Alginate beads, as well as microcapsules based on alginate, cellulose sulphate and polymethylene-co-guanidine, were produced at diameters of 0.4, 1.0 and 1.5 mm. These standard materials were tested, by independent laboratories, in regards to water activity, bead or capsule size, mechanical resistance and transport behaviour. The water activity and mechanical resistance were observed to increase with bead and capsule size. Transport properties (ingress) were assessed using a variety of low molar mass and macromolecular probes. It was observed that the penetration of Vitamin B12 increased with bead diameter, as did dextran penetration. However, for the membrane-containing microcapsules, larger membrane thickness, observed for the larger capsules, retarded ingress. The authors, who are part of a European working group, recommend that permeability be assessed either using a large range of probes or a broad molar mass standard, with measurements at one or two molar masses insufficient to simulate the behaviour in application. Mechanical compression is seen as a good means to estimate elasticity and rupture of beads and capsules, with the sensitivity of the force transducer, which can vary from microN to tens of N, required to be tuned to the anticipated bead or capsule strength. Overall, with the exception of the mechanical properties, the precision in the inter-laboratory testing was good. Furthermore, the various methods of assessing transport properties agreed, in ranking, for the beads and capsules characterized, with gels having smaller radii being less permeable. For microcapsules, the permeation across the membrane dominates the ingress, and thicker membranes have lower permeability.
Renken A, Hunkeler D. Renken A, et al. J Microencapsul. 2007 Feb;24(1):20-39. doi: 10.1080/02652040601058418. J Microencapsul. 2007. PMID: 17438940
Wandrey C, Espinosa D, Rehor A, Hunkeler D. Wandrey C, et al. J Microencapsul. 2003 Sep-Oct;20(5):597-611. doi: 10.1080/0265204031000148022. J Microencapsul. 2003. PMID: 12909544
Schuldt U, Hunkeler D. Schuldt U, et al. J Microencapsul. 2007 Feb;24(1):1-10. doi: 10.1080/02652040601058350. J Microencapsul. 2007. PMID: 17438938
Martins E, Poncelet D, Rodrigues RC, Renard D. Martins E, et al. J Microencapsul. 2017 Dec;34(8):754-771. doi: 10.1080/02652048.2017.1403495. Epub 2017 Nov 21. J Microencapsul. 2017. PMID: 29161939 Review.
Whelehan M, Marison IW. Whelehan M, et al. J Microencapsul. 2011;28(8):669-88. doi: 10.3109/02652048.2011.586068. Epub 2011 Nov 2. J Microencapsul. 2011. PMID: 22047545 Review.
Rathnayake D, Mun HS, Dilawar MA, Baek KS, Yang CJ. Rathnayake D, et al. Life (Basel). 2021 May 24;11(6):476. doi: 10.3390/life11060476. Life (Basel). 2021. PMID: 34073875 Free PMC article. Review.
Nguyen DH, Seok WJ, Kim IH. Nguyen DH, et al. Animals (Basel). 2020 May 30;10(6):952. doi: 10.3390/ani10060952. Animals (Basel). 2020. PMID: 32486180 Free PMC article. Review.
Thorne MF, Simkovic F, Slater AG. Thorne MF, et al. Sci Rep. 2019 Nov 29;9(1):17983. doi: 10.1038/s41598-019-54512-4. Sci Rep. 2019. PMID: 31784621 Free PMC article.
Spasojevic M, Bhujbal S, Paredes G, de Haan BJ, Schouten AJ, de Vos P. Spasojevic M, et al. J Biomed Mater Res A. 2014 Jun;102(6):1887-96. doi: 10.1002/jbm.a.34863. Epub 2013 Jul 24. J Biomed Mater Res A. 2014. PMID: 23853069 Free PMC article.