Department of Chemical and Biological Engineering, University of Colorado, 3415 Colorado Ave., Boulder, Colorado 80309-0596, USA.
Department of Mechanical Engineering, University of Colorado, 1111 Engineering Dr., Boulder, Colorado 80309-0596, USA.
J Mater Chem B. 2020 Apr 8;8(14):2775-2791. doi: 10.1039/c9tb02963j.
Enzyme-sensitive hydrogels containing encapsulated chondrocytes are a promising platform for cartilage tissue engineering. However, the growth of neotissue is closely coupled to the degradation of the hydrogel and is further complicated due to the encapsulated cells serving as the enzyme source for hydrogel degradation. To better understand these coupled processes, this study combined experimental and computational methods to analyze the transition from hydrogel to neotissue in a biomimetic MMP-sensitive poly(ethylene glycol) (PEG) hydrogel with encapsulated chondrocytes. A physics-based computational model that describes spatial heterogeneities in cell distribution was used. Experimentally, cell-laden hydrogels were cultured for six weeks under free swelling or subjected daily to one-hour of dynamic compressive loading. Extracellular matrix (ECM) synthesis rates were used as model inputs, and the model was fit to the experimentally determined construct modulus over time for the free swelling condition. Experimentally, ECM accumulation comprising collagen II and aggrecan increased over time concomitant with hydrogel degradation observed by a loss in PEG. Simulations demonstrated rapid degradation in regions of high cell density (i.e., cell clusters) reaching complete degradation by day 13, which facilitated localized ECM growth. Regions of low cell density degraded more slowly, had limited ECM, and led to the decrease in construct modulus during the first two weeks. The primary difference between the two culture environments was greater ECM accumulation in the clusters under free swelling, which facilitated a faster recovery in construct modulus. By 6 weeks the compressive modulus increased 2.5-fold to 107 kPa under free swelling, but dropped 1.6-fold to 26 kPa under loading. In summary, this biomimetic MMP-sensitive hydrogel supports neocartilage growth by facilitating rapid ECM growth within cell clusters, which was followed by slower growth in the rest of the hydrogel. Subtle temporal differences in hydrogel degradation and ECM accumulation, however, had a significant impact on the evolving mechanical properties.
含包封软骨细胞的酶敏感水凝胶是软骨组织工程的有前途的平台。然而，新组织的生长与水凝胶的降解密切相关，并且由于包封的细胞充当水凝胶降解的酶源，因此变得更加复杂。为了更好地理解这些耦合过程，本研究结合实验和计算方法，分析了在含有包封软骨细胞的仿生 MMP 敏感聚乙二醇（PEG）水凝胶中，水凝胶向新组织的转变。使用了一种描述细胞分布空间异质性的基于物理的计算模型。实验上，在自由溶胀或每天进行一小时动态压缩加载的条件下培养细胞负载水凝胶 6 周。细胞外基质（ECM）合成率用作模型输入，并且将模型拟合到实验确定的自由溶胀条件下的构建体模量随时间的变化。实验上，随着 PEG 的损失，观察到水凝胶降解，同时 ECM 合成率增加，包括胶原 II 和聚集蛋白聚糖。模拟表明，在细胞密度高的区域（即细胞簇）迅速降解，到第 13 天完全降解，从而促进了局部 ECM 的生长。细胞密度低的区域降解较慢，ECM 有限，导致在前两周构建体模量下降。两种培养环境的主要区别是在自由溶胀下，簇中的 ECM 积累更多，从而更快地恢复构建体模量。到 6 周时，在自由溶胀下压缩模量增加了 2.5 倍，达到 107 kPa，但在加载下降低了 1.6 倍，达到 26 kPa。总之，这种仿生 MMP 敏感水凝胶通过促进细胞簇内 ECM 的快速生长来支持新软骨的生长，随后水凝胶的其余部分生长缓慢。然而，水凝胶降解和 ECM 积累的细微时间差异对不断变化的机械性能有重大影响。
