Using Nanofiber Antibiotic Sutures to Prevent Ophthalmic Infections
Sutures are medical devices applied on the site of injury or surgery. Sutures prevent infection, minimize scarring, and fasten recovery. As of today, the only drug-eluting sutures available for antibacterial application in most surgeries are coated with triclosan. It’s known that putting drugs directly into the suture rather than coating its surface provides better delivery functionally to microsurgical sutures. Insertion, rather than coating, sustains drug delivery and prevents suture thickness.
However, the incorporation of drugs into the suture has a negative effect on its strength. Furthermore, there is scarce market offering for drug-coated or drug-eluting sutures in the field of ophthalmology where very thin ones sutures are highly challenging to produce. In eye surgeries, antibiotic-loaded sutures would help facilitate infection prevention, but antibiotic eye drops are given to prevent infection due to their availability.
Researchers believe that antibiotic-eluting sutures would enhance our current abilities to prevent bacterial colonization. In this study, the researchers reported twisting hundreds of single drug-loaded electrospun nanofibers into one ultra-thin, multifilament suture capable of microsurgical ocular application. Sutures made from nanofiber showed no to very little loss in strength, unlike monofilament sutures which lost more than 50% of their strength. Moreover, nanofiber sutures retained strength when loaded with drugs. When a nanofiber antibiotic suture was used for 30 days in rat eyes, ocular infections were prevented after purposeful bacterial exposure.
What Makes a Perfect Suture?
The first recorded use of natural fibers as sutures were in 3500 B.C. In the 1970s, sutures became synthetic and absorbable. Today, advances in material science are seeking to develop the perfect suture for surgery. Nanofibers have become of high interest for ophthalmological applications due to their thin nature.
Perfect sutures should be absorbable, strong, sterile, and biologically inert in order to provide the best condition for fast wound healing and tissue repair. They must also degrade and lose strength when the surrounding tissue heals. Also, sutures should be capable of providing sufficient therapeutic delivery to the surgical site.
Due to the slow clinical implementation of technology, available sutures are limited and don’t have a high capacity to hold drugs. The current suture manufacturing process includes melt extrusion and is not compatible with many therapeutic moieties as they have low breaking strength in clinical applications.
Drug-coated sutures can be a great solution in many applications. However, in ophthalmology, drug-coated sutures are limited when it comes to their ability to meet diameter requirements, control of drug release, load sufficiency drugs, or scalable manufacturing. Drug-coated sutures are limited for use mostly in topical anti-infection applications which do not require a small diameter. Antibacterial coatings are only available with absorbable thread and are only for general surgery as these sutures are too large to meet the U.S.P diameter requirements in ocular surgeries.
With more than 12 million procedures are done per year using conventional nylon sutures to close ocular wounds, researchers conclude that there is a significant need for an antimicrobial suture in eye surgery. Non-absorbable nylon sutures are good for ocular surgery due to their biocompatibility and strength retention at the surgery site.
Most nylon sutures are used in various procedures such as keratoplasty where the sutures remain in the eye for around 12 to 24 months. Like other implantable devices, the risk of infection from these sutures are high. Infectious keratitis after keratoplasty, for example, has up to a 12% occurrence rate. Suture-related complications have devastating consequences in 20% to 50% of cases. Patients may suffer from poor vision outcomes, graft failure, and reintervention needs.
Local antibacterial functionality along with implantation of non-absorbable sutures crucial for avoiding post-surgical adverse outcomes. Local antibiotic delivery from the suture would provide bacterial inhibition directly at the vulnerable surgical incision and alleviate concerns of noncompliance with topical and burdensome antibiotic eye drops.
Antibiotic eluting sutures can reduce the need for postoperative oral antibiotic prescriptions which can often lead to the emergence of resistant organisms associated complications such as clostridium difficile infection, and even life-threatening diarrhea.
Antibiotic-eluting sutures can also decrease the risk of infection to the following surgeries: glaucoma, retinal detachment, vitrectomy, keratoplasty, and cataract surgeries. They can also prevent complications from the implantation of orbital plates, glaucoma drainage implants, keratoprostheses, and other ocular devices.
An antibacterial suture must be extremely tiny in size while retaining its high strength during the application. It must also provide sufficient release of antibiotic agents to reduce ophthalmic infection.
To provide an antibacterial alternative to nylon sutures in eye surgeries, the researchers manufactured sutures made from polycaprolactone (PCL) and levofloxacin (Levo). Levo is a third-generation fluoroquinolone and an ophthalmic antibiotic usually used for treating bacterial conjunctivitis. PCL is a biocompatible polymer that is capable of long-term degradation as it has been used in other medical devices and sutures for 30+ years.
In the study, researchers evaluated the antibiotic-eluting suture size, pharmacokinetics, biocompatibility, and effects in a rat model of bacterial keratitis. The researchers hypothesized that sutures composed of twisted PCL with Levo nanofibers would provide suitable strength at surgical sites for an extended duration of time while also delivering antibiotics in a sufficient and controlled manner to prevent postoperative suture colonization and eye infection.
The ocular sutures available today are known to cause vision-threatening complications such as endophthalmitis and microbial keratitis. It has also been reported that 50% of infections always follow penetrating keratoplasty procedures. 65% of sutures become loose or broken 3 years after they are used in cataract surgeries. When removed, 40% of sutures have caused bacterial contamination to the patient’s eye. Implantation devices such as keratoprostheses when fixed with sutures have infection rates of more than 17%.
The study has found that antibiotic-eluting sutures offer a reduction to suture-related infections caused by surgery or device implantation. The problem is there are no available antibiotic-eluting sutures in ophthalmology, so the researchers designed one for this study.
Using a novel electrospinning system for nanostructured drug-eluting sutures, the researchers evaluated its potential for development and use for ocular surgeries. They found that it enables facile fabrication and twisting of aligned drug-loaded nanofibers into the ultra-thin multifilament sutures of specific diameters.
The novel design is both controlled and versatile and even allows for reproducible manufacture of drug-eluting sutures from a wide range of specifications and formulations. They can also be lengthened, which increases the distance between the grounded parallel collectors. This suture was fabricated using a manufacturing platform that exceeds the U.S.P specification for sized strength suitable for ophthalmic use.
The suture also demonstrated biocompatibility when compared to conventional nylon sutures. In fact, it retained 96% of its breaking strength over the span of 31 days. It also delivered Levo at detectable levels in rat eyes for 30 days. Furthermore, the Levo-eluting multifilament nanofiber sutures prevented multiple bacterial infections for 1 week and were more effective than the postoperative antibiotic drops.
When it was incorporated with small molecule drugs such as steroids, analgesics, and immunosuppressants with different chemical properties, it continued to surpass its breaking strength requirements. This indicated a huge potential to improve surgical outcomes and alleviate patient concerns in various ocular procedures where most lead to complications and reoperations.
At the moment of the study, this is the first report of the drug-loaded suture to surpass U.S.P breaking strength specifications. Just like other studies, micron-sized electrospun PCL monofilament sutures lost more than 50% of their strength upon the administration of Levo. It showed that the procedure could enhance nanofiber molecular orientation. It can also increase the tensile strength with reduced diameter due to molecular confinement. This was not observed in PCL fiber via the melt flow extrusion process which is the process used in manufacturing sutures today. Furthermore, PCL crystallinity even increased along with a decrease in molecular weight.
In the study, the researchers tried to maximize the fiber crystallinity and suture strength using electrospinning nanofibers which are composed of low molecular weight PCL. They also tried twisting individual nanofibers into multifilament sutures and this provided extra resistance to breakage, knot security, and structural reinforcement. The ribbon-shaped appearance of the fibers suggests that twisting also led to stretching of the nanofibers which improved their tensile strength and polymer chain alignment.
The researchers also found that increased twisting resulted in a more compact nanofiber group. This allowed more fibers to be incorporated into one single suture which can then improve its breaking strength and drug loading capacity. Collectively, these factors contributed to the manufacture of drug-loaded, multifilament nanofiber PCL structures with unprecedented strength.
The hydrophobic nature of PCL nanofibers manufactured via the process separates the drug and polymer. This is why the strength of multi-filament PCL is the same without the drug and with the inclusion of therapeutic prophylactic agents with different molecular structures.
The researchers hypothesize that prior to PCL degradation, the drug delivery profile of small molecules from the multifilament PCL structures will depend on the distribution and solubility of the drugs in the nanofiber.
It can be expected that hydrophobic drugs will have an overall slower release rate than Levo, but provide burst release during the postoperative period. The burst release of an antibiotic is very critical for the prevention of immediate postoperative infection when wounds are healing and most vulnerable to bacterial infiltration. Sutures are always be susceptible to bacterial colonization for as long as they remain implanted.
Given the tiny diameter of sutures used in ophthalmic procedures, it’s not unexpected that there is a lot of difficulty in coating the sutures. If it’s done via conventional methods, it may provide insufficient drug delivery. The researchers conducted this study so that they can evaluate suture strength retention and degradation while assessing late-stage wound sealing and tissue healing.
Materials and Methods
Manufacture of Suture
Polymer solutions were created from a dissolution of 80 kDa PCL with the drug in HFIP by shaking overnight at room temperature. PCL concentration was maintained at 10% in relation to solvent for multifilament sutures. Levofloxacin, moxifloxacin, dexamethasone, rapamycin, and bupivacaine were dissolved at either 8%, 16%, 24%, or40% (w/w) in relation to the polymer.
The solution was then electrospun via pumping at 450L/h through a 20 G blunt-tip needle with an applied voltage of 17 kV at a distance of 13cm from a set of parallel grounded collectors to form 17-cm long parallel nanofibers.
One collector was rotated clockwise 1575 times before the removal of the suture from the collectors. Electrospinning time was 30, 60, 90, and 135 seconds for 21, 28,38, and 48 m diameter multifilament sutures, respectively. Monofilament structures were created via application of 5 kV to a 15%PCL solution (w/w) in HFIP flowing at 1 mL/h toward a static, grounded collector 15 cm away.
Size – The suture diameter was determined using light microscopy using a 20xobjective of an Eclipse TS100 and calibrated spot 5.2. Basic imaging software. The sutures were measured at three different parts at least 2xm apart. They used additional experimentation when the average diameter was within ±0.5 m of the specified diameter.
Morphology – Suture morphology was analyzed through SEM at 1kV using an LEO Field Emission SEM (Zeiss, Oberkochen, Germany) Prior to imaging, the samples were dried and sputter-coated with 10nm of Au/Pd.
Breaking Strength – Sutures (n =3–4 for each condition) were clamped vertically then pulled until breaking at a rate of 2.25 mm/min using a DMA 6800. Breaking strength is defined as the load required to break the suture.
Strength Retention – PCL /8% Levo and PCL/16% Levo sutures (n=5) were sectioned into two halves. Breaking strength of one segment was measured based on the equation while the other segment was submerged in 1xDulbecco’s Phosphate Buffered Saline and shaken at 224 rpm at 37C for 31 days. Sutures were all dried before breaking strength testing.
Drug Loading – Drug loading was determined by submerging 15mm of suture in acetonitrile and sonicating for 30 minutes before evaluation via HPLC. Samples were then injected into a SymmetryTM 300 C185 m column with a mobile phase of 0.1% v/v trifluoroacetic acid at a flow rate of 1 mL/min. Levo elution was detected at an excitation wavelength of 295 nm and an emission wavelength of 496 nm.
Animal studies – All the animals used in the study were all cared and the experiments conducted were all in accordance with the protocols approved by the Animal Care and Use Committee of the John Hopkins University, in accordance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research.
Implantation of sutures in the eye increases the risk of vision complications and eye infections. This is largely because the surface of sutures is vulnerable to bacterial colonization and proliferation.
The researchers have developed a nanostructured multifilament suture that can be loaded with high levels of small molecule drugs whilst retaining its high breaking strength. This is compatible with several ophthalmic antibiotics and surpasses clinical strength requirements while delivering sufficient levels of antibiotics. The researchers believe that this drug-eluting suture platform has the potential to improve the clinical outcomes of various surgical procedures.