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Title: Synthesis and characterisation of thermo-responsive microcarriers based on poly(N-isopropylacrylamide)
Author: Zhang, Jinnan
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2013
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Cell cultivation on the surface of microcarriers in suspension bioreactors is widely used for large scale expansion of anchorage-dependent cells. Proteolytic enzyme treatment is most commonly used to detach cells from colonized microcarriers for cell harvest or scale-up. One advantage of thermo-responsive microcarrier is that it may facilitate non-enzymatic harvest of cultured cells. In this thesis, the synthesis and characterisation of thermo-responsive microcarriers grafted with poly(N-isopropylacrylamide) (PNIPAAm) is discussed, potentially allowing cell-harvesting by temperature alternation, and without enzyme treatment. To prepare the PNIPAAm-grafted microcarriers, three grafting techniques were employed, namely grafting-to, grafting-from and seeded polymerization. In each technique, the methods for regulating PNIPAAm grafting parameters, including grafting density, grafted amount, molecular weight, and molecular architecture were investigated, as the grafting parameters may affect the efficiency of cell attachment, proliferation and detachment. In the grafting-to technique, chain transfer polymerization was employed to synthesise and control the molecular weight of end-functionalized PNIPAAm polymers. The grafting density of PNIPAAm was regulated by changing the feed concentration of the polymers. However, the grafting density (or amount) was limited by steric hindrance and reached a plateau at around 27 μmol/g (300 mg/g). Apart from linear PNIPAAm molecular architecture, the methods for preparation of looped and double-layer looped PNIPAAm chain were also investigated. In the grafting-from technique, an azo-initiator, 4,4'-azobis(4-cyanovaleric acid) (ACVA), and an atom transfer radical polymerization (ATRP) initiator, 2-bromopropionyl bromide, were immobilized respectively onto the surface of microspheres before PNIPAAm chains were grown from these initiators. Both initiator grafting density and PNIPAAm molecular weight can be regulated. The achievable PNIPAAm grafted amount (∼697 mg/g) is much higher than that prepared by the grafting to technique. Different preparations of PNIPAAm molecular architecture were investigated, namely linear PNIPAAm chain, cross-linked PNIPAAm hydrogel and branched PNIPAAm chain. Using seeded polymerization, cross-linked PNIPAAm hydrogel (up to 143mg/g) was grafted to the carboxyl functionalized polystyrene microspheres. The grafted amount of PNIPAAm hydrogel can also be regulated. The PNIPAAm-grafted microspheres with different grafting parameters were cultured with cells to investigate cytotoxicity, cell attachment and cell detachment behaviour. All the PNIPAAm-grafted microcarriers with appropriate treatments are not found to inhibit cell proliferation. However, cell attachment and thermally-induced detachment behaviour were not observed on these microcarriers. Potential reasons for these observations were systematically discussed. Finally, two conceptual bioreactor designs, using thermo-responsive microcarriers, are described which would be suitable for future detailed evaluation.
Supervisor: Cui, Zhanfeng ; Moloney, Mark ; Ye, Hua ; Field, Robert Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Tissue engineering ; Biomedical engineering