchange resin is herein referred to as the contaminant-lean ion exchange resin. After, the exchange, the ion exchange resin is herein referred to as the contaminant-rich ion exchange resin. Advantageously, the anion exchange resin may be regarded as having a dual function: quenching the reaction by removing the oxoanions and at the same time removing other residual impurities (i.e. other contaminants). As such, it is not required to add a separate quenching agent such as glycerol or ethylene glycol or any other aliphatic polyol to stop (i.e. quench) the oxidation reaction. It may be appreciated that by exchanging the contaminants for the anion of the anion exchange resin, the pH of the oxidized oligosaccharide product drops. This drop in pH is however countered by the presence of the cation exchange resin, that exchanges its proton (H+) for the cations present in the crude oxidized oligosaccharide that served as counterions for the oxoanions. Examples of such cations are alkali metals and alkaline earth metals such as sodium (Na+), calcium (Ca+), lithium (Li+) and the like. This exchange increases the pH. Thus, by using both the anion and cation exchange resins, a relatively neutral pH (e.g. a pH in the range of 5 to 9, preferably 6 to 8) can be maintained during steps ii) and iii). As such, a purified oxidized oligosaccharide product can be obtained having a relatively neutral pH as well, which may for example be in the range of 5 to 9, preferably 6 to 8. In a preferred embodiment, the ion exchange resin composition comprises a mixture of the anion exchange resin and a cation exchange resin. This means that in step ii), the two types of resins are contacted simultaneously with the crude oxidized oligosaccharide product. This maintains the relatively neutral pH during step ii). The two types of resins may however also be contacted with the crude oxidized oligosaccharide product sequentially. In such embodiments, it is preferred that the anion exchange resin is used first, and the cation exchange resin is used second. The resin that is used first (typically the anionic resin) may be separated from the oxidized oligosaccharide product, before the resin that is used second (typically the cationic resin), is used. However, it may also be that the two resins are used in sequence without any intermediate separation from the oxidized oligosaccharide product. If the two types of resins are contacted sequentially with the crude oxidized oligosaccharide product, the time period between this contact should be sufficiently short to avoid unwanted degradation of the oxidized oligosaccharide due to a high or low pH. It some embodiments, charcoal may be used to further purify the crude oxidized oligosaccharide product. The charcoal can be used in combination with the ion exchange composition, but also before or after the ion exchange composition is used. In step (ii), the anionic exchange resin is preferably used in a ratio vis- à-vis the oxoanion used in step i) in the range of 1:1 to 3:1, expressed as the molar ratio of exchangeable anions of the anionic exchange resin versus the molar amount of the oxoanion. More preferably, this ratio is in the range 1.25:1 to 2:1, most preferably about 1.9:1. The weight of resin to be used can be calculated based on the exchange capacity of the resin. The exchange capacity of the resin can be converted to Eq / g (1 eq of exchangeable anion equals 1 mol exchangeable anion). The ratio between the moles of oxoanion and the resin Eq / g (which correspond to the moles of the exchangeable anions) gives the amount of resin in grams. Good results were obtained if the cation exchange resin and the anion exchange resin are used in step ii) in a weight ratio the range of 6:1 to 1:6, whilst best results were obtain with a weight ratio in the range of 2:1 to 1:2 such as 1:1. Step (iii), i.e. the step comprising separating the oxidized oligosaccharide from the contaminant-rich ion exchange resin composition to obtain a purified oxidized oligosaccharide product, can be carried out using standard solid- liquid separation techniques known in the art, including filtration. The process may further comprise removing the solvent by drying. The drying may for instance comprise spray drying or lyophilization, preferably by lyophilization. Product and its applications A further aspect of the present invention is the oxidized oligosaccharide product that is obtainable by the method as described herein. Accordingly, the present invention is further directed to the purified oxidized oligosaccharide product (also referred to as the oxidized oligosaccharide product or the oxidized oligosaccharide) that comprises less than 5 wt%, preferably less than 2 wt% oxoanions, based on the total weight of the product. Moreover, the product may comprise less than 0.05 wt% of one or more oxidation quenching agents, such as thiols (e.g. N-acetylmethionine), sulfites (e.g. sodium sulfite), alcohols (e.g. glycerol, ethylene glycol, etc.) and the like, preferably less than 0.05 wt% of glycerol and / or less than 0.05 wt% of ethylene glycol, based on the total weight of the product. The amount of oxoanions in the purified oxidized oligosaccharide product (also referred to as residual oxoanions) herein referred to is as determined by reacting the residual oxoanions with iodide at pH 3.3 and spectrophotometrically measuring the amount of resulting triiodide using at 352 nm. Yet another property of the product is that it may exhibit an endotoxin content of less than 200 endotoxin units (EU) / g, preferably less than 100 EU / g, more preferable less than 10 EU / g, as determined by the LAL-test. Such low endotoxin levels can be obtained because long exposure to water is not required (contrary to e.g. in dialysis). Yet another property of the product is that it may exhibit a degree of oxidation in the range of 12 to 25%, preferably 15 to 22%, based on the hydroxyl group content of the oligosaccharide. The degree of oxidation herein referred to is as determined by reacting the oxidized oligosaccharide and 2,4-dinitrophenylhydrazine (DNPH), which results in the formation of an orange precipitate, and spectrophotometrically measuring the amount of unreacted DNPH remaining in the supernatant using at 357 nm. The degree of oxidation when expressed as a percentage is herein based on the hydroxyl group content of the oligosaccharide and can be calculated based on the aldehyde content in mmol / g and the hydroxyl content in mmol / g. The oxidized oligosaccharide as used in the present invention may be oxidized chondroitin sulphate, oxidized alginate, oxidized starch, oxidized pectin, oxidized dextran, oxidized hyaluronic acid, or a combination thereof. Oxidized dextran such as oxidized beta-cyclodextrin is particularly preferred for its application in medical purposes. The oxidized oligosaccharide in accordance with the present invention can be used in composition such as hydrogels and / or bio-adhesives that are formed from protein-based polymers and the oxidized oligosaccharide product. The oxidized oligosaccharide in accordance with the present invention are particularly suitable for such compositions, as these compositions have medical applications for which a control over the degree of contamination such as the oxoanions, endotoxin content, glycerol, ethylene glycol etc. is important. Accordingly, a further aspect of the present invention is directed to a composition, preferably a hydrogel and / or bio-adhesive comprising a protein-based biopolymer and the purified oxidized oligosaccharide product. The protein-based biopolymer may comprise elastin, silk, collagen, fibrin and / or gelatin. In particular embodiments, the protein-based biopolymer comprises gelatin, more preferably modified gelatin. Examples of modified gelatins that can be used include those selected from the group consisting of gelatin- methacryloyl (GelMA), gelatin-desaminotyrosine (GelDAT), gelatin- desaminotyrosyl tyrosine (GelDATT), gelatin-tyramine (GelTyr), gelatin-caffeic acid (GelCA), Gelatin-dihydrocaffeic acid (GelDHC), gelatin-gallic acid, gelatin- tannic acid, gelatin-p-coumaric acid, gelatin-cinnamic acid, gelatin-sinapic acid, gelatin-rosmarinic acid, gelatin-dihydroxybenzoic acids, gelatin-ferulic acid, gelatin-dopamine, gelatin-norbornene, gelatin-alkyne, gelatin-maleimide, gelatin- tetrazine, gelatin-azide, gelatin-divinyl sulfone, gelatin-thiol (GelSH) and combinations thereof. In embodiments of the present invention, the composition comprising the oxidized oligosaccharide product is the hydrogel as described in WO 2020 / 249695 (which is incorporated herein in its entirety). In WO 2020 / 249695, a hydrogel for in-vivo release of medication is described that comprises oxidized β- cyclodextrin (oβ-CD) as oxidized oligosaccharide. Although WO 2020 / 249695 describes that the hydrogel comprises at least one medication and is cross-linked, it may be appreciated that the hydrogel in accordance with the present invention does not need to comprise a medication, nor does it require to be cross-linked. Thus, it may be the same type of hydrogel as described in WO 2020 / 249695, but it may also differ from this in that it does not comprise the medication and / or is not cross- linked. Hence, in a particular embodiment of the composition, which may preferably be the hydrogel, the composition comprises: (i) a protein-based biopolymer functionalized with a functionalization agent that can form guest-host interactions with oxidized β-cyclodextrin; and (ii) the purified oxidized oligosaccharide product. The functionalization agent that can form guest-host interactions with oxidized β-cyclodextrin may be based on a phenol moiety. As described in WO 2020 / 249695, the gelatin may for instance be functionalized at its amine-groups with tyramine, or at its carboxylic acid groups with hydroxyphenyl propionic acid such as 3-(4-hydroxyphenyl)-propionic acid. In a preferred embodiment, the gelatin is gelatin functionalized with 3-(4-hydroxyphenyl)-propionic acid (also abbreviated as gelDAT), and purified as for instance described in NL 2036417. A specific example of guest-host interactions with oxidized β-cyclodextrin and gelatin comprising a phenol moiety is also described in Thi et al., RSC Adv., 2017, 7, 34053–34062. The composition may be provided in a dry state or in a wet state. When the composition is in a dry state, it generally comprises the purified oxidized oligosaccharide product in an amount of up to 32 wt%, preferably up to 22 wt% in embodiments wherein the composition does not comprise a medicament, based on the total weight of the composition. In embodiments wherein the composition comprises a medicament, the composition in dry state typically comprises the purified oxidized oligosaccharide product in an amount of up to 15 wt%, such as in the range of 8 to 14 wt%, preferably 11-12 wt%. A dry state of the composition may for instance be a dried variant of the ring element as described in WO 2020 / 249695. As used herein, dry state refers to the present of water in an amount of less than 20 wt%, preferably less than 14 wt%, and typically about 12 wt% or less, based on the total weight of the composition. When the composition is provided in a wet state, it comprises water and has the physical appearance of a swollen hydrogel. A specific embodiment is for instance the swollen ring element as disclosed in WO 2020 / 249695. In such embodiments, the amount of the purified oxidized oligosaccharide product is generally below 10 wt%, preferably between 4-5 wt%, based on the total weight of the composition. As used herein, wet state refers to the present of water in an amount of more than 12 wt%, preferably more than 50 wt%, based on the total weight of the composition. The composition may further comprise one or more of medicaments, immunomodulators, cells (including immune cells), cell aggregates, spheroids, organoids, and the like, and combinations thereof. The present inventors found that the high purity of the purified oxidized oligosaccharide product is particularly favorable for hydrogels that are used for culturing of cells, cell aggregates, spheroids and / or organoids. These cultured cells, cell aggregates, spheroids and / or organoids can for instance be used in immunotherapies, and / or in in vitro applications such as culturing cells, drug screening, drug testing, drug development, disease models, and culturing and testing organ-on-chip. Cell aggregates refer to groups of cells that adhere to each other but do not necessarily form any specific or organized structure. These clusters can form in vivo or be created in vitro. They are typically used in research to study cell-cell interactions, behavior, and communication in a three-dimensional context. Cell aggregates can provide a more physiologically relevant environment compared to traditional two-dimensional cell cultures. The hydrogel according to the present invention can be applied for this. Spheroids are a type of cell aggregate that forms a spherical shape. These three-dimensional structures are typically created by culturing cells in non- adhesive conditions, which encourage the cells to cluster together and form a sphere. Spheroids can be used in cancer research, drug testing, and tissue engineering because they mimic the microenvironment of tissues more accurately than two-dimensional cell cultures. The spherical shape allows for gradients of nutrients, oxygen, and waste products, which can more closely resemble the conditions found in living tissues. Organoids are generally considered to be three-dimensional, miniaturized, and simplified versions of organs produced in vitro (in a laboratory setting) that replicate some of the organ's key structures and functions. Organoids are typically derived from stem cells and cultured to grow into small organ-like structures. Organoids can be generated from pluripotent stem cells or adult stem cells. While organoids do not fully replicate the complexity of whole organs, they are typically considered to form organized, three-dimensional structures that exhibit some of the same cellular diversity, architecture, and functions of the organs they model. The organoids that can be cultured and comprises in the hydrogel in accordance with the present invention include brain organoids, liver organoids, kidney organoids, and intestinal organoids. These organoids, and others, can be used to study the development and progression of diseases, such as cancer, by observing how the disease manifests and progresses in a controlled environment and are considered to provide a more accurate model than traditional cell cultures for testing the efficacy and toxicity of new drugs. Moreover, organoids are explored for their potential in transplant therapies to replace damaged tissues or organs. This is considered promising as organoids derived from a patient's own cells can be used to tailor treatments to the individual's specific genetic makeup and disease characteristics. Organoids are thus in vitro model systems that mimic the structural and functional characteristic of organs, with great potential for application in regenerative medicine, drug screening and personalized therapy. Organoids are usually cultured in complex animal-derived extracellular matrices (ECM), such as Matrigel (i.e. solubilized basement membrane matrix secreted by Engelbreth- Holm-Swarm (EHS) mouse sarcoma cells produced by Corning Life Sciences), see for instance Heo et al, International Journal of Stem Cells, 15 (2022) 1, 60–69. However, the diverse and undefined constituents, as well as the inherent heterogeneity of Matrigel, limit the reproducibility of organoid culture experiments across different batches. Therefore, a hydrogel matrix that can mimic the functional and mechanical characteristics of native ECM, such as cell-adhesion and viscoelasticity could address many of the challenges facing organoid cultures. Because the hydrogel in accordance with the present invention (e.g. based on gelDAT and oxidized beta-cyclodextrin), with or without photocrosslinking) has important features, like cell adhesion sequences and viscoelasticity, it forms an ideal matrix for the culture of organoids. In addition, the low contamination and low endotoxin content is favorable for this application. The hydrogel according to the present invention for culturing of cells, cell aggregates, spheroids and / or organoids can be considered and / or referred to as a dynamic extracellular matrix. Such a matrix is viscoelastic and is able to undergo changes in response to cellular activities or external stimuli. This can include changes in its physical properties (e.g., stiffness, degradability) or chemical composition (e.g., release of growth factors). The hydrogel as a dynamic extracellular matrix provides an environment that stimulates the growth of cells, cell aggregates, spheroids and / or organoids, i.a. by providing physical pressure. The hydrogel according to the present invention for culturing of cells, cell aggregates, spheroids and / or organoids preferably comprises the protein-based biopolymer in an amount of 2 to 15 wt%, more preferably 5 to 10 wt%, and the purified oxidized oligosaccharide product in an amount of 1 to 8 wt%, more preferably 2 to 6 wt%, based on the total weight of the hydrogel. The protein-based biopolymer may be selected from the group of biopolymers as described herein- above, but preferably it comprises a functionalized gelatin as described in WO 2020 / 249695, more preferably gelatin functionalized with 3-(4-hydroxyphenyl)- propionic acid. The hydrogel is preferably crosslinked by using a biocompatible photoinitiator such as riboflavin and sodium persulfate, as also described in WO 2020 / 249695, and irradiating the (still uncrosslinked) hydrogel for a irradiation time in the range of 2 to 15 minutes, preferably 3 to 10 minutes such as 3, 5 or 10 minutes, to achieve crosslinking and obtaining the crosslinked hydrogel. These parameters were found to lead to good features like viscoelastic properties and other mechanical properties for the culturing. The composition preferably comprises the purified oxidized oligosaccharide in an amount of 0.1 % to 10% by weight of the composition, preferably in the range of 2% to 6% by weight of the total composition. The medicament that can be included in the compositions of the present invention herein refers to substances or preparations used for the treatment, mitigation, or prevention of disease. Medicaments encompass a wide range of products, including pharmaceuticals, vaccines, biologics, drugs and the like. Examples of medicaments that can be included in the hydrogels of the present inventions include analgesics, antibiotics, antivirals, antifungals, antihypertensives, antidiabetics, anti-inflammatory agents such as corticosteroids, radiosensitizers and immune checkpoint inhibitors. Radiosensitizers are substances that make cancer cells more sensitive to radiation therapy. The primary goal of using radiosensitizers is to enhance the effectiveness of radiation therapy, allowing for lower doses of radiation to be used or increasing the efficacy at standard doses. This can help in minimizing damage to surrounding healthy tissues while maximizing the destruction of cancer cells. Examples of radiosensitizers include certain chemotherapy drugs (like 5- fluorouracil), hypoxic cell sensitizers (like nimorazole), and newer targeted agents like PARP inhibitors. Immune checkpoint inhibitors are a class of medicaments that help to activate the immune system to recognize and attack cancer cells. Typically, immune checkpoints are molecules on certain immune cells (like T cells) that need to be activated (or inactivated) to start an immune response. Cancer cells can exploit these checkpoints to avoid being attacked by the immune system. Checkpoint inhibitors work by blocking these checkpoint molecules, effectively taking the brakes off the immune system and allowing it to target and destroy cancer cells. Typical checkpoints targeted by these inhibitors are CTLA-4 (Cytotoxic T-Lymphocyte-Associated Protein 4, inhibited by for instance ipilimumab, PD-1 (Programmed Cell Death Protein 1) and PD-L1 (Programmed Death-Ligand 1), inhibited by for instance pembrolizumab, nivolumab atezolizumab and durvalumab. Checkpoint inhibitors are applicable in cancer treatment and have shown significant efficacy in various cancers such as melanoma, non-small cell lung cancer, renal cell carcinoma, and others. The hydrogel of the invention can be used in methods to treat these cancers. Antidiabetics that may be included in the composition, preferable the hydrogel, include glucagon-like peptide-1 (GLP-1) receptor agonists which are a class of medications that mimic the action of the naturally occurring hormone GLP- 1. These agonists bind to and activate the GLP-1 receptor, which is involved in the regulation of glucose metabolism. The activation of GLP-1 receptors has several beneficial effects in the management of type 2 diabetes and other metabolic disorders. In embodiments of the present invention, the composition comprising the oxidized oligosaccharide product is the hydrogel as described in WO 2021 / 250205 (which is incorporated herein in its entirety). In WO 2021 / 250205, an adhesive drug carrier comprising an injectable hydrogel is described, which hydrogel comprises a protein-based polymer functionalized with a functionalization agent that is able to form guest-host interactions with oxidized β-cyclodextrin, a protein-based polymer bearing quinone and / or catechol groups, and oxidized β- cyclodextrin (oβ-CD) as oxidized oligosaccharide. Although WO 2021 / 250205 describes that the hydrogel further comprises at least one medication, the presence of the medicament of the present invention is not required. Moreover, WO 2021 / 250205 describes that the hydrogel comprises a combination of two protein- based polymer, the hydrogel in accordance with the present invention may also only comprise the protein-based polymers bearing quinone and / or catechol groups, i.e. the (i) protein-based polymer functionalized with a functionalization agent that is able to form guest-host interactions with oxidized β-cyclodextrin as described in WO 2021 / 250205 (i.e. gelDAT) may be absent. Such hydrogels may for instance be used as bioadhesives and / or sealant in biomedical applications. Hence, in a particular embodiment of the composition, the composition comprises: (i) a protein-based biopolymer bearing quinone and / or catechol groups; and (ii) the purified oxidized oligosaccharide product. The protein-based biopolymer bearing quinone and / or catechol groups may for instance be made by modification of the protein-based biopolymer with 2- (3,4-dihydroxyphenyl)ethylamine hydrochloride (dopamine), 3,4-dihydroxy-L- phenylalanine (DOPA), 3-(3,4-dihydroxyphenyl)-2-propenoic acid (CA, caffeic acid), 3-(3,4-dihydroxyphenyl)propanoic acid (DHC, dihydrocaffeic acid), 3,4,5- trihydroxybenzoic acid (gallic acid), (R)-4-(1-hydroxy-2-(methylamino)ethyl)-1,2- benzenediol (epinephrine), or (R)-4-(2-amino-1-hydroxyethyl)-1,2-benzeendiol (norepinephrine). Specific example of suitable biopolymers in this respect are gelatin-caffeic acid (GelCA), gelatin-dihydrocaffeic acid (GelDHC), gelatin- rosmarinic acid and gelatin-dihydroxybenzoic acids (depending on the specific compound). In a preferred embodiment, the gelatin is gelatin functionalized with caffeic acid (GelCA), dihydrocaffeic acid (GelDHC), rosmarinic acid and / or dihydroxybenzoic acids, and purified as for instance described in NL 2036417. In embodiments of the present invention, the composition comprises the oxidized oligosaccharide product in combination with one or more protein-based polymers with catechol and quinone functionality wherein the polymer is substituted with at least two different functionalization agents, each having a 3,4- dihydroxyphenyl group and each having a different resistance to oxidation, wherein the at least 2 different functionalization agents are oxidized to a different degree as described in WO 2023 / 002017 (which is incorporated herein in its entirety). In WO 2023 / 002017 as composition that is applicable as a bio-adhesive is described. In embodiments of the present invention, the composition comprising the oxidized oligosaccharide product is the bio-adhesive as described in WO 2023 / 002017. The functionalization agents herein can for instance be selected from the group consisting of 2-(3,4-dihydroxyphenyl)ethylamine hydrochloride (dopamine), 3,4-dihydroxy-L-phenylalanine (L-DOPA), 3-(3,4- dihydroxyphenyl)propanoic acid (dihydrocaffeic acid, DHC), 3-(3,4- dihydroxyphenyl)propenoic acid (caffeic acid, CA), and 3-(3,4-dihydroxyphenyl)-2- hydroxyprop-2-enoic acid (hydroxycaffeic acid, HCA). In a preferred embodiment, the protein-based polymers are a combination of at least DHC and CA. The compositions comprising the oxidized purified oxidized oligosaccharide product, such as the hydrogels and bio-adhesives, as described herein can be used in medicals methods comprising administering the composition subcutaneous. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term "and / or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and / or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. EXAMPLES The invention can be illustrated by the following non-limiting examples. Example 1 – Synthesis and purification of oxidized β -cyclodextrin β-cyclodextrin (β-CD) (25.00 g) was suspended in water, followed by adding sodium periodate (NaIO4) in a [NaIO4]:[β-CD] in varying molar ratios, see Table 1. The reaction was then stirred at room temperature for at least 2 hours protected from light. Afterwards, the mixture was transferred to a beaker containing Amberlite™ FPA22 OH- resin (resin-OH) and Amberlite™ CG50 resin (resin-H+) in varying molar ratios, see Table 1, and stirred. After 30 minutes the suspension was filtered, and the filtrate was then lyophilized to obtain the product as a white powder. Finally, the lyophilized oβ-CD is assayed for pH, the residual iodate impurities (IO3-), for the degree of oxidation (DoO) using a DNPH (2,4- dinitrophenylhydrazine) assay. Table 1 shows the results. Table 1 [NaIO4]: [resin-OH]: [resin-H+]: NaI IO3- DoO (%; Example [β-CD] O4] [resin-OH] pH molar ratio molar ratio (%) mmol / g) molar ratio 21.17%; 3.97 1-1 3:1 1.8 1.2 0.25 5.75 mmol / g 1-2 2.5:1 1.8 1.1 0.71 17.72% 5.87 16.10%; 3.12 1-3 2.2:1 1.8 1.1 0.99 5.82 mmol / g 1-4 2:1 1.8 1.1 0.57 14.50 6.59 The amount of the residual iodate impurities (IO3-)in the purified oxidized oligosaccharide product was determined by reacting the residual iodate impurities (IO3-) with iodide at pH 3.3 and spectrophotometrically measuring the amount of resulting triiodide at 352 nm. The degree of oxidation was determined by reacting the oxidized oligosaccharide and 2,4-dinitrophenylhydrazine (DNPH), which results in the formation of an orange precipitate, and spectrophotometrically measuring the amount of unreacted DNPH remaining in the supernatant using at 357 nm. Based on the results listed in Table 1, the experimental conditions selected for the synthesis and purification are: i) Periodate equivalent to β-CD ([NaIO4]:[β-CD]): 2:1; ii) Equivalent of anion resin to periodate ([resin-OH]: NaIO4] molar ratio): 1.9:1; iii) Equivalent of cation resin to anion resin ([resin-H+]:[resin-OH] molar ratio): 1:1. Example 2 – Synthesis and purification of oxidized β -cyclodextrin including use of charcoal β-cyclodextrin (β-CD) (25.00 g) was suspended in water, followed by adding sodium periodate (NaIO4) in a [NaIO4]:[β-CD] in 2:1. The reaction was then stirred at room temperature for at least 2 hours protected from light. Afterwards, the mixture was transferred to a beaker containing Amberlite™ FPA22 OH- resin (resin-OH) and Amberlite™ CG50 resin (resin-H+). The molar ratio of anion resin to periodate ([resin-OH]: NaIO4] molar ratio) was 1.9:1, whilst the weight ratio of cation resin to anion resin ([resin-H+]:[resin-OH]): 1:1. Charcoal was optionally added to compare this to the absence of charcoal, see Table 2. After mixing with the resins and optional charcoal, the mixtures were stirred. After 30 minutes stirring, the suspension was filtered, and the filtrate was then lyophilized to obtain the product as a white powder. Finally, the lyophilized oβ-CD is assayed for pH, the residual iodate impurities (IO3-), for the degree of oxidation (DoO) using a DNPH (2,4-dinitrophenylhydrazine) assay. Table 2 shows the results. Table 2 Charcoal IO3- --IO3 Example IO3 (%) DoO (%; mmol / g) pH used (mg / mL) (mg / g) 2-1 No 0.048 0.48 4.64 15.73%; 2.89 mmol / g 6.44 2-2 No 0.054 0.54 5.45 15.57%; 2.88 mmol / g 6.48 2-3 No 0.056 0.57 5.72 16.20%; 3.00 mmol / g 6.51 2-4 No 0.050 0.50 4.96 16.18%; 2.99 mmol / g 2.22 2-5 Yes ≤0.016 ≤0.16 ≤ 1.6 15.52%; 2.84 mmol / g 6.38 2-6 Yes 0.022 0.22 2.22 16.03%; 3.06 mmol / g 6.23 2-7 No 0.17 1.72 17.19 17.14%; 3.17 mmol / g 7 Example 3 – 2.1. Anion exchange resin acetate (AER-H3C2O2-) vs Anion exchange resin bicarbonate (AER-HCO3-) in combination with cationic Amberlite IRC-120 H+Experiments have also been performed to determine the most effective ionic form of anion exchange resin. The commercially available Amberlite IRA-402 was converted from the chloride form to the acetate (AER-H3C2O2-) or carbonate (AER-HCO3-) form. During a typical experiment, oxidized β-CD (oβ-CD) after synthesis was mixed with AER-H3C2O2- or AER-HCO3-. After agitating the resulting suspension, the iodate content was quantified. The results are included in Table 3. Table 3 Sample IO3- (mg / mL) IO3- (%) IO3- (mg / g)Amberlite acetate (H3C2O2-) 1.59 15.93 159.33 Amberlite bicarbonate (HCO3- ) 0.18 1.77 17.66Table 3 shows the iodate content of oβ-CD after quenching and purification using the acetate and bicarbonate resin. After treating the reaction solution with the resins there was a sharp decrease in iodate content. The reaction solution treated with Amberlite acetate had a content of iodate of 1.59 mg / mL (159.33 mg / g on oBCD). After treating oβ-CD with Amberlite bicarbonate the residual amount of iodate was 0.18 mg / mL (17.66 mg / g of oBCD). From this it can be concluded that stirring the reaction mixture with Amberlite resins is an effective manner to remove iodate from the mixture, and the amberlite bicarbonate is more efficient than the acetate resin. As the use of Amberlite acetate or Amberlite bicarbonate leads to the exchange of iodate with acetate or bicarbonate in the reaction solution, after drying either sodium acetate or sodium bicarbonate are left in the product. To prevent this and a high pH during drying, Amberlite IRC-120 H+was used in tandem with an anion exchange resin. This resin exchanges Na+ions (from NaIO4) in the solution for H+ions. This leads to the formation of either acetic acid or carbonic acid in the solution. As carbonic acid dissociates to CO2 and water, and acetic acid is volatile, it was thought that both carbonate or acetate could be removed from the reaction mixture. However, after NMR analysis the presence of acetic acid was observed in the NMR spectra of the reaction quenched with Amberlite acetate. This means that spray-drying does not remove all acetic acid from the product. The presence of bicarbonate (or carbonic acid) could not be determined from the NMR spectra of the reaction purified with Amberlite bicarbonate; however, the purified oβ-CD after dissolution in water appeared turbid. As OH-form anion resin thereof is preferred. Example 4 – Removal of different oxoanions using an anion exchange resin and a cation exchange resin Experiments have also been performed to confirm effective removal of hypochlorite (-OCl) and permanganate (MnO4-) oxoanions with a combination of an anion exchange resin (OH-) and a cation exchange resin (H+). A solution of sodium hypochlorite (NaOCl) and a solution of potassium permanganate (KMnO4) in water were prepared at 2% (w / v). Each solution was mixed with Amberlite™ FPA22 OH- resin (resin-OH) and Amberlite™ CG50 resin (resin-H+) at a ratio [resin-OH]:[oxoanion] of 1.9:1 and a ratio [resin-H+]:[resin-OH] of 1.6:1. At fixed time points aliquots from the solutions were collected and the absorbance measured with a plate reader. The absorbance of NaOCl aliquots was measured at 292 nm, while the absorbance of KMnO4 aliquots was measured at 530 nm. After 15 min of contact with the resins, NaOCl concentration decreased from 2% (w / v) to 0.036% (w / v) and further to 0.007% (w / v) after 120 min. The concentration of KMnO4 in solution dropped from 2% (w / v) to 0.004% (w / v) in 15 min, and to 0.0018% (w / v) after 180 min. Comparative example 1 – dialysis β-cyclodextrin (25.00 g) was suspended in 400 mL demineralized water followed by addition of sodium periodate (NaIO4) in a [NaIO4]:[β-CD] 3:1 molar ratio. The reaction was stirred for 2h protected from light followed by quenching with glycerol. The oxidized β-cyclodextrin was then dialyzed against water using a dialysis membrane with an MWCO of 500 Da (Spectrum Labs) for two days at room temperature to purify the product. Oxidized β-cyclodextrin powder was obtained by freeze-drying. Residual impurities and degree of oxidation are reported in Table 4. Comparative example 2 – quenching and purification using one resin β-cyclodextrin (25.00 g) was suspended in 400 mL demineralized water followed by addition of sodium periodate (NaIO4) in a [NaIO4]:[β-CD] 3:1 molar ratio. The reaction was stirred for 2h protected from light. The oxidized β-cyclodextrin was then purified from excess periodate using an anion exchange resin (FPA22) in the acetate form, as described in the prior art from Domb AJ et al., Journal of Polymer Science Part A: Polymer Chemistry. 1996, May;34(7):1229-36. The oxidized β-cyclodextrin powder was obtained by freeze-drying. The residual impurities and DoO are reported in Table 4. Table 4 Comparative Purification IO3- (%; mg / mL) DoO* (%) Example method 1 Dialysis 24.73; 0.25 mg / mL 19.61 Anion resin (FPA 2 10.10%; 0.10 mg / mL 17.90 22 in acetate form) *The difference in DoO is due to the interference of residual (per)iodate with the DNPH assay used for the determination of DoO. By comparing Example 1 and 2 with Comparative Examples 1 and 2, the following conclusions can be drawn. Invention compared to dialysis: - the purification method disclosed here is much quicker and does not require large volumes of water; - the periodate oxidation does not require a separate quenching step with glycerol and therefore does not add any additional contaminants; - higher yield as cyclodextrin is a small molecule that can easily pass through dialysis membranes; - potential bacterial contamination due to the long exposure to water (at least 2-3 days). Compared to single ion exchange resin technology: - one step, - less time consuming, and - neutral pH avoids any potential degradation of the periodate oxidized product. - the product contains less residual iodate and this ensure stability of the oxidized compound during storage and at high temperature. Comparative example 3 – quenching and purification using one resin Cl-form anion resin and one OH-form resin Amberlite IRA-402 has a low affinity for the hydroxide ion, meaning that this resin has a higher affinity for the iodate ions. Therefore, the commercially available Amberlite IRA-402 was converted from the chloride form to the hydroxide form (A-OH). The effectivity of both resins was assessed. During a typical experiment, oxidized β-CD (oβ-CD) after synthesis was mixed with A-Cl. The same was done with A-OH. After agitating the resulting suspension, the iodate content was quantified. The hydroxide form A-OH showed a fast decrease of periodate, and was more effective than A-Cl. However, as iodate is exchanged for hydroxide the pH of the oxidized β-CD solution increases. During the purification the final pH value was 12. Additional experiments were performed to determine if oβ-CD is stable at high pH values. These showed that prolonged exposure to a high pH environment had a negative effect on the stability of oβ-CD.

Claims

Claims 1. A method for preparing a purified oxidized oligosaccharide product comprising the following steps: i) contacting an oligosaccharide and an oxidizing agent comprising an oxoanion such as a perhalogenate, in a solvent, to obtain a crude oxidized oligosaccharide product comprising oxidized oligosaccharide and contaminants; ii) contacting the crude oxidized oligosaccharide product and a contaminant-lean ion exchange resin composition comprising a combination of an anion exchange resin and a cation exchange resin to obtain a contaminant-rich ion exchange resin composition; iii) separating the oxidized oligosaccharide from the contaminant-rich ion exchange resin composition to obtain a purified oxidized oligosaccharide product.

2. The method according to claim 1, wherein the solvent comprises water.

3. The method according to any of the previous claims, wherein the oligosaccharide in step i) is dissolved in the solvent in a concentration of 0.5-40, preferably 2-10, more preferably about 5 wt%, based on the weight of the solvent.

4. The method according to any of the previous claims, wherein the oxoanion is selected from the group consisting of fluorine-based salt including hypofluorite (FO−), chlorine-based salts including hypochlorite (ClO−), chlorite (ClO2−), chlorate (ClO3−), perchlorate (ClO4−), bromine-based salts including bromate (BrO3−), perbromate (BrO4−), iodine-based salts including hypoiodite (IO−), iodite (IO2−), iodate (IO3−), periodates such as metaperiodate (IO4−), mesoperiodate (IO53−) and orthoperiodate (IO65−), astatine-based salts including astatate (AtO3−), perastatate (AtO4−), manganate-based salts such as permanganate (MnO4-) and combinations thereof, preferably from the group consisting of perchlorate (ClO4−), perbromate (BrO4−), periodates such as metaperiodate (IO4−), mesoperiodate (IO53−) and orthoperiodate (IO65−), and combinations thereof, more preferably wherein theoxoanion comprises a periodate, even more preferably a metaperiodate (IO4–) most preferably sodium metaperiodate (NaIO4).

5. The method according to any of the previous claims, wherein the molar ratio of oxoanion to the oligosaccharide is in the range of 0.5:1 to 8:1, preferably in the range of 1:1 to 3:

1.

6. The method according to any of the previous claims, wherein the anion exchange resin is in OH- form.

7. The method according to any of the previous claims, wherein the molar ratio of exchangeable anions in the anion exchange resin used in step ii) to the oxoanion used in step i), is in the range of 1:1 to 3:1, preferably in the range of 1.25:1 to 2:1, more preferably about 1.9:

1.

8. The method according to any of the previous claims, wherein step iii) comprises bringing the pH of the purified oxidized oligosaccharide in the range of 5 to 8.5, preferably in the range of 5.5 to 7.5, more preferably about 6 by addition and separation of an additional amount of cation exchange resin.

9. The method according to the previous claim, wherein the cation exchange resin in step ii) and step iii) are of the same type, and wherein preferably the cation exchange resin in in H+form.

10. The method according to any of the previous claims, wherein the cation exchange resin and the anion exchange resin are used in step ii) in a weight ratio in the range of 6:1 to 1:6, preferably in the range of 2:1 to 1:2, more preferably in a 1:1 weight ratio.

11. The method according to any of the previous claims, wherein the oligosaccharide comprises an oligosaccharide containing 3-30, preferably 3-20 monosaccharide units, more preferably having 1,4- and / or 1,6 glycosidic linkages, still more preferably selected from oligosaccharides derived from chondroitinsulphate, alginate, starch, pectin, dextran, hyaluronic acid, still more preferably a cyclodextrin, most preferably wherein the oligosaccharide comprises β-cyclodextrin.

12. The method according to any of the previous claims, further comprising, following step iii), a step iv) of removing the solvent, which preferably comprises spray drying or lyophilization, preferably by lyophilization.

13. A purified oxidized oligosaccharide product that is obtainable in accordance with any of the previous claims, said product comprising less than 5 wt%, preferably less than 2 wt% oxoanions, preferably wherein said product comprising less than 5 wt%, preferably less than 2 wt% perhalogenate, based on the total weight of the purified oxidized oligosaccharide product.

14. The purified oxidized oligosaccharide product according to claim 13, wherein the product comprises 0.05 wt% of one or more oxidation quenching agents, preferably less than 0.05 wt% of glycerol and / or less than 0.05 wt% of ethylene glycol, based on the total weight of the product.

15. The purified oxidized oligosaccharide product according to any of claims 13-14, wherein the product exhibits an endotoxin content of less than 200 endotoxin units (EU) / g, preferably less than 100 EU / g, more preferable less than 10 EU / g, as determined by the LAL-test.

16. The purified oxidized oligosaccharide product according to any of claims 13-15, wherein the oxidized oligosaccharide exhibits a degree of oxidation in the range of 12 to 25%, preferably in the range of 15-22%, based on the hydroxyl group content of the oligosaccharide.

17. A composition comprising a protein-based biopolymer and the purified oxidized oligosaccharide product according to any of claims 13-16.

18. The composition according to claim 17, wherein the protein-based biopolymer comprises gelatine, preferably modified gelatin, more preferably modified gelatin selected from the group consisting of gelatin-methacryloyl(GelMA), gelatin-desaminotyrosine (GelDAT), gelatin-desaminotyrosyl tyrosine (GelDATT), gelatin-tyramine (GelTyr), gelatin-caffeic acid (GelCA), Gelatin- dihydrocaffeic acid (GelDHC), gelatin-gallic acid, gelatin-tannic acid, gelatin-p- coumaric acid, gelatin-cinnamic acid, gelatin-sinapic acid, gelatin-rosmarinic acid, gelatin-dihydroxybenzoic acids, gelatin-ferulic acid, gelatin-dopamine, gelatin- norbornene, gelatin-alkyne, gelatin-maleimide, gelatin-tetrazine, gelatin-azide, gelatin-divinyl sulfone, gelatin-thiol (GelSH) and combinations thereof.

19. The composition according to any of claims 17-18, wherein the hydrogel is cross-linked, preferably photocrosslinked.

20. The composition according to any of claims 17-19, further comprising a medicament, immunomodulators, cells (e.g. immune cells), cell aggregates, spheroids, organoids, or a combination thereof.

21. The composition according to any of claims 17-20, wherein the protein- based biopolymer comprises gelatin functionalized with 3-(4-hydroxyphenyl)- propionic acid, wherein the purified oxidized oligosaccharide product comprises β- cyclodextrin, and wherein the composition is preferably not photocrosslinked.

22. The composition according to any of claims 17-21, for use in the culturing of cells, cell aggregates, spheroids and / or organoids, comprising the protein-based biopolymer in an amount of 2 to 15 wt%, preferably 5 to 10 wt%, and the purified oxidized oligosaccharide product in an amount of 1 to 8 wt%, preferably 2 to 6 wt%, based on the total weight of the hydrogel, and wherein the composition is preferably a photocrosslinked hydrogel.

23. The composition according to any of claims 17-22, wherein the composition is in a dry state and comprises a medicament, and wherein the composition comprises the purified oxidized oligosaccharide product in an amount of up to 32 wt%, preferably up to 22 wt%, based on the total weight of the composition.

24. The composition according to any of claims 17-22, wherein the composition is in a wet state and comprises a medicament, and wherein the composition comprises the purified oxidized oligosaccharide product in an amount of up to 15 wt%, preferably in the range of 8 to 14 wt%, more preferably 11-12 wt%, based on the total weight of the composition.

25. The composition according to any of claims 17-20, wherein the protein- based biopolymer comprises gelatin functionalized with caffeic acid, dihydrocaffeic acid, rosmarinic acid and / or dihydroxybenzoic acids, preferably with caffeic acid, and wherein the purified oxidized oligosaccharide product comprises β-cyclodextrin.

26. The composition according to any of claims 17-20 and 25, wherein the protein-based biopolymer comprises gelatin that is functionalized with at least 2 different functionalization agents each having a 3,4-dihydroxyphenyl group and each having a different resistance to oxidation and wherein the at least 2 different functionalization agents are oxidized to a different degree, and wherein the purified oxidized oligosaccharide product comprises β-cyclodextrin; preferably wherein the protein-based biopolymer comprises gelatin that is functionalized with dihydrocaffeic acid and caffeic acid.