Bionic Design And Performance Of Functional Sutures For Tendon Repair

More than 30 million tendon injuries are reported globally every year.Because of tendons' inferior intrinsic healing,surgery becomes the most common treatment for ruptured tendons,and surgical sutures are indispensable medical devices for closing tendons and skin incisions.Stable fixation and functional reconstruction of the tendons is the ultimate goal for tendon repairs.Absorbable sutures are not selected for tendon closure due to their dramatic degradation of tensile strength before tendon healing.Therefore,non-absorbable sutures,like the new generation ultrahigh molecular weight polyethylene(UHMWPE)based sutures,continue to be the mainstream materials for tendon repair.Nevertheless,their clinical treatment effects are still far from ideal,manifesting in poor knot security,a high re-tear rate,inferior tissue healing,and inflammatory and infection reactions.The fundamental reasons for these issues consist of the following several points:a.the high stiffness and low friction coefficient of UHMWPE fibers often result in stress shielding and knot slippage;b.most sutures maintain traditional thin,round,and solid structures,which lead to stress concentration at the anchor sites,high re-tear rates,and poor tissue integration;c.the existing sutures lack interaction with tendons owing to their inert surfaces,and inflammation and infection occurred with a high probability.Therefore,the medical profession throws out an urgent requirement to further optimize the orthopedic sutures for improving tendon repairs.Except for stable tendon fixation and functional reconstruction,good healing and regeneration of the skin incisions is an important guarantee for tendon healing.Although most sutures can provide sufficient mechanical support for wound closure,their clinical therapeutic effects are unsatisfactory because of their disability in actively participating in wound healing.Complications related to skin wounds only in Achilles tendon repairs account for 1.4%-18%,which undoubtedly hinders tendon healing and brings extra pressure on patients and society.Unfortunately,so far,the treatments of tendons have not yet paid enough attention to the healing and regeneration of the skin incisions.Aim at the above-mentioned clinical problems,a series of functional sutures were designed and constructed by adjusting material composites,optimizing braiding structures,and decorating suture surfaces to enhance the therapeutic effects of the sutures for tendon healing.Additionally,the mechanical and biological properties of the sutures and their influence mechanism were mainly investigated.The main research contents and results are as follows:(1)The mechanical limitation of single-component sutures in fixing the injured tendons is a nonnegligible clinical issue.UHMWPE and polycaprolactone(PCL)multifilaments were selected as raw materials to co-braid partially absorbable sutures.A series of sutures with uniform structures and similar diameters were prepared by adjusting the ratio and distribution of the two materials.The influence mechanism of the material composition on the mechanical properties was investigated via systematic in vitro evaluation.The results showed that PCL/UHMWPE partially absorbable sutures compensated for the mechanical shortcomings of the single-component sutures.Especially,P/U=50/50 possessed high tensile strength up to 180 N and similar stress-strain behavior to natural tendons,which could efficiently alleviate stress shielding.Meanwhile,the synergistic effect of high strength of UHMWPE and large friction coefficient of PCL endowed P/U=50/50 with superior knot strength and the ability to resist knot slippage.The loop strength of P/U=50/50 was 2.40 times and1.61 times that of pure PCL suture and UHMWPE suture,demonstrating significantly improved knot security.Besides,after the creep behaviors,bending stiffness,cyclic stretching,and degradation experiments and compared with clinically commonly used polyester(PET)sutures,the mechanical adaptability between the PCL/UHMWPE sutures and tendon fixation was further clarified.Furthermore,the mathematical relationship between material ratio and breaking strength/elongation was revealed via the theoretical analysis.The viscoelastic model was employed to predict the creep behaviors of the sutures.It was believed that this work could provide experimental and theoretical references for the design and development of partially absorbable sutures.(2)In view of the clinical bottleneck of the severe cutting behaviors on tendons and inability in tissue ingrowth caused by traditional thin,round,and solid suture structures,the bamboo-inspired hollow and porous tape sutures were firstly designed and constructed.Based on the excellent mechanical properties of the PCL/UHMWPE partially absorbable sutures,PCL and UHMWPE were still selected as raw materials to develop the bamboo-inspired sutures.Thanks to the high melting points difference(?80 ?)between UHMWPE and PCL,UHMWPE and PCL worked as braiding yarn and axial yarn separately to construct the bamboo-inspired partially absorbable tape sutures through the combination of triaxial braiding and melting technologies.The coalescent PCL along the axial direction could form bonding points(like "nodes" in the bamboo)to stabilize the hollow structure and prevent the porous wall from collapsing.Three types of 1.60 mm bamboo-inspired partially absorbable tape sutures were fabricated by controlling the number of PCL yarns and the braiding angles.Besides,the structure parameters and mechanical properties of the sutures were systematically characterized,and their structure-performance relationship was discussed.The results demonstrated that sufficient axial yarns and bonding points at each braiding point were essential for the sutures to maintain stable hollow and porous structures.The suture mass decreased at least 28%owing to the porous structure,reducing the in vivo introduction of foreign body materials.Notably,the bamboo-inspired partially absorbable tape sutures not only hold high strength,similar stressstrain behavior to natural tendons,and excellent handling performance but also had significantly less cutting action on tendons(about 38% to that of traditional sutures),which could considerably decrease the re-tear rate.Moreover,the animal in vivo experiment disclosed that the porous structure offered enough space for tissue ingrowth and collagen deposition,potentially promoting tendon healing.This study may provide new insights into the design and construction of novel orthopedic sutures.(3)The inert surface of the sutures often results in poor load transfer along the suture and high re-tear rates.Besides,the inflammation and infection induced by sutures also hinder tendon healing greatly.Herein,multifunctional sutures were constructed by decorating the suture surfaces with bioactive materials.In brief,the bamboo-inspired sutures were firstly activated by polydopamine(PDA).Then bio-sourced polyelectrolytes chitosan(CS)and gelatin(GE)were alternatively deposited onto the suture surfaces.Finally,the sutures were crosslinked by tannic acid(TA),forming CS/GE-TA modified functional sutures.The mechanism of the CS/GE-TA multifunctional coating layer on load transfer,anti-inflammatory and antibacterial abilities,and tissue integration was explored through in vitro and in vivo experiments.The results showed that the functional sutures presented a 3.32 times increase in pull-out force from the tendon because TA endowed the sutures with superior tissue adhesion and enhanced interaction with tendons.Interestingly,the pull-out force was remarkably higher than those sutures coated with cyanoacrylate adhesive,and equipped with better handling performances.It indicated that the CS/GE-TA decorated functional sutures greatly improved load transfer along the axial direction of the sutures while reserving the original good mechanical properties.What's more,the multifunctional sutures possessed excellent antiinflammatory and antibacterial performances without involving any additional drugs.In vivo evaluations further proved that the functional sutures could improve tissue healing by reducing inflammatory,increasing collagen deposition,and vessel formation,showing profound significance in facilitating the functional reconstruction of the tendons.(4)Excellent healing and regeneration of the skin incisions can provide tendon healing with much protection.Although most sutures are strong enough for wound closure,their clinical therapy effects are barely satisfactory owing to the inability to actively participate in wound healing and the prone surgical site infection.The electroactive and antibacterial sutures were fabricated by decorating the suture surfaces with CS/GE-TA and polypyrrole(PPy)composite coating to solve the issues mentioned above.Firstly,several 3-0 PPDO conductive sutures were prepared via regulating the polymerization time of PPy,and the influence of the polymerization time on the conductivity and strength of the sutures were investigated.A short polymerization time would result in insufficient polymerization and poor conductivity;however,a too-long polymerization time certainly would damage the tensile strength,making the sutures unable to meet the requirements of clinical use.The conductivity and tensile strength of the sutures could be well balanced by setting the polymerization time at 12 h.Subsequently,the mechanism of the CS/GE-TA and PPy composite coating on the conductive stability,antibacterial performance,and wound healing was preliminarily explored through in vitro and in vivo experiments.The results demonstrated that the composite coating overcame the inherent brittleness of single-component PPy and improved the compatibility between the electroactive coating and the flexible suture,ensuring the conductivity stability under knotted and the continuous electrical signal transmission among cells.Additionally,thanks to the synergistic antibacterial effects of the composite coating,the sutures possessed a superior antibacterial rate of over 99%.Furthermore,in vivo studies revealed that electroactive and antibacterial sutures could dramatically enhance wound healing and skin regeneration.In summary,aimed at the shortcomings of the existing surgical sutures for tendon repairs,the functional sutures for tendon fixation and skin incision closure were separately designed and constructed to improve the clinically therapeutic effect of tendons.The mechanical and biological performances of the sutures and their influence mechanisms were deeply investigated through systematic in vitro and in vivo experiments.Moreover,the potential of these sutures in enhancing tendon healing and function reconstruction was analyzed and evaluated.The relevant results of this research can offer new ideas and approaches for the development of the sutures.