Laser-tissue soldering with biodegradable polymer films in vitro: film surface morphology and hydration effects.

BACKGROUND AND OBJECTIVE: Previous research introduced the concept of using biodegradable polymer film reinforcement of a liquid albumin solder for improvement of the tensile strength of repaired incisions in vitro. In this study, the effect of creating small pores in the PLGA films on the weld breaking strength is studied. Additionally, the effect of hydration on the strength of the reinforced welds is investigated. STUDY DESIGN/MATERIALS AND METHODS: A 50%(w/v) bovine serum albumin solder with 0.5 mg/mL Indocyanine Green dye was used to repair an incision in bovine aorta. The solder was coagulated with an 806-nm CW diode laser. A poly(DL-lactic-co-glycolic acid) (PLGA) film was used to reinforce the solder (the controls had solder but no reinforcement). Breaking strengths were measured acutely and after hydration in saline for 1 and 2 days. The data were analyzed by ANOVA (P < 0.05) and multiple comparisons of means were performed using the Newman-Keuls test. RESULTS: The creation of pores in the PLGA films qualitatively improved the film flexibility without having an apparent adverse effect on the breaking strength, while the actual technique of applying the film and solder had more of an effect. The acute maximum average breaking strengths of some of the film reinforced specimens (114.7 g-134.4 g) were significantly higher (P < 0.05) than the acute maximum average breaking strength of the unreinforced control specimens (68.3 g). Film reinforced specimens were shown to have a statistically significantly higher breaking strength than unreinforced controls after 1- and 2-day hydration. CONCLUSIONS: Reinforcement of liquid albumin solders in laser-assisted incision repair appears to have advantages over conventional methods that do not reinforce the cohesive strength of the solder in terms of acute breaking strength and after immersion in moist environments for short periods of time. Using a film with the solder applied to one surface only may be advantageous over other techniques.

New range of light-activated surgical adhesives for tissue repair.

A new range of light-activated surgical adhesives has been developed for laser-assisted tissue repair. The surgical adhesive is composed of a polymer scaffold doped with the traditional protein solder mix of serum albumin and a chromophoric dye. Problems associated with inflexibility in conforming to various tissue geometries, instability in a hydrated environment due to non-uniform tissue adhesive denaturation, and solubility in physiological fluids prior to denaturation are overcome using the adhesives. The new adhesives offer a viable alternative to conventional fasteners, including sutures, staples and clips, currently used for tissue repair. In addition, it could be possible to use patches prepared from the adhesive in the field as a simple and effective method to stop bleeding and repair tissue quickly in an emergency situation. First, studies were conducted to improve the flexibility of traditional protein solders to enable them to be tailored to a wide range of clinically relevant geometry's including tubes, crescents and tape. Second, the creation of a chromophore concentration gradient across the thickness of the adhesive was investigated as a means to allow control of the heat source gradient through the adhesive. Increased deposition of the laser energy near the vital solder/tissue interface was thus achieved. Finally, predenaturation of the adhesive was investigated as a means for enhancing its stability in a hydrated environment thus improving the handling characteristics of the adhesive for clinical application. The application of the new surgical adhesives to augment laser tissue repairs is shown to enhance edge co-optation, improve repair strength and to reduce thermal tissue injury. The moldable, absorption controllable and flexible nature of the new adhesives greatly improves the clinical applicability of laser-solder tissue repair.

Improved vascular tissue fusion using new light-activated surgical adhesive on a canine model.

Newly developed light-activated surgical adhesives have been investigated as a substitute to traditional protein solders for vascular tissue fusion without the need for sutures. Canine femoral arteries (n = 14), femoral veins (n = 14), and carotid arteries (n = 10) were exposed, and a 0.3-0.6 cm longitudinal incision was made in the vessel walls. The surgical adhesive, composed of a poly(L-lactic-co-glycolic acid) scaffold doped with the traditional protein solder mix of bovine serum albumin and indocyanine green dye, was used to close the incisions in conjunction with an 805 nm diode laser. Blood flow was restored to the vessels immediately after the procedure and the incision sites were checked for patency. The new adhesives were flexible enough to be wrapped around the vessels while their solid nature avoided the problems associated with "runaway" of the less viscous liquid protein solders widely used by researchers. Assessment parameters included measurement of the ex vivo intraluminal bursting pressure 1-2 h after surgery, as well as histology. The acute intraluminal bursting pressures were significantly higher in the laser-solder group (>300 mmHg) compared to the suture control group (<150 mmHg) where four evenly spaced sutures were used to repair the vessel (n = 4). Histological analysis showed negligible evidence of collateral thermal damage to the underlying tissue in the laser-solder repair group. These initial results indicated that laser-assisted vascular repair using the new adhesives is safe, easy to perform, and contrary to conventional suturing, provides an immediate leak-free closure. In addition, the flexible and moldable nature of the new adhesives should allow them to be tailored to a wide range of tissue geometries, thus greatly improving the clinical applicability of laser-assisted tissue repair.

Novel solid protein solder designs for laser-assisted tissue repair.

BACKGROUND AND OBJECTIVES: Previous studies have shown that the application of chromophore-enhanced albumin protein solders to augment laser tissue repairs significantly improves repair strength, enhances edge co-optation, and reduces thermal tissue injury. These investigations are furthered with this in vitro study conducted to assess a new range of specially designed chromophore-enhanced solid protein solders manufactured and tested for application during laser-assisted tissue repair. STUDY DESIGN/MATERIALS AND METHODS: The experimental study was divided into three parts. In the first part of the study, the creation of a chromophore concentration gradient across the thickness of the solid protein solder was investigated as a means to improve control of the heat source gradient through the solder during laser irradiation. In the second part of the study, predenaturation of the solid protein solder was investigated as a means for enhancing the stability of the solder in physiological fluids before irradiation. Finally, in the third part of the study, the feasibility of using synthetic polymers as a scaffold for traditional albumin protein solder mixes was investigated as a means of improving the flexibility of the solder. RESULTS: Uniform denaturation across the thickness of the solder was achieved by controlling the chromophore concentration gradient, thus ensuring stable solder-tissue fusion when the specimen was submerged in a hydrated environment. Predenaturation of the solid protein solder significantly reduced the solubility of the solder, and consequently, improved the handling characteristics of the solder. The solder-doped polymer membranes were flexible enough to be wrapped around tissue, whereas their solid nature avoided problems associated with "runaway" of the less viscous liquid solders currently used by researchers. In addition, the solder-doped polymer membranes could be easily tailored to a wide range of geometries suitable to many clinical applications. CONCLUSION: The novel solid protein solder designs presented here add a new dimension to tissue repair as their flexible, moldable, and absorption controllable nature, greatly improves the clinical applicability of laser-assisted tissue repair.

Biodegradable polymer film reinforcement of an indocyanine green-doped liquid albumin solder for laser-assisted incision closure.

BACKGROUND AND OBJECTIVE: The purpose of this study was to determine whether solid material reinforcement of a liquid albumin solder coagulum could improve the cohesive strength of the solder and, thus, the ultimate breaking strength of the incision repair in vitro. STUDY DESIGN/MATERIALS AND METHODS: A 50%(w/v) bovine serum albumin solder with 0.5 or 2.5 mg/ml indocyanine green (ICG) dye was used to repair an incision in bovine aorta. The solder was coagulated with an 806-nm continuous wave diode laser. A 50-micrometer-thick poly(DL-lactic-co-glycolic acid) film was used to reinforce the solder (the controls had solder but no reinforcement). Acute breaking strengths were measured, and the data were analyzed by Student's t-test. RESULTS: Observations of the failure modes indicate cohesive strength reinforcement of the test specimens vs. the controls. The 2.5 mg/ml ICG reinforced solder was stronger than the controls without reinforcement (P < 0.05) for all laser powers tested. There was no difference between the test specimens and the controls with 0.5 mg/ml ICG solder for low laser powers, but at higher laser powers, the reinforced solder was stronger than the controls (P < 0.05). CONCLUSION: Reinforcement of liquid albumin solders in laser-assisted incision repair seems to have advantages in terms of acute breaking strength over conventional methods that do not reinforce the cohesive strength of the solder.

Optimal parameters for laser tissue soldering: II. Premixed versus separate dye-solder techniques.

BACKGROUND AND OBJECTIVE: Laser tissue soldering by using an indocyanine green (ICG)-doped protein solder applied topically to the tissue surface and denatured with a diode laser was investigated in Part I of this study. The depth of light absorption was predominantly determined by the concentration of the ICG dye added to the solder. This study builds on that work with an in vitro investigation of the effects of limiting the zone of heat generation to the solder-tissue interface to determine whether more stable solder-tissue fusion can be achieved. STUDY DESIGN/MATERIALS AND METHODS: An alternative laser tissue soldering technique was investigated, which increased light absorption at the vital solder-tissue interface. A thin layer of ICG dye was smeared over the surface to be treated, the protein solder was then placed directly on top of the dye, and the solder was denatured with an 808-nm diode laser. Because laser light at approximately 800 nm is absorbed primarily by the ICG dye, this thin layer of ICG solution restricted the heat source to the space between the solder and the tissue surfaces. A tensile strength analysis was conducted to compare the separate dye-solder technique with conventional techniques of laser tissue soldering for which a premixed dye-solder is applied directly to the tissue surface. The effect of hydration on bond stability of repairs formed by using both techniques was also investigated using tensile strength and scanning electron microscopy analysis. RESULTS: Equivalent results in terms of tensile strength were obtained for the premixed dye-solder technique using protein solders containing 0.25 mg/ml ICG (liquid solder, 220 +/- 35 N/cm(2); solid solder, 602 +/- 32 N/cm(2)) and for the separate dye-solder technique (liquid solder, 228 +/- 41 N/cm(2); solid solder, 578 +/- 29 N/cm(2)). The tensile strength of native bovine thoracic aorta was 596 +/- 31 N/cm(2). Repairs created by using the separate dye-solder technique were more stable during hydration than their premixed dye-solder counterparts. The conventional premixed dye-solder was simpler and approximately twice as fast to apply. The separate dye-solder technique, however, increased the shelf-life of the solder, because the dye was mixed at the time of the experiment, thus conserving its spectral absorbency properties. CONCLUSION: Two laser-assisted tissue soldering techniques have been evaluated for repairing aorta incisions in vitro. The advantages and disadvantages of each of these techniques are discussed.

Optimal parameters for laser tissue soldering. Part I: tensile strength and scanning electron microscopy analysis.

BACKGROUND AND OBJECTIVES: The use of liquid and solid albumin protein solders to enhance laser tissue repairs has been shown to significantly improve postoperative results. The published results of laser-solder tissue repair studies have, however, indicated inconsistent success rates. This can be attributed to variations in laser irradiance, exposure time, solder composition, chromophore type, and concentration. An in vitro study was performed using indocyanine green-doped albumin protein solders in conjunction with an 808 nm diode laser to determine optimal laser and solder parameters for tissue repair in terms of tensile strength and stability during hydration. STUDY DESIGN/MATERIALS AND METHODS: Twenty-five different combinations of laser irradiance (6.4, 12.7, 19.1, 25.5, 31.8 W/cm2) and exposure time (20, 30, 40, 50, 100 or 40, 60, 80, 100, 200 seconds) were used. The effect of changing bovine serum albumin (BSA) concentration (25% and 60%) and indocyanine green (ICG) dye concentration (2.5 mg/ml and 0.25 mg/ml) of the protein solder on the tensile strength of the resulting bonds was investigated. The effect of hydration on bond stability was also investigated using both tensile strength and scanning electron microscopy analysis. RESULTS: Tensile strength was observed to decrease significantly with increasing irradiance. An optimum exposure time was found to exist where further irradiation did not improve the tensile strength of the bond. Tensile strength was found to be greatly improved by increasing the BSA concentration. Finally, the lower ICG dye concentration increased the penetration depth of the laser light in the protein solder leading to higher tensile strengths. The strongest repairs were formed by using 6.4 W/cm2 irradiation for 50 seconds with a protein solder composed of 60% BSA and 0.25mg/ml ICG. In addition, the solid protein solder provided more stable adhesion to the tissue than did the liquid protein solder when the tissue was submerged in a hydrated environment. CONCLUSIONS: This study greatly enhances the current understanding of the various factors affecting the soldering process. It provides a strong basis for optimization of the laser light delivery parameters and the solder constituents to achieve strong and reliable laser tissue repairs.

Photothermal effects of laser tissue soldering.

Low-strength anastomoses and thermal damage of tissue are major concerns in laser tissue welding techniques where laser energy is used to induce thermal changes in the molecular structure of the tissues being joined, hence allowing them to bond together. Laser tissue soldering, on the other hand, is a bonding technique in which a protein solder is applied to the tissue surfaces to be joined, and laser energy is used to bond the solder to the tissue surfaces. The addition of protein solders to augment tissue repair procedures significantly reduces the problems of low strength and thermal damage associated with laser tissue welding techniques. Investigations were conducted to determine optimal solder and laser parameters for tissue repair in terms of tensile strength, temperature rise and damage and the microscopic nature of the bonds formed. An in vitro study was performed using an 808 nm diode laser in conjunction with indocyanine green (ICG)-doped albumin protein solders to repair bovine aorta specimens. Liquid and solid protein solders prepared from 25% and 60% bovine serum albumin (BSA), respectively, were compared. The efficacy of temperature feedback control in enhancing the soldering process was also investigated. Increasing the BSA concentration from 25% to 60% greatly increased the tensile strength of the repairs. A reduction in dye concentration from 2.5 mg ml(-1) to 0.25 mg ml(-1) was also found to result in an increase in tensile strength. Increasing the laser irradiance and thus surface temperature resulted in an increased severity of histological injury. Thermal denaturation of tissue collagen and necrosis of the intimal layer smooth muscle cells increased laterally and in depth with higher temperatures. The strongest repairs were produced with an irradiance of 6.4 W cm(-2) using a solid protein solder composed of 60% BSA and 0.25 mg ml(-1) ICG. Using this combination of laser and solder parameters, surface temperatures were observed to reach 85+/-5 degrees C with a maximum temperature difference through the 150 microm thick solder strips of about 15 degrees C. Histological examination of the repairs formed using these parameters showed negligible evidence of collateral thermal damage to the underlying tissue. Scanning electron microscopy suggested albumin intertwining within the tissue collagen matrix and subsequent fusion with the collagen as the mechanism for laser tissue soldering. The laser tissue soldering technique is shown to be an effective method for producing repairs with improved tensile strength and minimal collateral thermal damage over conventional laser tissue welding techniques.

Optical and thermal characterization of albumin protein solders.

The effect of temperature on the optical and thermal properties of pure and indocyanine green-doped albumin protein solders as a function of wavelength has been studied between 25 degrees C and 100 degrees C. An increase in the group refractive index by up to 4% and a decrease in absorption coefficient (approximately 800 nm) by up to 8%, after denaturing the solder specimens in a constant-temperature water bath at temperatures of 60-100 degrees C, were not significant. The reduced scattering coefficient, however, increased rapidly with temperature as the solder changed from being a highly nonscattering medium at room temperature to a highly scattering medium at temperatures close to 70 degrees C. The thermal conductivity, thermal diffusivity, and heat capacity increased by up to 30%, 15%, and 10%, respectively. Finally, the frequency factor and activation energy were measured to be 3.17 x 10(56) s(-1) and 3.79 x 10(5) J mol(-1), respectively, for liquid protein solders (25% bovine serum albumin) and 3.50 x 10(57) s(-1) and 3.85 x 10(5) J mol(-1), respectively, for solid protein solders (60% bovine serum albumin). Incorporation of dynamic optical and thermal properties into modeling studies of laser tissue interactions could have a significant influence on the determination of the expected zone of damage.