The synthesis of hydrogels still uses frequently starting materials based on petroleum. Since it is a non-renewable resource, the deposits will be exhausted sooner or later, besides the exploitation of fossil oil is environmentally harmful and contributes to climate change. For these reasons, alternative materials for hydrogels, which should be renewable and sustainable, have to be investigated. The focus of this thesis was to synthesize \textit{in situ} gelling hydrogels based on polysaccharides and to characterize their properties.
Cellulose and chitosan were chosen as starting materials, because they belong to the most abundant biopolymers on earth and therefore are readily available and also renewable. Additionally, cellulose sulfate (CS) is capable of attracting growth factors and chitosan has antimicrobial properties, all characteristics that are desirable for hydrogels in tissue engineering applications.
First of all, the sulfation of the cellulose was performed. Celluloses of two origins, sulfite pulp and cotton linters, were used to investigate the influence of the DP towards the DS_sulf. By applying direct sulfation and acetosulfation as sulfation methods a broader range of DS_sulf from 0.8 to 2.0 has been achieved. Values below 1.0 were obtained by acetosulfation for both celulloses without a remarkable influence of the DP. In direct sulfation, DS_sulf values of 1.1 to 2.0 could be achieved by adjusting the ratio of chlorosulfuric acid and anhydroglucose unit of cellulose between 3.0:1; 4.5:1 and 6.0:1. Starting from a ratio of 4.5:1 the effect of the DP is obvious, the longer chains of the cotton linters reach lower DS_sulf values compared to the sulfite pulp. Due to the longer cellulose chains the accessibility of the sulfating agent is limited, this is in particular noticeable at higher DS_sulf once the outer and easier accessible positions are sulfated.
The determination of the sulfur content, to quantify the DS_sulf, in CS became a key challenge in this thesis. At the beginning the sulfur content was determined by elemental analysis, because it is the common method in the literature. It is fast, does not require any pretreatment and only a low amount of sample is needed. However, it was found that the carbon and sulfur content in a sample did not interact logically. Theoretically, the relative carbon content should decrease and the relative sulfur content should rise with increasing DS_sulf, because pure cellulose does not contain sulfur. In practice, the percentage for carbon and sulfur increased both or the decrease of the carbon content was lower than expected for the increase in sulfur, so that the resulting DS_sulf was not the same when calculated with sulfur or carbon content. Hence additional direct methods of measuring the DS_sulf, which do not need a calibration sample, had to be investigated to validate the DS_sulf. ICP-OES and precipitation as BaSO4 were chosen as further methods and showed consistent results, but differed considerably from the findings of the elemental analysis. In contrast to elemental analysis, the other methods involve the digestion of the sample. So it could be possible that by using elemental analysis the reaction is incomplete and therefore the result is non-reproducible, however, it is not very likely. Although elemental analysis is faster and straightforward, it is recommended to use ICP-OES or precipitation as BaSO4 when determining the DS_sulf to receive reliable results.
The introduction of aldehyde groups in CS was necessary to provide reactive groups for the later hydrogel formation. In cellulose chemistry the widely known Malaprade reaction, an oxidation using sodium periodate, was performed. This oxidation stops after forming the aldehyde and does not further oxidize through to the carboxylic acid, additionally the reaction is possible in water which is essential for CS, since it is only soluble in water and it was aimed for a homogeneous reaction. A requirement for the oxidation is the presence of vicinal diols. The carbon-carbon bond is cleaved and an aldehyde is formed at each hydroxyl group. The maximum DS_ald of 2, which is possible for pure cellulose, cannot be reached, because the prior introduced sulfate groups reduce the number of vicinal OH groups. Although acetosulfation only took place at C6-position and all vicinal diols were still available, the DS_ald reached not more than 0.35. It is possible that steric hindrance through the sulfate group is the reason for these low values. Overall the DS_ald of the oxidized cellulose sulfate (oCS) ranged from 0.09 to 0.35; with increasing DS_sulf the DS_ald decreased.
The oCS are intended to be used in medical applications, accordingly their toxicity had to be investigated. Indeed, oCS are just one component for the hydrogel synthesis, but in case of an incomplete gelation the single components have to be non-toxic as well. In general, all oCS were non-toxic at low concentrations (0.5 mg/ml) yet an increasing concentration or a DS_ald of 0.3 and higher resulted in toxic effects. Aside from that, a coherence between M_w and toxicity was ascertained. The toxicity increased when the M_w of the oCS was 70 kDa or less.
The second component which is required for the hydrogel formation is a chitosan. The amino group of the chitosan can react with the aldehyde of the oCS by forming an imine. That way both biopolymers are crosslinked and result in a hydrogel. The used chitosan needs to be soluble under physiological conditions, consequently pure chitosan is not suitable since it is only water-soluble under acidic conditions. Hence, three chitosan derivates -- chitosan acetate, chitosan lactate and carboxymethyl chitosan (CMCh) -- were chosen which fulfill the criterion of solubility. To examine their aptitude for hydrogel formation 10 mg of each chitosan derivative and 10 mg of oCS were solved separately in 0,5 ml phosphat buffered saline. Afterwards, a chitosan derivative and oCS were mixed together while stirring until a reaction took place. In case of chitosan acetate and chitosan lactate a white sediment was the result, whereas the use of CMCh led to a colorless hydrogel, on this account all further studies were performed with CMCh.
To establish a basis for a targeted hydrogel synthesis, the storage modulus G' was investigated regarding selected parameters of the hydrogel: DS_ald, M_w, mixing ratio and time for gelation. The cross-linking was conducted with four mixing ratios of oCS:CMCh (1:1; 1:3; 1:5; 1:10) and correlated to the amount of substance of the aldehyde of the oCS and the corresponding amount of required CMCh. Since the CMCh solution has a high viscosity, the range of mixing ratios was limited. The total amount of substance was the same for all gels to ensure the comparability of the different hydrogels and the mixed volumes had always a 1:1 ratio to guarantee a fine blending of the components. Like presumed, G' increased with increasing DS_ald. The DS_ald is the specifying magnitude for the cross-linkage of the hydrogel, because the frequency of the aldehyde group is much lower compared to the frequency of the amine group of CMCh. Aldehyde and amine interact by forming an imine bond, the more of these bonds are formed the stiffer the resulting hydrogel becomes and as a result of that, G' increases as well. Furthermore, the storage modulus rises with rising M_w, the reason for it is that with increasing chain length the possibility of polymer entanglement increases. This physical type of cross-linkage makes for a stiffer gel too and therewith a higher G' is the consequence. With respect to the mixing ratio, maximum values for G' are attained if the ratio is shifted towards the component which defines the number of cross-linkages. During the study of the time for gelation it appeared that the time is not an independent parameter, in fact the time depends on the DS_ald. Samples with the shortest time for gelation showed the highest values for G', additionally they had the highest value for DS_ald. Thus, the time for gelation can not be considered when adjusting G'.
Further research on the hydrogels presented in this work needs to focus on investigating the toxicity and long-term behavior of the hydrogel, like stability and degradation, as well as its impact on tissue regeneration. The bioactivity and harmlessness of the hydrogel need to be ensured before it can be utilized in tissue engineering for example as cartilage tissue.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:88682 |
| Date | 18 December 2023 |
| Creators | Strätz, Juliane |
| Contributors | Fischer, Steffen, Beyer, Mario, Zhang, Kai, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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