Comparison of different thread products for facial rejuvenation: Materials and barb designs

In recent years, the thread‐lifting technique has become widely used in clinical settings. Several thread products are used in clinical practice, and there are many differences between products in terms of many aspects.

tissue suspended semipermanently. 6,9 However, due to factors such as individual differences in patients or clinicians' experiences, the performance of the products cannot be evaluated objectively.
With numerous brands' establishment and their commercial promotion, an increasing number of threads are being used for threadlifting techniques, [10][11][12][13] and the product's properties differ in many aspects ( Figure 1C). Regarding the product materials, polypropylene (PP) was the first material used in face-lifting sutures 5 ; it is a nonabsorbable material that can take more than a year to degrade completely. Absorbable materials, such as polycaprolactone (PCL), polylactic acid (PLA), polyglycolic acid (PGA), and polydioxanone (PDO), have recently been used in this field ( Figure 1D).
The research published by Matthias et al. evaluated six barbed PDO thread products in terms of material quality and biocompatibility, 10 and they found significant differences between thread products. However, there have been no studies comparing the Materials and Barb Designs of different types of threads. In this study, we evaluated a variety of commercial threads for facial lifting. The main scales include mechanical properties and biocompatibility. This preliminary study may provide additional objective nonhuman evidence for future investigations and clinical choices.

| Preparation of threads
In this study, we purchased six different threads. They all had barbs, and the material, gauge, and barb shape were different. We numbered them (T1-T6) for the following tests. All threads were purchased from the manufacturers, and the basic information was provided by the instructions or the products' official websites.

| Macroscopy observation and scanning electron microscopy (SEM) examination
To determine the differences in the appearance of the threads, macroscopic observation photographs were taken, and SEM was performed. The threads were cut into pieces approximately 3 mm long, and the samples included at least one barb; at the same time, the cross-sections of the threads were also observed. The samples were sputtered with gold and then imaged using 20 kV SEM (INSPECT F, FEI, the Netherlands).

| Tensile testing
To determine the mechanical properties of the threads, the Young's moduli and maximum loading forces of six barbed threads (n = 5) were evaluated in a standard tensile test configuration using a compression and tension tester (Instron 5565, US). We conducted the experiment according to previously described methods. 10 Briefly, the samples were at least 70 mm long with barbs, and at least 20 mm from each end was inserted into the clamps to prevent the samples from slipping out. A free length of 30 mm was marked for testing. The Young's modulus was calculated from the linear range of the stressstrain curve during tensile testing using a speed ramp of 30 mm/min with respect to the thread's diameter. We recorded the absolute maximum values of the stress in Newtons before breakage as the

| Animal experiments and histochemical observation
To determine the biocompatibility in vivo, we conducted animal experiments. All applicable institutional and/or national guidelines for the care and use of animals were followed. Approval was obtained from the Experimental Animals Ethics Committee before commencement of the study. Seventy-two female Sprague-Dawley rats, 8 weeks old and 160-200 g in body weight, were divided into six groups. The rats were kept in separate cages at room temperature under a 12-hour light/dark cycle and given water and an appropriate diet. After intraperitoneal anesthesia with 40 mg/kg pentobarbital sodium, the hair in the dorsal region was shaved, and a 40-mm-long thread was placed laterally to the spinal cord on the right side. At the same time, as in the control group, we also used a needle to pierce the skin on the left side without any further operations ( Figure S1, which demonstrates the methods of thread injection). Threads were injected by an experimental operator as described by the manufacturer's instructions.

| Statistical analysis
All quantitative data are presented as the mean ± SD based on data derived from experiments in each group. Statistical comparisons were carried out using one-way analysis of variance (ANOVA) followed by Tukey's multiple comparisons test using GraphPad Prism (version 9.0.0 for MacOS, GraphPad Software, USA, www.graph pad. com). In all analyses, p < 0.05 was considered statistically significant.

| Macroscopy observation and SEM
As shown in the Table 1, we collected six barbed thread products of similar gauge (five threads were USP 2-0, and one was USP 0)

| Mechanical properties
The Young's modulus was evaluated by observing the slope of the linear range in the stress-strain diagram ( Figure 3A). The diameter of each sample was measured before the test, and they were different in The maximum loading force is also recorded. This value indicates the maximum force that the thread can withstand. Although it is difficult to generate such a large force on the thread in the tissues, this value can compare the ability of each thread to resist breakage to a certain extent ( Figure 3B). All groups showed statistically significant differences except one pair (T3 versus T4

| Biocompatibility testing
In all thread-injected groups' sections, a capsule-like structure was observed around the thread ( Figure 4A). A round capsule-like structure could be observed in most groups (T1, T2, T3, T5, T6), and in T4, the capsule-like structure was approximately square, which exactly corresponded to the cross-sectional shape of the threads.

F I G U R E 3
Tensile test results of (A) Young's moduli and (B) maximum force before breakage. Data are represented as the mean and standard deviation. Each dot on bar charts represents independent sample measurement. Statistical significance was calculated using ordinary one-way ANOVA with Tukey's multiple comparisons test. (*p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001; n.s. = not significant).

(B) (A)
immediate effect, and relative safety. However, based on clinical experience and reviews, serious long-term studies and peer-reviewed data regarding longevity and patient satisfaction are still lacking, 1,7 and there is still much debate about effectiveness and mechanisms of this technique. This clinical procedure is currently widely used because it is less invasive when compared with the open procedure; thus, our research on barbed threads is still meaningful.
Barbed threads, which are used in the thread-lifting procedure, can draw the soft tissue in a certain direction, allowing a good suspension effect to be achieved immediately. 6,14 Although, as the thread degrades, the local tissue will be stimulated to produce many fiber-like structures surrounding the thread, and these structures can have a certain promoting effect on maintaining the lifting effect. 9,15 With the development of materials science, many threads and su- Compared with nonabsorbable materials, consumers are more likely to choose absorbable materials that have better biocompatibility and manageable risks. The earliest barbed thread was made of nonabsorbable materials. 5 For example, PP is nondegradable in tissues and can exist in the body for more than 1 year. At present, most barbed thread products are made of absorbable materials, such as PDO, which are completely degraded within a year. At the same time, their lifting effect will be weakened as the thread degrades.
Due to the degradability of absorbable threads, inexperienced physicians may prefer to choose such threads. In addition, nonabsorbable threads are associated with higher risk of complications, and it is more difficult to manage it. 16 Second, we believed the thread's barbs should have a suitable shape and anchoring ability, which requires the barbs to have a certain grasping force in the tissue that cannot deform easily, and the barbs should not cause damage to the local tissue. There are two main barb designs: In the first, the barb is cut onto the thread body; in the second, the barb is integrally formed on the thread body by molding methods. The results of previous experiments showed that press-molded barbs did not have better strength or lifting ability, 10 but in our study, the threads with press-molded barbs (T4 & T6) showed an advantage in maximum force before breaking. We suspect that this result may be related to thickness weakness at the barb positions.
Additionally, the material should have suitable mechanical properties: elasticity and strength. Threads that are too flexible (low modulus) may not provide a good lift effect, and those that are too stiff (high modulus) may produce a foreign body sensation when muscle and tissue move. Notably, we found that barbed thread diameters did not strictly conform to the USP standard. However, the diameter is not one of the factors that determines the Young's modulus, so we can ignore the differences of diameters. Threads with a larger modulus can provide greater stress when they change to the same length. The strength of the thread is related to the types and shapes of the samples, so the diameter and barbs' designs will greatly affect the result of the maximum forces. Interestingly, we found that it was difficult to generate such a large force in the tissues to break the suture; further experiments are needed to determine whether the suture can be anchored locally under such a large force.
Regarding the biological stimulation of threads, many published articles have studied the histochemical changes of various barbed threads in animals. The tissue reactions can be observed and analyzed from various scales, including fibrous capsule formation, 17 collagen fibril density, 18 collagen amount, 9 and number of myofibroblasts. 15 In this study, all materials we tested led to good results in which capsulelike structures formed, and no severe acute inflammatory response or granuloma production was observed. Compared with the subcutaneous tissue without thread implantation, the thread groups were richer in collagen in dermis. This may be due to the fibrous encapsulation produced by the local tissue's immune response. 18 The biological response produced by materials may be different, which requires detection of more indicators for analysis in the future.
Another limitation of the study is that animal experiments were not conducted over long enough periods of time to observe the complete degradation of the thread. Also, we only tested threads of four materials, and some other materials on the market were not included, such as PLLA. In the future, more comprehensive experiments are needed to confirm whether a material can be well used in thread-lifting treatment.
In clinical practice, there are more factors that will influence the treatment effect, such as the design of the thread arrangement. It is necessary to conduct clinical trials on various thread products.
When the amount of data is sufficient and the methods are objective enough, the therapeutic effect of a product, including the complications, can be completely evaluated. Thus, a preclinical in vitro experiment may only provide a preliminary evaluation of the biological safety and the mechanical quality of a product, which can provide a certain reference for the selection of products or evidence to explain the different clinical outcomes.

| CON CLUS ION
In this experiment, we tested the barbed thread products and found there were differences in mechanical properties, and the biocompatibility was tested at the same time. All these materials showed good biocompatibility and the potential to stimulate collagen production and can be used safely with certain effects. These methods used in this experiment may provide a reference to the future clinical selection of products. However, more long-term evaluations and testing are needed to evaluate the clinical efficacy.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare that they have no conflict of interest.

E TH I C A L A PPROVA L
The animal experiments were conducted in accordance with the guidelines outlined in the "Principles of Laboratory Animal Care" (NIH) and were approved by the Ethics Committee of West China Hospital of Stomatology, Sichuan University (WCHSIRB-D-2021-114).Statement of human and animal rights: All applicable institutional and/or national guidelines for the care and use of animals were followed.

I N FO R M E D CO N S E NT
For this type of study, informed consent is not required.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.