Calcium Concentration Effects on the Mechanical and Biochemical Properties of Chondrocyte-Alginate Constructs

Alginate gel crosslinked by calcium ions (Ca2+) has been widely used in cartilage tissue engineering. However, most studies have been largely performed in vitro in medium with a calcium concentration ([Ca2+]) of 1.8 mM, while the calcium level in the synovial fluid of the human knee joints, for example, has been reported to be 4 mM or even higher. To simulate the synovial environment, the two studies in this paper were designed to investigate how the alginate scaffold alone, as well as the chondrocytes seeded alginate gel responds to variations in medium [Ca2+]. In Study A, the mechanical properties of 2% alginate hydrogel were tested in 0.15 M NaCl and various [Ca2+] (1.0, 1.8, and 4 mM). In Study B, primary bovine chondrocytes were seeded in alginate gel, and biochemical contents and mechanical properties were determined after incubation for 28 days in three [Ca2+] (1.8, 4, and 8 mM). For both studies, it was found that the magnitude of the complex shear modulus (|G*|) at 1 Hz doubled and the corresponding phase angle shift angle (delta) increased > 2 degrees as a result of the approximate 4-fold change in [Ca2+]. At high [Ca2+], the chondrocyte glycosaminogylcan ( GAG) production inside the chondrocyte-alginate constructs was suppressed significantly. This is likely due to a decrease in the porosity of the chondrocyte-alginate constructs as a result of compaction in structure caused by an increased crosslinking density with [Ca2+]. These may be important considerations in the eventual successful implementation of cartilage tissue-engineered constructs in the clinical setting.