# An Accelerated Aging Test to Compare the Thermal Stability over Time between the Mint Lift+
## 4 Discussion  
Both synthetic and natural polymers are vulnerable to degradation at varying levels during their lives, which may eventually lead to alterations in their mechanical, thermal, optical, functional, or other properties. Such vulnerability to the degradation plays a role in impairing the performance of polymers and thereby shortening their life cycles [34; 35; 36; 37; 38; 39]. The long-term behavior of polymers may be affected by diverse factors, such as their chemical properties, the manufacturing process, and the type and level of environmental exposure. Indeed, even the same polymers would perform differently if exposed to different conditions [40].  
The accelerated aging test is commonly performed to obtain the experimental data from a new product and, thereby, to estimate its clinical performance and shelf-life claims [41]. To our knowledge, this is the first study to assess the thermal stability of the Mint Lift(r) 17 and the Mint Lift(r) Fine over time as compared with the MEDI ROPE.  
Adequate tensile strength for tissue anchorage _as well as_ durability is considered a requirement for thread-lifts, for which both larger cross-sectional diameters and irregular shapes serve as factors causing a tissue reaction, and barbs are involved in the even distribution of the tensile strength in the dermal and subcutaneous layers [42]. There are several literature studies advocating the effects of barbed thread-lifts; these effects are classified as mechanical and biochemical ones. The former arises from the helical and bidirectional arrangement of barbs, which is followed by the generation of the tensile force, applied to both sides, allowing the barbs to act as a hook without a wire slip [43; 44]. The  
Figure 3: Microscopic findings observed at 20 weeks. At 20 weeks, the integrities of the Mint Lift® 17 and the Mint Lift® Fine were better preserved as compared with the MEDI ROPE.  
Figure 2: Microscopic findings observed at 0, 5, and 10 weeks. At 0, 5, and 10 weeks, the Mint Lift® 17 and the Mint Lift® Fine showed no notable differences in microscopic findings as compared with the MEDI ROPE.  
latter leads to neocollagenesis; [PERSON] et al. [45] used a rat experimental model and thereby inserted cog threads in the skin. These authors observed that myofibblasts were a key player in the contracture of the fibrous tissue at 4 weeks following the insertion of thread. In addition, histological examinations showed the formation of a homogeneous fibrous capsule around the thread, leading to the preservation of the traction and compactness of tissues. Moreover, there was an increase in the thickness of the dermal papillae on histopathology, which is suggestive of the interstitial growth of collagen components [7].  
A limited durability, in vivo, of thread-lifts deserves special attention; it may commonly arise from changes in their thermal stability over time. This is closely associated with clinical failure of the facial rejuvenation using them. Therefore, their clinical lifetimes should be prolonged through improvements in their thermal stabilities over time.  
To summarize, our results are as follows: First, between 0 and 14 weeks, the Mint Lift(r) 17 and the Mint Lift(r) Fine had significantly higher degrees of strength as compared with the MEDI ROPE (\(p<0.05\)). Second, at 0, 5, and 10 weeks, the Mint Lift(r) 17 and the Mint Lift(r) Fine showed no notable differences in microscopic findings as compared with the MEDI ROPE. Third, at 20 weeks, the integrity of the Mint Lift(r) 17 and the Mint Lift(r) Fine was better preserved as compared with the MEDI ROPE.  
However, our results cannot be generalized, because we failed to perform histologic and molecular analyses in vivo. It is mandatory to consider inherent differences in the elastic and thread-holding properties between in vitro and in vivo conditions. This deserves further studies.