Hollow fiber membrane diffusive permeability regulates encapsulated cell line biomass, proliferation, and small molecule release
Introduction
The cell encapsulation approach, which uses a variety of semipermeable materials to isolate transplanted cells from host tissues, has been used to deliver a wide range of small molecules to a variety of sites [1], [2]. Hollow fiber membranes (HFM), originally developed for ultrafiltration applications, where among the first materials used for such applications [3], [4], [5]. Among the more common HFM materials employed is a copolymer of acrylonitrile and vinylchloride (PAN-PVC) [6], [7], [8], [9], including formulations with such solvents as dimethylsulphoxide (DMSO) [10], [11], [12], [13], dimethylformamide (DMF) [10], dimethylacetamide (DMAC) [10], and a number of additives [14], [15].
A large number of studies suggest that such materials allow sufficient exchange of soluble factors to maintain cell viability in vitro and in a variety of tissue locations over extended periods of time [1], however, the influence of HFM transport on encapsulated cell biomass, proliferation and the sustained release has not been fully examined [10], [16], [17], [18], [19], [20]. Indeed, molecular weight cutoff, that is, the molecular size at which 90% of a species is rejected under hydraulic pressure, is most often the only transport data provided in such publications [10], [11], [21], [22], [23], [24].
Accordingly, the objective of the current study was to evaluate the influence of HFM permeability on several biological parameters including encapsulated cell biomass, the number of proliferating cells, and the release of cell derived small molecules using a model cell line. Using a dry-jet, wet spinning technique, we fabricated a variety of PAN-PVC HFM of similar wall dimensions, but with widely different transport properties. PC-12 cells, a well-studied model cell line known to secrete neurotransmitters [10], [25], [26], [27], were encapsulated within the various HFM and cultured in vitro for 4 weeks. Using HPLC and various histological methods, we quantified the viable cell mass, the number of mitotic figures, and the amount of dopamine released from the various encapsulation membranes, and then compared the results with transport performance. The results indicated that the size of the encapsulated cell biomass was related to the quantity of dopamine released and increased as a function of the increasing membrane diffusive permeability. In addition, we found that the percentage of viable cells and the overall appearance of the biomass architecture were not significantly affected by differences in HFM transport properties. The results suggest that encapsulation membrane diffusive transport may be used to control the dosage of small molecules derived from such cell containing implants at steady state.
Section snippets
PAN-PVC solution preparation
Phase-inversion membranes were fabricated from a random copolymer of acrylonitrile and vinylchloride (PAN-PVC) (Dynel, Union Carbide). Several formulations were prepared including a solution of 15% polymer (wt to vol) (grams per 100 ml of solvent) in either dimethylformamide (DMF), dimethylacetamide (DMAC), or dimethylsulphoxide (DMSO) (Mallinckrodt Specialty Chemicals Co), a solution consisting of 15% PAN-PVC, 6% dextrose (J.T. Baker Chemical Co.), 5% deionized water in DMF (wt to vol), and a
Generation and analysis of membranes with varying transport properties
A series of HFM were formed using various starting polymer solutions that were of similar size but varied in wall thickness (Table 1). Observations of the wall architecture using scanning electron microscopy revealed a number of different morphologies (Fig. 1). The HFM formed from DMF, DMAC and DMSO had wall architectures that were anisotropic and consisted of a fingerlike macrovoid structure (Figs. 1A–C). The HFM produced with additives were less anisotropic and possessed areas of more
Discussion
Cell encapsulation takes advantage of cells as a renewable and sustainable resource for the production and delivery of soluble small molecules while physically isolating the cells from interacting directly with host tissues. The key function of the encapsulation membrane is to promote cell viability for long periods, which allows the device to serve as a sustained release source of small biologically active molecules at particular tissue sites. To perform this function, molecules must
Acknowledgements
We would like to thank the W.M. Keck Foundation of Los Angeles for funding support.
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