## 4 Discussion  
The result confirmed that there are differences in cell proliferation when hamstring tenocytes from different ages of donors are exposed to different concentrations of dexamethasone and ascorbic acid. Hamstring tenocytes from an old donor are not always more susceptible to dexamethasone and ascorbic acid treatment. An optimal dose of ascorbic acid in decreasing the cytotoxic effect of dexamethasone can be screened by a high throughput microfluidic platform. Tenocytes from an old donor have the best proliferation when they are exposed to 0.5 mg/mL dexamethasone and 250 mg/mL ascorbic acid, while tenocytes from a young donor have more resistance toward the cytotoxicity of 2.5 mg/mL dexamethasone when they are treated along with 500 mg/mL ascorbic acid. Hamstring tendon is commonly used in ACLR. Quadrupled hamstring autograft is believed to provide the best overall strength when compared to other types of autograft [24; 25]. However, there is still controversy regarding the best graft choice in older patients, because [PERSON] et al. proved cell density and proliferation potential decrease with age [26]. On the other hand, hamstring tendonitis are commonly treated by corticosteroid injection [27], which is believed to reduce tenocytes proliferation, viability, and collagen synthesis as well as to increase markers of apoptosis [27; 28]. Among them, a high concentration of dexamethasone debates the pool of human tendon stem cells, suppresses type I collagen, and enhances fatty and cartilage-like tissue changes that can lead to tendon ruptures, while low concentrations of dexamethasone stimulate cell proliferation [19]. Ascorbic acid is critical in collagen secretion and biosynthesis. The standard growth medium with fetal bovine serum containing 25 mg/mL ascorbic acid was found to be the most suitable formulation [29]. Additionally, 1250 mg/mL ascorbic acid played an important role during cell culture [30], and 50 mg/mL ascorbic acid is believed to support proper collagen synthesis [31]. Furthermore, cotreatment of tenocytes with ascorbic acid protects against the glucocorticoid-induced inhibition of ERK and Akt  
Figure 7: Real-time changes of cell index when tenocytes were exposed to 5 mg/mL of dexamethasone and different concentrations of ascorbic acid. (**a**) Cell index when tenocytes were exposed to 50, 250, and 500 mg/mL ascorbic acid in the 20-year-old patient. (**b**) Cell index when tenocytes were exposed to 50, 250, and 500 mg/mL ascorbic acid in the 41-year-old patient. * \(p\) < 0.05.  
activation and the reduction in cell number induced by chemical inhibition of ERK, and, to a lesser extent, inhibits Akt signaling [21]. Therefore, the current study used three concentrations (50, 250, and 500 ug/mL) of ascorbic acid to see the effects in reducing the cytotoxicity of dexamethasone. The objective was to determine the optimal concentrations and the formulation of ascorbic acid when used with dexamethasone and their different effects on tenocytes from young and old donors. To screen the best treatment combination, conventionally, large numbers of cells and large volumes of reagents are needed. However, harvested cells are sometimes limited because most of them should be used in reconstruction. With the development of integrated microfluidic devices, precise, rapid, and reproducible measurements on small reagent volumes can be achieved [32]. [PERSON] et al. published their result that ascorbic acid reduces the cytotoxic effects of analgesics and NSAIDs [22]. In current study, the reduction of the cytotoxic effect of dexamethasone with cotreatment with ascorbic acid in a specific concentration was also observed in the microfluidic platform. In addition, tenocytes performed differently in young and old donors. The microfluidic platform provides an opportunity for rapid and low-cost screening of the best treatment protocol among each individual, making personalized medicine possible. There are still limitations about this study.