Study Of Laser Surface Modification In Biomedical Applications

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How laser surface modification occurs

Selection of a surface material that has suitable electrical, thermal, magnetic and optical properties and adequate resistance to degradation, corrosion and wear, is very important to its functionality. When the surface of an engineering component cannot sufficiently withstand external environmental conditions or forces it is exposed to then that component is likely to fail in performing its intended function. Therefore surface requirements are very important in the field of engineering. Surface engineering is a branch of materials science that focuses on study of solid matter surfaces. One of the main targets of surface engineering is to improve functionality of products. The main potentials of surface engineering are: improved functionality, reduced waste, reduced power consumption, likelihood of creating new and better products, resolve some of the formerly challenging engineering problems, and preservation of scarce materials (University of Sheffield, 2017).

Laser surface modification is a technology used to produce hybrid materials that have wide-ranging and improved functionality (Kwok, 2012; Vilar, 2016). This technology makes it possible to modify materials so as to attain desired surface properties. The technology is applicable to different materials including metals, ceramic, plastics/polymers and composites (Mittal and Bahners, 2014). Some of the applications of laser surface modification are: improved mechanical properties (fatigue and hardness), increased corrosion resistance, increased adhesion, improved electrical properties, improved fabricability/machinability, improved tribological properties (abrasive and water resistance), improved heat or thermal resistance properties, increased strength, reduced friction, increased resistance to chemical attack, and improved aesthetic look, among others.

One of the fields that have found comprehensive applications of laser surface modification is the healthcare industry. This technology has numerous biomedical applications, especially in the manufacture of biomaterials such as implants. Surface properties of biomaterials play a very important role in determining how implants respond to the biological environment. Techniques of laser surface modification improve tribological and mechanical properties of materials (Adesina, Popoola and Fatoba, 2016). This makes them suitable for making desired biomaterials that perform the intended function effectively without having negative impacts on the patient. The effectiveness of biomaterials is largely influenced by the surface properties of these biomaterials, including surface morphology, mechanical properties, composition, microstructure, surface free energy and wettability (Bandyopadhyay et al., 2011). Therefore laser surface modification is very essential in improving surface properties of biomaterials so as to make them compatible with surrounding body tissues and cells (Bandyopadhyay, Sahasrabudhe and Bose, 2016). Laser surface modification is also used to make other biomedical products, including equipment and devices used in healthcare facilities. In general, laser surface modification optimizes performance of a material for a given biomedical application (Brown and Arnold, 2010). This report discusses laser surface modification in biomedical applications.

Basically, the process of laser surface modification involves directing a laser beam to a material substrate (Earl et al., 2015). This interaction between the laser beam and material substrate modifies the surface properties of the material thus making it suitable for the intended function. Lasers are commonly used in surface modification of different materials because of their high energy density, directionality and high coherence (Ganjali, 2015). Lasers are also among the most powerful and versatile tools used in material processing. These lasers produce high density heating sources that can be easily controlled and finely manipulated. Laser surface modification process takes place within a definite environment, which can be protective, vacuum or filled with processing gases. A resonator is used to generate light, which is then directed onto the material’s surface through optical transmission systems, such as fiber optics or mirror systems. The process starts from the required power density (i.e. a specified average optical output energy) then the distribution of power intensity all over the beam is adjusted using beam shaping or beam focusing optics, such as beam integrators, scanner units, mirrors or lenses (Bell, Dong and Li, (n.d.)).  As the laser beam moves over the material surface, it is possible to generate a track pattern on the surface of the material. Interaction time also has to be calculated using the feed rate and the beam’s cross-section. In simple terms, interaction time is the duration of directing the laser onto the surface of the material being modified. This time is very useful in ensuring that the desired surface properties are achieved. Relative movement of the beam is also achieved by using robots, portal systems or translation stages, depending on the geometry of the component and type of laser surface modification being used. At the end of interaction time, the surface will be ready with the desired properties.

Material Properties Influenced by Laser Surface Modification

There are several material properties that can be changed with laser surface modification (Ahmad, 2011). These properties include: surface depth, hardness, roughness, thermal fatigue, absorptivity, corrosion resistance, adhesion, thermal and electrical conductivity, fabricability/machinability, water resistance, abrasion, strength, chemical attack resistance, aesthetic look, etc. These properties are very important in biomedical applications and that is why laser surface modification has become a very crucial topic in healthcare industry.

There are two main categories of laser surface modification techniques: those that change the composition of the material and those that do not change the composition of the material. The following are some of the laser surface modification techniques commonly used in biomedical applications:

This technique involves melting the surface using a laser beam then re-solidifying it rapidly without adding any external material elements directly with an aim of modifying the surface’s chemical composition. This technique is accomplished through several processes. It starts with region near the surface reaching melting point followed by movement of liquid/solid interface. After that, elements start diffusing in the liquid phase. Inter-diffusion continues them the material starts re-solidifying very quickly, creating a modified layer on the material surface (Biswas, 2007). This technique is widely used in biomedical applications because of the following advantages: it does not use foreign materials that could stress force or early material failure; it produces a homogenous surface; it uses less parameters hence results are repeatability; it is cheaper because it uses less energy and materials than other laser surface modification techniques; it produces surfaces with great corrosion resistance properties; and it creates crack-free surfaces (Baker, 2009; Chikarakara, 2012).

This technique uses high power density obtained from laser beam sources and focused on the material surface. The power is used for melting alloying elements that are externally added and underlying substrate (Chikarakara, 2012). The addition of extraneous alloying elements changes the surface’s chemical composition. The alloying element can be deposited through two main ways: pre-deposition and co-deposition. Pre-deposition is where the alloying element is added before laser irradiation, whereas co-deposition is where the alloying element is added directly during laser irradiation. The allow element is usually deposited in form of powder, gas or wire (Brandl et al., 2008; Filip, 2011).

This technique aims at reducing grain sizes of materials, which then increases the tribological properties, hardness and strength of the material (Basu, Katti and Kumar, 2009). Glazing is a process used to make solids with no crystalline structures. The technique of laser blazing is performed by rapidly scanning a beam with adequate intensity across the material surface. The material gains adequate heat through conduction. This is then followed by cooling the surface rapidly resulting to sufficient microstructural modification. This change in microstructure of the material improves the material’s compressive strength, corrosion and wear resistance, and hardness properties (Matthews, Ocelik and Hosson, 2007).  

This is a melting process where a substrate material is fused with a material having different metallurgical properties, as show in Figure 1 below (University of North Dakota, 2017). It is as good as laser surface alloying only that substrate dilution is very minimal (less than 10%) while dilution in laser surface alloying is more than 10%. The main advantages of laser surface cladding are: great flexibility, optimum bonding, low thermal load and minimal distortion on the material being modified. After cladding, the material requires very minimal treatment. However, this technique also has some shortcoming, including: high cost, producing uncontrollable cracks and poor reproducibility. Uncontrollable cracks are produced because the melted pool’s cooling rate is usually very high, while poor reproducibility because of difficulties in ensuring that the material dissolves uniformly.  

Laser Surface Modification Techniques

This technique also modifies the material’s surface structure. The techniques uses laser of high power intensity in combination with appropriate overlays to produce shock waves containing high pressure on the material’s surface. A coat of black paint is usually applied on the surface then it is covered with a layer of glass or water. When the coated surface is struck by the laser beam, the black paint becomes heated and vaporizes instantly. The laser beam radiation that remains gets absorbed by the vapor to produce plasma. The plasma generates a high pressure on the material surface. The plasma, in form of a shock wave, gets transmitted into the material and induces compressive stress that penetrates into and strengthens the material’s surface (Rozmus-Gornikowska, 2010). This can cause changes to the microstructure of the material, increase dislocation density, change the material surface’s roughness and introduce compressive stresses in the material surface. However, this technique has minimal impact on the corrosion and wear properties of the material, compared to other laser surface modification techniques.

Figure 2 below shows different classifications of laser surface modification techniques, depending on their power density and interaction time (Chikarakara, 2012)

Figure 2: Classification of laser surface modification techniques

There are several advantages of laser surface modification techniques. Some of these include the following:

Laser surface modification techniques use less energy than conventional surface modification methods such as flame hardening and carburizing. These techniques restrict heating only to the needed area and to a small volume and shallow layer, thus preventing energy wastage and reducing energy consumption. The techniques also cause very small dimensional changes or deformations of the material hence reducing the need for final machining of the material by grinding. This also saves energy.

Laser surface modification techniques can be used on any type, shape or size of material, including those with amorphous and non-equilibrium structures. The power input can also be widely varied by adjusting the power source of laser, using focusing lenses with different focusses or gradations of defocus. The laser beam can also be easily switched between workstations using simple optical devices.

Laser surface modification techniques can be automated to improve precision of surface modification process and increase quality of the final product. The beam can be directed over the material surface using a computer thus eliminating possibilities of inaccuracies. Automation also ensures that the material surfaces are accurately modified with desired width and depth of the layer.  

Laser surface modification techniques produce surfaces with standardized and fine-grained microstructures. This means that the microstructures remain intact hence the material maintains its original internal mechanical and tribological properties.

These techniques have very minimal thermal damage to the material being damaged. The amount of heat that is subjected to the material is predetermined and controlled, leaving no room for exposing the material to excess heat. This also helps in achieving and maintaining the desired properties of the material.    

This is another major advantage of laser surface modification techniques because the distortion and grain growth of these techniques are negligible, compared to conventional methods. If the process is performed until appropriate conditions, the distortion and grain growth cannot be noticed, which helps in maintaining the mechanical and tribological properties of the material (Aqida et al., 2008).

Laser surface modification techniques are cost-effective methods both for mass production or individual production of components (Montealegre et al., 2010). These techniques reduce total cost of modifying surface properties through consumption of less energy, requirement of few workers, use of less space and use of few equipment.

Laser surface modification techniques are also less time-consuming as they can modify surfaces to attain desired properties very quickly. These techniques are automated and therefore very minimal time is required to complete the surface modification process. The processing speeds can also be increased thus reducing interaction time.

There are also some limitations associated with laser surface modification techniques. First, sometimes the laser beam’s energy distribution can be non-homogenous. This may affect the evenness and quality of the final component that is being modified. Second, those techniques that change the composition of the material can also cause microstructural structure. If this happens, unintended properties of the material may be altered thus affecting the functionality of the material. Third, laser’s absorption by the material surface may be poor resulting into failure to attain desired surface properties. Last but not least, these processes are relatively new and therefore many people are not familiar with them. This makes it quite challenging to resolve some of the unanticipated issues that may arise.

The main variables in laser surface modification process are: laser power, beam configuration and diameter, velocity of work-piece, condition of substrate (absorptivity, temperature and roughness), composition of alloy element, and work-piece’s thermo-physical properties (Majumdar and Manna, 2009). Besides this, there are several other factors that influence the final properties of the material surface that is being modified. These factors include the following:

Biomedical applications require materials with particular kinds of microstructure. The microstructure influences the response of material to laser surface modification process. The microstructure of the material also helps in selecting the most appropriate laser surface modification technique. Therefore original properties of the material has a significant impact on the final properties of the material after it has been modified by laser beam.

There are two main operating modes that are used in laser surface modification: pulsed mode and continuous mode. Pulsed mode is usually complex and has more variable parameters than continuous mode. The operating mode chosen also influences absorptivity, grain structures or refinement, melt depths, residence time, etc.

The amount of laser power used has direct impacts on the resultant structure of the biomedical materials being modified. The power has to be directed to the exact spot so as to prevent wastage. In general, laser power and intensity influences the anticipated temperature increase of the spot and the material’s melting intensity.

Residence time refers to the total amount of time that the material is in contact with the laser beam. This time is determined by the sample speed and size of laser beam spot. Residence time has to be predetermined so as to avoid premature or over modification of the material surface.

Overlap refers to the ratio of spot’s diameter and the distance between 2 successive laser spots. The overlap percentage has to be controlled because it has an impact on the work-piece’s heat build-up that can cause preheating of the material prior to succeeding tracks.

For the desired surface properties to be achieved, energy density must be controlled. This ensures that the surface gets the right amount of energy per unit time and area.

Biomaterials refer to synthetic materials that are used for manufacturing devices aimed at restoring or replacing functions of body tissues. There are 4 classes of biomaterials, which are based on the compatibility of these materials with the body tissues that surround them (BIOFABRIS, 2014). These classes are:

These are biomedical implants that are not in direct contact with the surrounding tissues. The separation is done by a soft tissue layer. There is no contact within the osteogenesis and the implant induces the layer by releasing ions, monomers or corrosion products.

These are biomedical implants that are in direct contact with the surrounding tissues. There is contact within the osteogenesis even though no chemical reaction takes place between the implant and the tissues.  

These are biomedical implants that interact directly with the surrounding tissues, and they interfere with osteogenesis directly. They promote osteoconduction because the implant is bound to the tissues’ mineral components.

These are biomedical implants that are in direct contact and interaction with surrounding tissues. With time, the body starts degrading, solubilizing or phagocytizing the implants.

Metals

Metallic biomaterials are the most common biomedical implants. They are used in orthopedics, maxillofacial, dental and cardiovascular surgeries. They comprise of metals and metal alloys, such as steel, titanium, cobalt, etc. They have great biocompatibility, thermal conductivity, fatigue and static strength, hardness, shock resistance and low friction properties.

They have great wear resistance, hardness, corrosion resistance and biocompatibility properties. They are also mainly used for making dental restorative materials such as dentures, cements and crowns. They are also used for joint replacements, augmentation and bone repairs. Examples of ceramic biomaterials are zirconia and alumina.

Different types of polymers are used for making biomaterials that have varied application including tracheal tubes, facial prostheses, liver, kidney and heart components, dentures, knee and hip joints, and medical sealants, adhesives and coatings. They have great biocompatibility and corrosion resistance properties. Examples of polymers are polyethylene, polytetrafluoroethylene and acrylic.  

These are biomaterials comprising of one type of material. They are largely used in dentistry as dental cements or restorative materials, joint replacement, bone repair, etc.

These are biomaterials derived from plants or animals. Their potential has led to extensive study of biomimetic. They are usually more compatible with body tissues and cells than synthetic biomaterials.

Laser surface modification has facilitated development of numerous biomedical implants. These implants have a variety of applications, including the following:

This largely comprises of hip replacement and knee implants. Hip replacement involves partial or total hip joint replacement through orthopedic surgery. Biomedical implants are used to replace the thigh bone (upper femur) and hip bone (mating pelvis) area, and other components of the hip. Laser surface modification enhances manufacturing of femoral ball, femoral stem and polymeric socket, which are used for making artificial hips. In knee implants, biomaterials are used for manufacturing polymeric materials or metal alloys that replace thighbone (lower femur), shinbone (tibia) and kneecap (patella) that are used in total knee arthroplasty. Some of the diseases treated by orthopedic biomaterials include: bone fractures, chronic joint pain, scoliosis, osteoarthritis, spinal stenosis, etc.

Biomedical materials are used to make different heart implants such as pacemakers, valves, vessels, artificial hearts, stents, etc. These devices are used for treatment of different heart problems such as valvular heart disease, heart failure, angina pectoris, ventricular tachycardia, etc.

Biomedical implants are used to restore or replace teeth and this is very common nowadays.

These implants are used for treating disorders that affect the brain and major senses. They are used for treating visual impairments (including glaucoma, cataract and keratoconus), hearing loss problems (such as otosclerosis and otitis media), and several neurological issues (such as depression, Parkinson’s disease and epilepsy). They include artificial cornea and eye and contact lenses.

Prosthetics (cosmetic implants) have varied applications especially in restoring aesthetic form of damaged body parts. They are largely used in mastectomy (that is caused by breast cancer), modifying body parts (such as buttocks and chins), and correcting disfigurements. Examples of cosmetic implants are breast implants, injectable filler, ocular prosthesis, nose prosthesis, etc.  

The basic use of contraception implants is to prevent pregnancy. They are also used for treating certain conditions like menorrhagia. An example of contraception implants are intra uterine devices.

Biomaterials (suture materials) are used to enhance wound closure and fracture fixation. They include wires, screws, rods, plates and nails made from metals and polymers.

Biomedical implants are also being used for quick and controlled delivery of drugs to targeted points in the body.

Other biomaterials are used for and/or as: vocal cords, artificial skin, spinal cord, pancreas, kidney, liver, lung, cochlear implants, spine screws, ear tubes, etc.

Conclusion

The potential of laser surface modification in biomedical applications is great and that is why it has attracted the interest of many stakeholders in the healthcare industry. Laser surface modification makes it possible, easier and cost-effective to build customized and complex biomedical structures. This process is applicable to different biomaterials including metals, polymers, ceramics and composites. It creates biomedical components that have suitable tribological and mechanical properties, and which are biocompatible with the body tissues and cells of the implant recipients.  

Biomedical applications require components that have specific surface properties, including high corrosion resistance, improved mechanical properties, increased adhesion, improved electrical properties, improved fabricability/machinability, improved tribological properties (abrasive and water resistance), improved heat or thermal resistance, increased strength, reduced friction, increased resistance to chemical attack, and improved aesthetic look. In most cases, components whose surfaces have these properties are very expensive hence there is need to develop approaches of reducing these costs. Laser surface modification is a widely used method of improving material surface properties for biomedical applications because of its capability of producing biomedical components quickly, efficiently and cost-effectively. There are different techniques of laser surface modification. Each technique has unique advantages and limitations hence it is important to explore them before selecting the best technique. If properly used and with the ongoing research and development programs and projects, laser surface modification will continue improving lives of people all over the world.

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