Laser Assisted Soldering: MicrodropletAccumulation With a Microjet Device

Eric K. Chan, PhD,1* Quiang Lu, MS,2 Brent Bell, BS,3

Massoud Motamedi, PhD,3 Christopher Frederickson, PhD,2

Dennis T. Brown, PhD,5 Ian S. Kovach, MD, PhD,4 and Ashley J. Welch, PhD6

1Indigo Medical, Inc., Cincinnati, Ohio 452422MicroFab Technologies, Inc., Plano, Texas 75074

3University of Texas Medical Branch, Galveston, Texas 775504Massachusetts General Hospital, Boston, Massachusetts 021445North Carolina State University, Raleigh, North Carolina 27695

6The University of Texas, Austin, Texas 78712

Background and Objective: We investigated the feasibility of amicrojet to dispense protein solder for laser assisted soldering.Study Design: Successive micro solder droplets were depositedon rat dermis and bovine intima specimens. Fixed laser expo-sure was synchronized with the jetting of each droplet. Afterphotocoagulation, each specimen was cut into two halves at thecenter of solder coagulum. One half was fixed immediately,while the other half was soaked in phosphate-buffered salinefor a designated hydration period before fixation (1 hour, 1, 2,and 7 days). After each hydration period, all tissue specimenswere prepared for scanning electron microscopy (SEM).Results: Stable solder coagulum was created by successive pho-tocoagulation of microdroplets even after the soldered tissueexposed to 1 week of hydration.Conclusions: This preliminary study suggested that tissue sol-dering with successive microdroplets is feasible even with fixedlaser parameters without active feedback control. Lasers Surg.Med. 23:213–220, 1998. © 1998 Wiley-Liss, Inc.

Key words: endoscopic surgery; laparoscopic surgery; laser-tissue interaction;tissue soldering

INTRODUCTION

One major drawback of laser tissue solderingis the lack of a reliable method to assure propersolder coagulation at the tissue. In most cases,visual feedback is used to determine the status ofthe coagulation process [1–4]. The term ‘‘occur-rence of desiccation’’ is often used in the literatureto describe the visual cue used for deciding thecoagulation extent. ‘‘Occurrence of desiccation’’ isthe point where the solder starts appearing pale/diffuse and perhaps there is slight solder/tissuediscoloration during the coagulation process. Sur-face temperature is another feedback parameterthat has been investigated [5–7]. Since photoco-agulation is a rate process, which means it de-pends on both the heating time and temperature

[8], infrared radiometry has been used to monitorsolder coagulation by advancing the laser beamwhen the solder surface reaches a certain tem-perature [9–11]. However, the most crucial regionof the whole process, the solder-tissue interface, isburied at the bottom of the solder. Therefore, sur-

Contract grant sponsor: Office of Naval Research Free Elec-tron Laser Biomedical Science Program; Contract grant num-ber: N00014-91-J-1564; Contract grant sponsor: TexasHigher Education Coordinating Board; Contract grant num-ber: BER-ATP-253; Contract grant sponsor: Albert W. andClemmie A. Caster Foundation.

*Correspondence to: Dr. Eric Chan, Indigo Medical, Inc.,10123 Alliance Road, Cincinnati, OH 45242.E-mail: echan@eesus.jnj.com

Accepted 29 July 1998

Lasers in Surgery and Medicine 23:213–220 (1998)

© 1998 Wiley-Liss, Inc.

face information may provide a reliable estimateof coagulation extent at the solder-tissue interfaceonly when the solder thickness is consistent andthe optical and thermal properties of the solderand the tissue remain constant. In a previousstudy, we have demonstrated that under-coagu-lated solder may result in premature solder de-tachment due to poor solder-tissue fusion [12].This detachment phenomenon is more susceptiblewhen the tissue substrate has a smooth surfaceand the welded tissue is exposed in a prolongedperiod to a hydrated environment.

Under-coagulation can be avoided when thesolder thickness is small compared to solder ab-sorption depth. Recently, we reported a study us-ing accumulative micro solder droplets to performtissue repair [13]. Acute failure strength of thesolder was 257 N/cm2, significantly higher thanthe failure strength of the tissue substrate nativeaorta (76 N/cm2). However, solder droplet was de-posited manually with a precision micro-syringeand then laser irradiated one droplet at a time.This technique worked well in our in vitro model,but the whole process was very time consuming.This small droplet soldering technique could bemade feasible in a clinical setting by automatingsolder deposition with a microjet device. The mi-crojet technology has been used in non-weldingprojects to deposit small amounts of biological flu-ids [14–16]. The microjet can precisely controldroplet volume as small as tens of a nanoliter [17].The microjet technology may provide a viablefluid dispensing technique for laser soldering.

In this study, the feasibility of dispensing inalbumin-based solder with a microjet was inves-tigated. The stability of solder-tissue fusion wasevaluated by exposing the soldered specimens to ahydrated environment.

MATERIAL AND METHODSSolder Composition

The protein solder was made based on therecipe from Pohl and Bass [18]. It was composedof 25% human serum albumin and 0.5% sodiumhyaluronate dissolved in deionized water. An ab-sorbing dye, Indocyanine Green (ICG), with a con-centration of 2.5 mg/mL was added to enhancesolder light absorption from a diode laser at 808nm. Similar solder formulation has been used inother studies [1,9,19].

Successive Small Droplet Deposition

The setup of the microdroplet experiment isshown in Figure 1. The microjet nozzle was aimed

perpendicularly at the tissue substrate. The opti-cal fiber was oriented at about 30° from the mi-crojet and aligned with the nozzle target region. A60 mm diameter orifice was used in conjunctionwith a solenoid driven microjet (MicroFab Tech-nologies, Inc., Plano, TX). The microjet controllerregulated the delay and timing of the microjet andthe laser. The microjet settings used in this studywere driving voltage of 35 V, pulse duration of 6.4ms, back-pressure of 70 PSI, and repetition rate of1 Hz. The microjet was not able to dispense theviscous solder. Therefore, a 1:8 dilution withdeionized water was used. ICG was added afterdilution of the solder. The microdroplet volumewas 0.2–0.3 mL/drop with a droplet diameter ofabout 700 mm. The laser source was a 808 nmdiode laser. A pulse generator triggered both thelaser and the microjet controller. Laser light wascoupled into a 400 mm fiber. Since the optical fiberwas aimed at an angle, the laser spot diameter atthe target was 2.5–3.5 mm. Each droplet was ex-posed to a stationary laser beam with a fixed laserexposure of 140 W/cm2 for half a second. Since thelaser beam diameter exceeded the solder dropletdiameter, not all of the laser irradiance was ab-sorbed by the solder droplets. Five microdropletsof solder were deposited and coagulated on eachtissue specimen. Both rat skin dermis and bovineintima were used for these experiments.

Tissue Fixation and Hydration

Following the experiments, a full thicknessincision was cut at the center of the solderedspecimen exposing the solder-tissue cross-section.One half was immediately fixed in 1% glutaralde-hyde and the other was soaked in phosphate-buffered saline (PBS) for the prescribed observa-

Fig. 1. Setup for microjet laser soldering experiment.

214 Chan et al.

tion period before fixation. The observation peri-ods were 1 hour, 1, 2, and 7 days. One specimenfrom each tissue type was used for each hydrationperiod.

SEM Analysis

The tissue preparation for SEM analysis in-cluded specimen dehydration in graded ethanols(25, 50, 75, 95, 100, and 100%), critical point dry-ing, and specimen coating with a thin layer ofgold-palladium. SEM analysis was performed ona Philips 515 scanning electron microscope.

RESULTS

Successive Small Droplet Skin Specimens

Small protein volume was achieved with themicrojet setup as illustrated in Figure 1. We useda microscope to monitor the jetting process. Wenoted that when the micro solder droplets depos-

ited on a tissue substrate, the solder would re-main in its liquid form. Liquid solder left unco-agulated would eventually dehydrate and bestuck on the substrate surface. Once rehydrated,however, the uncoagulated dried protein solderwould dissolve in water.

In this study, the thermal coagulum wascomposed of five solder droplets with thickness of50–70 mm. The control and hydrated specimenswere well attached to the tissue substrate. Theleft panel of Figure 2 shows an oblique view of thesolder-tissue cross section. The solder was wellcoagulated onto the dermis by the laser radiationbetween each droplet. There was some vaporvacuoles that likely resulted from the boilingof solder mixture during photocoagulation. Athigher magnifications, fusion of solder and der-mal substrate was observed (Fig. 2, right panel).Figure 3 shows the difference between native skincollagen fibers (left panel) and the fusion of solder

Fig. 2. Successive droplet rat skin specimen, after 1 day of hydration. As shown in the right hand panel, fusion of solder-tissueinterface was achieved. Original magnification: left, ×44; right, ×200. Scale bar: left, 500 mm; right, 111 mm.

Laser Assisted Soldering 215

and collagen fibers (right panel). Using fusion ofcollagen fibrils as an indication for tissue damage,the extent of underlying coagulation was approxi-mately 250–400 mm. Direct light absorption byaorta tissue at 810 nm is minimal. Therefore,most of the thermal damage was likely a result ofthermal diffusion from the heated solder. Figure 4shows that the solder was still attached to a ratdermis specimen after 7 days of hydration.

Successive Small Droplet Intima Specimens

The overall solder coagulum thickness wasabout 50–100 mm. All the control and hydratedspecimens adhered well to the aortal surface.Steam vacuoles were often noted at the solder(Fig. 5, left panel). Fusion of solder and aortalcollagen fibers produced a stable solder-tissuebond (Fig. 5, right panel).

DISCUSSION

In a previous study, we noted that when ahigh temperature gradient generated across thesolder thickness by using conventional solderingtechnique, even though the solder surface may ap-pear to be coagulated, under-coagulation of soldercould occur at the solder-tissue interface. Under-coagulated solder-tissue interface can cause pre-mature solder detachment in a hydrated environ-ment [12]. Soldering with microdroplets mini-mizes solder temperature gradient that assures amore uniform and thorough solder-tissue coagu-lation providing a more predictable solder adhe-sion to the tissue substrate. The microjet allowsmicrodroplets of solder to be deposited at the tis-sue substrate. Each droplet can be totally coagu-lated by fixed laser parameters. To establish asizable coagulum, successive droplets can be de-

Fig. 3. Successive droplet rat skin specimen, after 1 day of hydration. Original magnification: ×5,000. Left panel is nativecollagen fibers and right panel is fused solder-collagen matrix. Scale bar: 5 mm.

216 Chan et al.

posited. Complete coagulation avoids the detach-ment problem associated with under-coagulatedsolder at the solder-tissue interface.

For conventional laser soldering, tissue ther-mal damage depends on how solder is distributedover tissue and the amount of laser energy ab-sorbed by the solder and the surrounding tissue.Light dosimetry control can be quite subjective.Excessive tissue heating may happen during sol-dering especially when laser energy is not care-fully regulated. Poppas et al. [9] conducted astudy by using a temperature-controlled lasersystem to regulate light dosimetry for soldering.However, like most conventional soldering stud-ies, they also used visual feedback for end-pointcontrol. In a porcine skin model, they reportedcoagulation depth ranging from 2–3 mm depend-ing on the controlled temperature. It should benoted that they used a 1.3 mm laser and solder

without dye added, so the underlying tissue wouldalso directly absorb the laser irradiation.

The microjet soldering technique utilizespulses of laser energy to coagulate each micro sol-der droplet. From a previous study, the opticaldepth of solder with the same ICG concentrationwas determined to be 32 mm [20]. In this study,the laser spot size of 3 mm is much larger than thesolder optical depth. Therefore, the thermal diffu-sion time, t, can be expressed by

t =d2

4a,

where d is the optical depth of solder and a is thethermal diffusivity [21]. Using water thermal dif-fusivity, a 4 0.15 mm2/sec, the thermal diffusiontime is approximately 1.7 msec. Since the pulse

Fig. 4. Successive droplet rat skin specimen after 7 days of hydration. The arrows define the solder-tissue interface. Originalmagnification: left, ×44; right, ×208. Scale bar: left, 500 mm; right, 125 mm.

Laser Assisted Soldering 217

length used in this study (0.5 sec) is longer thanthe diffusion time, the thermal energy penetratesbeyond the solder into tissue during each laserpulse. We used a laser irradiance of 140 W/cm2 toensure a reliable coagulum established betweenthe solder and tissue substrate within a short ex-posure time. As multiple solder droplets and laserpulses were delivered, more energy was diffusedinto the tissue substrate. We noted that the un-derlying tissue coagulation was approximately250–400 mm. The depth of damage is a result ofheat conducted from the irradiated droplets.Minimal thermal damage to underlying tissue isdesirable for dye-enhanced tissue soldering. Webelieve that the depth of coagulation with thistechnique can be minimized once the laser andsolder parameters are optimized. For example,minimizing irradiation time will reduce depth ofdamage.

Another factor that may have contributed to

solder heating is the dynamic changes of solderoptical properties. We noted that as each solderdroplet was photocoagulated, the green tint of theICG solder became darker, suggesting that mois-ture in the solder evaporated. As a result, localICG density increased, which made the solder co-agulum more efficient in absorbing laser light.Therefore, the temperature at the soldered areaprogressively increased with successive solderdroplet and laser irradiation. Eventually, conduc-tion of heat generated in the solder coagulumcaused coagulation of the underlying tissue. Thisfurther assured a reliable fusion between the sol-der coagulum and the underlying tissue, as wellas between successive solder droplets.

A solenoid driven microjet was used for theseexperiments. The smallest droplet volume depos-ited by this microjet was 0.2 mL. Moreover, whenrunning at frequency beyond 10 Hz, heat gener-ated by the jetting mechanism clogged the orifice

Fig. 5. Successive droplet bovine intima specimen, after 48 hours of hydration. Fusion of solder-tissue interface was noted.Original magnification: left, ×97; right, ×291. Scale Bar: left, 500 mm; right, 167 mm.

218 Chan et al.

with thermally denatured solder. Friction gener-ated by opening and shutting the valve at a highrepetition rate elevates solder temperature to apoint that caused coagulation of solder in the mi-crojet. The maximum jetting frequency withoutclogging was 2 Hz using a dilution ratio of 8:1.

A more promising technology of piezo-electric microjet may alleviate this problem. Thistechnology will allow a higher repetition ratewithout causing premature solder heating insidethe orifice. Unlike a mechanical valve used by thesolenoid microjet, this technology will employ apiezo-electric crystal to dispense the fluid. More-over, the droplet volume can be controlled moreprecisely since the piezo-electric microjet is ca-pable of jetting droplets in the order of a nano-liter.

It is our vision to develop an automated sol-dering system that can jet viscous solder. As men-tioned earlier, we have performed tissue repairsusing the accumulative small droplet techniquewith a microsyringe to deposit viscous solder [13].There are also reports that have shown higheralbumin density may provide more stable repairsin soldering [22–23]. Such a device may allow con-sistent microdroplets of viscous solder to be irra-diated by laser energy that ensures sufficient sol-der coagulation. Furthermore, this device may beused without active feedback control of laser ex-posure. Therefore, tissue repairs can be per-formed on hard-to-reach lesions where visual and/or other means of feedback are difficult.

This preliminary study has demonstratedthat the successive microdroplet method can pro-vide stable solder fusion to tissue substrates. Nev-ertheless, steam vacuoles in the solder coagulummay still produce suboptimal welds. A denselypacked coagulum likely provides a stronger weldthan one filled with vacuoles. Optimizing the la-ser parameters could minimize these vacuolesand reduce underlying tissue damage. Furtherstudy in characterizing weld strength using thistechnique is underway.

CONCLUSIONS

The feasibility of performing laser solderingby successively coagulating micro solder dropletshas been investigated. SEM analysis demon-strated that stable solder-tissue adhesion evenwhen the soldered tissue was exposed to a hy-drated environment. Underlying tissue damage of250–400 mm was noted in this study. Optimiza-tion of laser and ICG parameters may minimize

tissue thermal damage. This technology holdspromise to automating tissue soldering withoutusing an active feedback control mechanism tomonitor the soldering process.

ACKNOWLEDGMENTS

We thank Dr. Pascal Valax for his sugges-tions and ideas, Dr. Lawrence Bass, Dr. DieterPohl, and Dr. Dragon Dimitrov for the lessons onlaser soldering, and Mr. John Mendenhall for hisassistance on SEM analysis. Ashley J. Welch isthe Marion E. Forsman Centennial Professor inEngineering.

REFERENCES

1. Kirsch A, Miller M, Hensle T, Chang D, Shabsigh R, Ol-soon C, Connor J. Laser tissue soldering in urinary tractreconstruction: First human experience. Urology 1995;46:261–266.

2. Auteri J, Jeevanandam V, Oz M, Libutti S, Kirby T,Smith C, Treat M. Tracheal anastomosis using indocya-nine green dye enhanced fibrinogen with a near-infrareddiode laser. SPIE Proceedings 1990; 1200:60–63.

3. Bass L, Oz M, Auteri J, Williams M, Rosen J. Libutti S,Eaton A, Lontz J, Nowygrod R, Treat M. Laparoscopicapplications of laser-activated tissue glues. SPIE Pro-ceedings 1991; 1421:164–168.

4. Libutti S, Williams M, Oz M, Forde K, Bass L, WeinsteinS, Auteri J, Treat M, Nowygrod R. Preliminary resultswith sutured colonic anastomoses reinformed with dye-enhanced fibrinogen and a diode laser. SPIE Proceedings1991; 1421:169–172.

5. Springer T, Welch AJ. Temperature control during laservessel welding. Applied Optics 1993; 32:517–525.

6. Klioze S, Poppas D, Rooke C, Choma T, Schlossberg S.Development and initial application of a real time ther-mal control system for laser tissue welding. J Urol 1994;152:744–748.

7. Stewart R, Benhrahim A, LaMuraglia G, Rosenberg M,L’Italien G, Abbott W, Kung R. Laser assisted vascularwelding with real time temperature control. Lasers SurgMed 1996; 19:9–16.

8. Pearce J, Thomsen S. Rate Process Analysis of ThermalDamage. In: Welch AJ, van Gemert MJC, eds. ‘‘Optical-Thermal Response of Laser-Irradiated Tissue.’’ NewYork: Plenum Press, 1995; pp. 561–608.

9. Poppas D, Stewart R, Massicotte J, Wolga A, Kung R,Retik A, Freeman M. Temperature-controlled laser pho-tocoagulation of soft tissue: In vivo evaluation using atissue welding model. Lasers Surg Med 1996; 18:335–344.

10. Cilesiz I, Thomsen S, Welch AJ. Controlled temperaturetissue fusion: Argon laser welding of rat intestine in vivo,Part One’’ Lasers Surg Med 1997a; 21:269–277.

11. Cilesiz I, Thomsen S, Welch AJ, Chan E. Controlled tem-perature tissue fusion: Ho:YAG laser welding of rat in-testine in vivo. Lasers Surg Med 1997b; 21:278–286.

12. Chan E, Brown D, Kovach I, Welch AJ. Effects of hydra-tion on laser soldering. J Biomed Optic 1998 (in press).

13. Chan E, Welch AJ, Springer T, Shay E, Frederickson C,

Laser Assisted Soldering 219

Motamedi M. Accumulative small droplet laser soldering:Tensile strength and scanning electron microscopyanalysis. Proceedings of SPIE Biomedical Optics, 1998;3245D-40.

14. Wallace DB, Hayes DJ, Frederickson CJ, Howell G. Theapplication of ink-jet technology to neuroscience researchand biomedical research. In: Diller TE, Hochmuth RM,Cho YI, eds. Bioprocess Engineering Symposium. ASMEPress. 1989; 153–158.

15. Bernardini GL, Rampy BA, Howell GA, Hayes DJ, Fre-derickson CJ. Applications of piezoelectric fluid jettingdevices to neuroscience research. J Neurosci Methods1991; 38:81–88.

16. Jennett E, Motamedi M, Rastegar S, Frederickson CJ,Arcoria CJ. Dye enhanced ablation of dental tissue bypulsed laser. J Dent Res 1994; 73:1841–1847.

17. Frederickson D. Technical communication with EKC. 1996.18. Pohl D, Bass L. Technical communication with EKC.

1995.

19. Bass L, Treat M. Laser tissue welding: A comprehensivereview of current and future clinical applications. LasersSurg Med 1995; 17:315–349.

20. Chan E. Laser Tissue Welding: Effects of solder coagula-tion and tissue optical properties. Doctoral dissertation.The University of Texas at Austin. 1997.

21. Van Leeuwen TG, Jansen ED, Motamedi M, Borst C,Welch AJ. Pulsed laser ablation of soft tissue. In: ‘‘Opti-cal-Thermal Response of Laser-Irradiated Tissue.’’ WelchAJ, van Gemert MJC, eds. New York: Plenum Press,1995; pp. 709–763.

22. Massicotte JM, Stewart RB, Poppas DP. Designing purealbumin tissue solders for laser welding: The effect ofnative absorption on acute tensile strength. Lasers SurgMed. 1997; Supplement 9. Abstract 209.

23. Lauto A. Repair strength dependence on solder proteinconcentration: A study in laser tissue welding. LasersSurg Med 1998; 22:120–125.

220 Chan et al.