INTRODUCTION
Increasing age of population is a great success of numerous breakthroughs in life science and improved health care. For a child born in 2015, for example, an average global life expectancy of meanwhile 71.4 years is assumed which increased by around 8% in the last decade [1]. In accordance with enhanced life expectancy, however, age-related health problems continuously rise. In this regard, the gap between patients awaiting transplantation and appropriate donors consequently will get larger in the future [2]. To this end, there is a need for new strategies in regenerative medicine [3]. Biomaterial matrices were developed to foster tissue regeneration by mimicking the key characteristics of the extracellular matrix (ECM) [4]. Modern biomaterial research focuses on 3D scaffolds, which can be adequately adapted toward specific requirements of the target tissue [5]. In this regard, flexible material platforms are wanted, whose properties can be adjusted over a wide range and independently of each other [6].
In this context, the macromer-based material concept is promising due to the high flexibility of macromers in chemical design and processability [7]. Macromers are reactive oligo- or polymeric molecules which act as monomers and can therefore be polymerized/cross-linked into a polymeric network [8]. The key principle of this approach is the synthesis of chemically well-defined structures which allows for a more precise control over the resulting properties of the cross-linked polymeric network when compared to conventional polymers. For example, macromer chemistry can be adjusted in terms of chemical macromer composition, valence, content of cross-linkable functionalities and molecular weight. The versatility of macromer-derived materials greatly increases when different macromer types are combined which potentially enables precise material tunability on multiple levels. The design flexibility of macromer-based networks motivated the investigation of two different macromer-based material concepts with regard to macromer processability and material adjustability.
The following objectives were proposed:
1) To synthesize two sets of biodegradable, multi-valent macromers by using free-radical polymerization and ring-opening polymerization combined with established activation strategies. The synthesis setups will be tuned toward high macromer yields which will be required for processing into biomaterials with relevant sizes.
2) To physico-chemically characterize oligomeric macromers with regard to chemical composition, molecular weight and reactivity in order to yield well-defined macromer structures. NMR spectroscopy, gel permeation chromatography (GPC) and wet chemistry will be applied.
3) To characterize macromer processability into covalently cross-linked hybrid matrices. This work will focus on a soft macromer-cross-linked gelatin-derived hydrogel system for versatile biomedical applications as well as a rigid macromer/sol-gel glass hybrid material for hard tissue regeneration. Sets of different formulations will be investigated in order to characterize the range of macromer processability and to establish structure-property relationships.
4) To investigate strategies for the adjustment of material porosity. Besides the adaption via cross-linking density, porogen-leaching and 3D-printing approaches will be followed in order to introduce macroporosity and to enable a decoupling of porosity and chemical (nano)structure of the cross-linked network.
5) To determine key material properties relevant for regenerative applications, including mechanical properties by compression tests and oscillation rheology, in vitro matrix degradability, as well as material cytocompatibility in indirect and direct contact experiments.
6) To identify strategies for covalent functionalization of the hybrid materials. Post-fabrication functionalization via specifically introduced chemical functionalities is favored as it enables effective material decoration (almost) independent of the physico-chemical matrix properties.
SUMMARY OF DISSERTATION
The first material concept was based on anhydride-containing macromers which can be processed into hydrogel matrices by covalent cross-linking of amine-bearing macromolecules, such as gelatin [9–11]. The innovative aspect of this work was to decouple material functionalization from the physico-chemical properties of the cross-linked hydrogel network. To this end, a second chemical functionality was introduced which remained reactive in the hydrogel state and was therefore available for covalent post-fabrication functionalization strategies. Specifically, dual-functional macromers were synthesized by free-radical polymerization of maleic anhydride (MA) with diacetone acrylamide (DAAm) and pentaerythritol diacrylate monostearate (PEDAS) to yield oligo(PEDAS-co-DAAm-co-MA) (oPDMA) [12]. Amphiphilic oligomers (molecular weight (Mn) < 7.5 kDa) with anhydride contents of 7-20% were obtained. Fractions of chemically intact anhydrides of around 70% enables effective cross-linking with low molecular-weight gelatinous peptides (Collagel® type B, 11 kDa). Rigid two-component hydrogels (elastic modulus (E) = 4-13 kPa) with adjustable composition and physicochemical properties were formed. Reactivity of the incorporated methyl ketone functionality toward hydrazides and hydrazines was shown on the macromer level and in the cross-linked hydrogel by different strategies. Firstly, pre-fabricated hydrogels were successfully reinforced by secondary cross-linking with adipic acid dihydrazide (ADH). Secondly, pH-dependent immobilization of 2,4-dinitrophenylhydrazine (DNPH) to acid-soluble macromer derivatives as well as cross-linked oPDMA/COL matrices was demonstrated. Thirdly, reversible immobilization of a fluorescent hydrazide (AFH) was shown which was controlled by hydrogel ketone content, hydrazide ligand concentration and medium pH. This triple-tunability of hydrazide immobilization holds promise for adjustable and cost-effective hydrogel modification. Lastly, proof-of-concept experiments with hydrazido-functionalized hyaluronan (ATTO-hyHA) demonstrated the potential for covalent post-fabrication hydrogel decoration with ECM components. Hydrogel cytocompatibility was demonstrated and the introduction of DAAm into the hydrogel system resulted in superior cell material interactions when compared with previously established analogous ketone-free gels [13].
Limited ability of cells to migrate into deeper regions of these macromer-cross-linked gelatin-based gels further motivated the investigation of two different strategies to enhance hydrogel porosity [10,14]. On the one hand, the introduction of macropores was attempted by hydrogel fabrication in presence of poly(ethylene glycol) (Mn = 8000 Da, P8k). This polymer acted as porogen by phase separation during hydrogel formation. It was found that P8k was effectively extracted from the cross-linked matrix, while physico-chemical hydrogel properties remained unchanged. The second approach aimed at increasing mesh size of the cross-linked network by using hydrogel building blocks with increased molecular weights. In particular, high molecular-weight gelatin (160 Bloom, G160) was cross-linked by macromers with low MA content. Homogeneous and mechanically stable hydrogels were obtained and physico-chemical properties were determined. Successful optimization of hydrogel porosity was functionally shown by enhanced cell migration and improved release profile of incorporated nanoparticles [15].
In the second macromer-based material, hydrolytically degradable multi-armed macromers were covalently introduced into a tetraethoxysilane(TEOS)-derived silica sol in order to address the insufficient degradability of glass-based materials [16]. In detail, oligo(D,L-lactide) units were introduced into three- (TMPEO, Tx) and four-armed (PETEO, Px) ethoxylated alcohols by ring-opening polymerization, followed by activation with 3-isocyanatopropyltriethoxysilane (ICPTES) to yield TxLAy-Si and PxLAy-Si macromers [17,18]. A series of 18 oligomers (Mn: 1100-3200 Da) with different degrees of ethoxylation and varying length of oligoester units was synthesized. Applicability of a previously established indirect rapid prototyping method enabled fabrication of macromer/sol-gel-glass-derived class II hybrid scaffolds with controlled porosity [19]. Successful processability of a total of 85 different hybrid scaffold formulations allowed for identification of relevant structure-property relationships. In vitro degradation was analyzed over 12 months and a continuous linear weight loss (0.2-0.5 wt%/d) was detected which was controlled by oligo(lactide) content and matrix hydrophilicity. Compressive strength (2-30 MPa) and compressive modulus (44-716 MPa) were determined and total content, oligo(ethylene oxide) content, oligo(lactide) content and molecular weight of the oligomeric cross-linkers as well as material porosity were identified as the main factors determining hybrid mechanics by multiple linear regression. Cell migration into the entire scaffold pore network was indicated in cell culture experiments with human adipose tissue-derived stem cells (hASC) and continuous proliferation over 14 days was found.
Overall, two macromer-based material platforms were established in which material versatility was realized by three main principles: I) synthesis of macromers with different chemical composition, II) combination of macromers with a second oligomeric building block, and III) flexible processability of these dual-component hybrid formulations into porous scaffold materials. Precise adjustability of material properties as demonstrated in both concepts offers potential for application of these hybrid materials for a wide range of regenerative purposes.
REFERENCES
(1) World Health Statistics of the WHO. http://www.who.int/gho/publications/world_health_statistics/en/ 2017.
(2) OPTN/UNOS Public Comment. https://optn.transplant.hrsa.gov/ 2017.
(3) Puppi, D.; Chiellini, F.; Piras, a. M. M.; Chiellini, E. Prog. Polym. Sci. 2010, 35 (4), 403–440.
(4) Patterson, J.; Martino, M. M.; Hubbell, J. A. Mater. Today 2010, 13 (1–2), 14–22.
(5) Picke, A.-K.; Salbach-Hirsch, J.; Hintze, V.; Rother, S.; Rauner, M.; Kascholke, C.; Möller, S.; Bernhardt, R.; Rammelt, S.; Pisabarro, M. T.; Ruiz-Gómez, G.; Schnabelrauch, M.; Schulz-Siegmund, M.; Hacker, M. C.; Scharnweber, D.; Hofbauer, C.; Hofbauer, L. C. Biomaterials 2016, 96, 11–23.
(6) Loth, R.; Loth, T.; Schwabe, K.; Bernhardt, R.; Schulz-Siegmund, M.; Hacker, M. C. Acta Biomater. 2015, 26, 82–96.
(7) DeForest, C. A.; Anseth, K. S. Nat. Chem. 2011, 3 (12), 925–931.
(8) Nic, M.; Jirát, J.; Košata, B.; Jenkins, A.; McNaught, A.; Wilkinson, A. IUPAC, Research Triangle Park, NC 2014.
(9) Loth, T.; Hennig, R.; Kascholke, C.; Hötzel, R.; Hacker, M. C. React. Funct. Polym. 2013, 73 (11), 1480–1492.
(10) Loth, T.; Hötzel, R.; Kascholke, C.; Anderegg, U.; Schulz-Siegmund, M.; Hacker, M. C. Biomacromolecules 2014, 15 (6), 2104–2118.
(11) Kohn, C.; Klemens, J. M.; Kascholke, C.; Murthy, N. S.; Kohn, J.; Brandenburger, M.; Hacker, M. C. Biomater. Sci. 2016, 4, 1605–1621.
(12) Kascholke, C.; Loth, T.; Kohn-Polster, C.; Möller, S.; Bellstedt, P.; Schulz-Siegmund, M.; Schnabelrauch, M.; Hacker, M. C. Biomacromolecules 2017, 18 (3), 683–694.
(13) Sülflow, K.; Schneider, M.; Loth, T.; Kascholke, C.; Schulz-Siegmund, M.; Hacker, M. C.; Simon, J.-C.; Savkovic, V. J. Biomed. Mater. Res. A 2016, 104 (12), 3115–3126.
(14) Loth, T. Diss. Univ. Leipzig, Fak. für Biowissenschaften, Pharm. und Psychol. 2016.
(15) Schwabe, K.; Ewe, A.; Kohn, C.; Loth, T.; Aigner, A.; Hacker, M. C.; Schulz-Siegmund, M. Int. J. Pharm. 2017, 526 (1–2), 178–187.
(16) Rahaman, M. N.; Day, D. E.; Sonny Bal, B.; Fu, Q.; Jung, S. B.; Bonewald, L. F.; Tomsia, A. P. Acta Biomater. 2011, 7 (6), 2355–2373.
(17) Schulze, P.; Flath, T.; Dörfler, H.-M.; Schulz-Siegmund, M.; Hacker, M.; Hendrikx, S.; Kascholke, C.; Gressenbuch, M.; Schumann, D. Ger. Pat. No. DE102014224654A1 2016.
(18) Kascholke, C.; Hendrikx, S.; Flath, T.; Kuzmenka, D.; Dörfler, H.-M.; Schumann, D.; Gressenbuch, M.; Schulze, F. P.; Schulz-Siegmund, M.; Hacker, M. C. Acta Biomater. 2017, 63, 336–349.
(19) Hendrikx, S.; Kascholke, C.; Flath, T.; Schumann, D.; Gressenbuch, M.; Schulze, P.; Hacker, M. C.; Schulz-Siegmund, M. Acta Biomater. 2016, 35, 318–329.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:21530 |
| Date | 20 June 2018 |
| Creators | Kascholke, Christian |
| Contributors | Hacker, Michael C., Schulz-Siegmund, Michaela, Mäder, Karsten, Universität Leipzig |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/acceptedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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