Of Advanced Biomaterials

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laser.qxp 3/17/2005 10:56 AM Page 1 LASER PROCESSING OF ADVANCED BIOMATERIALS Lasers are ideal for depositing diamondlike carbon films, microfabricating “lab-on-a-chip” biosensors, and building medical device nanostructures. Roger J. Narayan*, Chunming Jin, Tim Patz, Anand Doraiswamy Georgia Institute of Technology, Atlanta, Georgia Rohit Modi, Douglas B. Chrisey U.S. Naval Research Lab, Washington, D.C. Yea-Yang Su Amorphous Oxide Technology, Atlanta, Georgia S. J. Lin Veterans General Hospital, Taipei, Taiwan Aleksandr Ovsianikov, Boris Chichkov Laser Zentrum Hannover, Hannover, Germany

ince their invention in the early 1960s, lasers have found many uses in the medical field. Most current medical applications of lasers involve service as a “uni- Sversal scalpel” in minimally invasive surgeries, 1 µm which offer little contact, little blood loss, shorter operating times, and less postoperative pain than Fig. 1 — Top: scanning electron micrograph of anodized conventional techniques. Laser in situ ker- alumina membrane. Middle: scanning electron micrograph atomileusis (LASIK) refractive corneal surgery, of anodized alumina membrane exposed to platelet rich coagulation for retinal detachment, and skin treat- plasma. The surface exhibits significant protein adsorp- tion and pore fouling. Bottom: scanning electron micrograph ments are only some of the current applications of anodized diamondlike carbon-coated alumina membrane for lasers in medicine. exposed to platelet rich plasma. The surface contains sodium In the future, lasers will also serve to create chloride crystals; however, the pores remain free of the fouling novel medical devices with unique biological seen in the middle image. functionalities. Microstructured and nanostructured biomaterials have recently been fabricated ical implants for over thirty years. The first carbon via pulsed laser deposition, laser direct writing, biomaterials were low-temperature isotropic (LTI) and two photon-polymerization processes. In this pyrolytic carbons. These dense, isotropic ma- article, we will review the current status of laser terials are deposited from a hydrocarbon gas in processing in the development of ceramic a fluidized-bed reactor at temperatures below nanocomposite films, tissue engineered ma- 1500ºC (2730°F). In 1969, Dr. De Bakey introduced terials, and nanostructured medical devices. a pyrolytic carbon-coated aortic valve prosthesis, which incorporated carbon-coated metal struts Diamondlike carbon thin films and a carbon-coated hollow ball. These carbon- Carbon biomaterials have functioned in med- coated valves demonstrated biocompatibility, in- *Member of ASM International ertness, and immunity to fatigue, and are the com- ADVANCED MATERIALS & PROCESSES/APRIL 2005 39 laser.qxp 3/24/2005 10:30 AM Page 2

Imaging that accelerates the release of silver ions. Several studies have shown that diamondlike carbon can greatly improve the hemocompati- Optical bility of a medical device. Figure 1 is a scanning Synchronized illumination laser electron micrograph of a diamondlike carbon- pulse coated anodized alumina membrane placed in UV transparent platelet-rich plasma. Unlike many metal or ribbon polymer surfaces, diamondlike carbon-coated Thin film, surfaces do not adsorb fibrous proteins or cellular < 200 nm µ material. In fact, only sodium chloride crystals z 50 m y were observed on the surface. In-vitro studies CAD/CAM motion-controlled x stage demonstrate that platelets are activated less often and platelets adhere less frequently on diamond- Fig. 2 — Left: schematic of matrix assisted pulsed laser evaporation-direct like carbon-coated surfaces than on titanium, ti- write (MAPLE-DW) system. Right: confocal micrograph of MAPLE-DW transferred B35 neuroblasts within an extracellular matrix scaffold. Axonal exten- tanium carbide, titanium nitride, or stainless steel sions were observed between neuroblasts on different deposition planes, which surfaces. Therefore, diamondlike carbon-coated were spaced 30 to 40 µm apart. materials are being considered for a variety of blood-contacting medical devices, including im- Beam expander plantable microdevices, sensors, coronary artery Near IR fs pulses stents, synthetic heart valves, left-ventricular as- Femtosecond (fs) oscillator sist devices, and artificial hearts. Scanner Matrix-assisted pulsed laser evaporation z CCD camera Matrix-assisted pulsed laser evaporation- y Ormocer direct write (MAPLE-DW) is a novel technique for micrometer-scale resolution transfer of small x amounts (one pico-liter) of polymers, proteins, Immersion liquid and eukaryotic cells. The MAPLE-DW system Mask contains an excimer laser, biomaterial-seeded quartz disk, and a computer-controlled X-Y trans- Glass plate lation stage (Fig. 2). Structure Near IR fs pulses The biomaterial is solvated in a transparent matrix and spin-coated onto a quartz disk. The biomaterial-seeded quartz disk is then placed in a Fig. 3 — Left: schematic of two photon polymerization system. Right: schematic position such that the biomaterial-coated side of laser orientation in two photon polymerization system. faces the substrate. A lens then focuses the laser ponents of choice to this day. More recently, di- pulse at the biomaterial/quartz disk interface. A amondlike carbon has been considered for med- low-fluence (low-energy) ArF laser pulse (l=193 ical prostheses and surgical instruments. nm, fluence=0.01-0.5 mJ/cm2) volatilizes some of Although no universally accepted nomencla- the matrix and propels the material from the bio- ture has been agreed upon, the term “diamond- material-seeded ribbon to the underlying sub- like carbon” (DLC) often refers to amorphous strate. The computer-controlled X-Y translation carbon thin films containing some sp3 hybridized stage enables a resolution of one micrometer. atoms. These materials exhibit atomic number Laser-based processing provides several ad- densities greater than 3.19 g/cm3, and often pos- vantages over inkjet, Langmuir-Blodgett, dip- sess densities closer to that of diamond (3.51 pen, and other solvent-based techniques. These g/cm3) than that of graphite (2.26 g/cm3). include: Diamondlike-carbon/metal-nanocomposite • Enhanced biomaterial-substrate adhesion. films demonstrate enormous potential for med- • Deposition can be carried out under ambient ical device coatings. Silver, platinum, titanium, conditions. and silicon have been incorporated into diamond- • The amount and location of transferred ma- like carbon films by a multi-target pulsed-laser terial can be quantitatively determined. deposition process. These composite films exhibit • Multilayered structures can be prepared with exceptional wear resistance, corrosion resistance, multiple quartz disks or segmented targets. and hardness, as well as improved adhesion to We have recently demonstrated three-dimen- medical alloys. sional transfer of B35 neuroblast cells by varying In addition, silver-platinum composite films the transfer energy and the extracellular matrix have shown unique antimicrobial properties, solidification. Neutral density filters attenuated which result from the release of silver ions. Silver the incident laser beam without significantly ions bind strongly to electron donor groups on changing the laser spot size. We initially deposited oxygen-, nitrogen-, or sulfur-containing enzymes, neuroblasts with a laser fluence capable of deeply and displace cations (e.g., Ca2+) necessary for en- penetrating the extracellular matrix. Another neu- zyme function. Films containing both silver and roblast array was then deposited directly on top platinum demonstrate enhanced antimicrobial of the initial array by attenuating the laser beam activity due to formation of a galvanic couple and lowering the laser fluence. This process was 40 ADVANCED MATERIALS & PROCESSES/APRIL 2005 laser.qxp 3/17/2005 10:57 AM Page 3

repeated several times, and resulted in a 100 µm thick ECM scaffold with neuroblasts located at Pulsed laser deposition various depths. Pulsed laser deposition (PLD) is a physical vapor deposition We envision several applications for the ma- process for creating diamondlike carbon thin films with high trix-assisted pulsed laser evaporation-direct write fractions of sp3 hybridized carbon atoms. A KrF (l=248 nm) or process in next generation medical devices: ArF (l=193 nm) laser is used to vaporize a target that contains • MAPLE-DW can create localized patterns of components of the desired film. When laser radiation is ab- enzymes, cells, or pharmacologic agents for im- sorbed by the target, electromagnetic energy is converted into plantable biosensors and drug delivery devices. electronic excitation, chemical energy, mechanical energy, and • MAPLE-DW can provide a unique approach thermal energy. These processes produce a plume of ionic, for fabricating customized three-dimensional cell- atomic, and molecular species with kinetic energies on the order seeded scaffolds for tissue engineering. of 100 to 1000 kT (2.5 to 25 eV), which propagates in a forward- peaked manner toward the substrate surface. Typical growth Two-photon polymerization rates of DLC films with the KrF laser (fluence=2-5 mJ/cm2) are We have recently demonstrated direct writing on the order of 0.01 nm/pulse. of three-dimensional medical device nanostruc- Deposition of diamondlike carbon films generally involves tures by two-photon polymerization (2PP). This ablation of a high-purity graphite target; other target materials technique involves the application of light to in- have included pressed diamond powder, glassy carbon, and duce several chemical reactions between starter polymers. Pure carbon sources lead to hydrogen-free films, and molecules and monomers in a transparent resin. hydrocarbon sources lead to films that contain significant The principal reaction chain in the case of 2PP by amounts of hydrogen or hydrocarbons. Pulsed laser deposition ultraviolet light can be expressed as follows: can produce DLC films that contain up to 80% sp3-hybridized carbon atoms. Diamondlike carbon exhibits cell compatibility, υ → corrosion resistance, lubricity, and wear resistance, and can 2h UV + starter starter (or ion) starter + monomer → polymer serve as a “hermetic seal” for metal, ceramic, or polymer bio- polymer + polymer → end of reaction materials.

Ultrahigh- Femtosecond laser pulses (60 fs, < 450 vacuum mW, 780 nm) are tightly focused by a high chamber numerical-aperture objective lens into a Substrate focal volume within the photosensitive resin (Fig. 3). A galvoscanner or a three-dimensional piezo stage serves to position the Excimer laser focus within the resin. The laser passes laser beam through the out-of-focus region without Target absorption. However, within the vicinity of the focal 20 nm volume, nonlinear absorption of the laser pulses breaks chemical bonds on starter photo- Left: schematic of pulsed laser deposition (PLD) system. Right: transmis- initiator molecules. In some systems, ions instead sion electron diamondlike carbon matrix. of radicals may be generated. The radicalized starter molecules can react with monomers, re- modified silicon alkoxides and controlled hydrol- sulting in formation of radicalized polymolecules. ysis. The organic components can subsequently The reactions cease when two radicalized poly- crosslink through ultraviolet, infrared, thermal, Ormocer molecules join together. or redox-initiated processes. consists The required structures are fabricated by The photoinitiator Irgacure 369 (Ciba Specialty moving the laser focus in three dimensions; the Chemicals, Basel, Switzerland) applied in Or- of a material is polymerized along the trace of the laser mocer device fabrication exhibits an absorption crosslinkable focus. After processing, the non-irradiated resin peak at 320 nm. Ormocer materials exhibit excep- network of is washed away by an alcohol solution. The quad- tional thermal and chemical stability, high adhe- ratic character of the two-photon absorption prob- sive strength, and transparency in the infrared re- organic and ability and the well-defined polymerization gion. Furthermore, its biological properties are inorganic threshold allow the diffraction limit to be over- well known, because Ormocer resins currently components. come. The result is achievement of resolutions serve as light-cured alternatives to amalgams in better than 100 nm in the polymerized structures. restorative dentistry. Material properties can be modified by adding new functional groups or by Ormocer hybrid polymer changing inorganic/organic network density. We have developed several three-dimensional The precision of two-photon polymerization nanostructured devices built of Ormocer, an or- along with the unique properties of Ormocer allow ganic-inorganic hybrid polymer created by Fraun- the development of a wide range of medical appli- hofer-Gesellschaft (Fraunhofer-Institut für Sili- cations. For example, we have developed mi- catforschung, Würzburg, Germany). Ormocer croneedle arrays for drug delivery. Micron-scale consists of a crosslinkable network of organic and needles can increase skin permeability and improve inorganic components. The inorganic components the transdermal delivery of macromolecules. crosslink through condensation of organically Two-photon polymerization can produce mi- ADVANCED MATERIALS & PROCESSES/APRIL 2005 41 laser.qxp 3/17/2005 10:57 AM Page 4

nesses that may exhibit improved fracture properties. Arrays of these hollow microneedles could provide painless injections to people suffering from diabetes, blood clotting, or other disorders. In addition, we have developed Ormocer tissue engineering scaffolds with unique geometries (Fig. 4). These cell-seeded scaffolds could potentially be stacked like building blocks to provide layer- by-layer growth of engineered tissues.

Future of laser biomaterials Healthcare consumers continue to demand ever more complex tools for diagnosis and treat- ment of medical conditions. In the coming decades, implantable sensors, drug delivery devices, and artificial tissues will become significant instruments for medical care. Lasers have demonstrated unique capabilities for thin film processing, direct writing, cutting, and microfab- 250 µm rication of materials in medical devices. We an- ticipate that laser processing of implantable medical devices and tissue-engineered materials will Fig. 4 — Ormocer pillar array created by two-photon polymerization. These become increasingly more significant. arrays are currently being investigated for drug delivery and tissue engineering.

croneedles with a larger range of sizes, shapes, For more information: Prof. Roger J. Narayan, School and materials than is possible with conventional of Materials Science and Engineering, Georgia Insti- stainless steel or titanium microfabrication tech- tute of Technology, Atlanta, GA 30332-0245; tel: niques. In particular, the technology enables fab- 404/894-2888; e-mail: [email protected]; rication of microneedles with large wall thick- Web site: www.gatech.edu.

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