**3. Effects of laser power**

Power is one of the most influential laser parameters when surface treating a material. Adjusting the power increases the surface irregularities of the material and therefore enhan‐ ces the topography properties of the substrate [25, 33]. In this chapter, the effects of four different powers on surface topographic and oxidation properties of titanium substrates are investigated, and the bioactivity of the treated substrates is examined through the use of simulated body fluid (SBF). Simulated body fluid (SBF), or formally known as hydroxyapa‐ tite, is a supersaturated insoluble calcium phosphate mineral, with the chemical composition of Ca10(PO4)6(OH)2. This fluid has a close similarity with human blood plasma and is the essential component of the biological hard tissues such as bones. Due to the high absorbance and catalytic properties of SBF, this fluid is commonly used to estimate the biocompatibility level of materials used in implant productions.

To treat the titanium substrates, a range of low to high average laser powers from 6 to 12W was used, while frequency and scanning speeds were kept constant in all cases. The effec‐ tive number of pulses for all results was kept constant to be 25 pulses for frequency of 100 kHz [36, 37]. **Table 1** introduces the parameters used in more detail.


**Table 1.** Laser parameters used in surface treating of Ti substrates.

#### **3.1. Surface topography analysis**

increases the wettability of surface of the material. Consequently, this leads to an increase in the apatite‐inducing ability of the material and greatly improves the biocompatibility of the

The nanosecond laser used for obtaining all the results discussed in this chapter was a Nd:YAG pulsed laser system (SOL‐20 by Bright Solutions Inc). The maximum output power is 20 W with a wavelength of 1064 nm and a repetition rate ranging between 10 and 100 kHz. This laser emits pulses of 6–35 ns pulse duration. The diameter of circular output beam from the laser is around 9 mm. The diameter of beam is reduced to 8mm by using an iris diaphragm before entering to galvo‐scanner. A two‐axis galvo‐scanner (JD2204 by Sino Galvo) with the input aperture of 10 mm and beam displacement of 13.4 mm was used for beam scanning since it has a high scanning speed (to 3000 mm/s). In order to focus the normal beam to the surface of Ti, scan lens of a focal length of 63.5 mm was used. The theoretical focused laser spot diame‐ ter (*d*0) is calculated to be 20 μm. During the experiment, the spot size may be bigger due to scatter and misalignment. The average laser fluence was 900 μJ at the frequency of 10 kHz.

The scanning parameters including scanning speed, and scanning configurations can be adjusted through the software operating the laser. When the combination of parameters is adjusted with this software, along with power and frequency set for laser irradiation, the

Power is one of the most influential laser parameters when surface treating a material. Adjusting the power increases the surface irregularities of the material and therefore enhan‐ ces the topography properties of the substrate [25, 33]. In this chapter, the effects of four different powers on surface topographic and oxidation properties of titanium substrates are investigated, and the bioactivity of the treated substrates is examined through the use of simulated body fluid (SBF). Simulated body fluid (SBF), or formally known as hydroxyapa‐ tite, is a supersaturated insoluble calcium phosphate mineral, with the chemical composition of Ca10(PO4)6(OH)2. This fluid has a close similarity with human blood plasma and is the essential component of the biological hard tissues such as bones. Due to the high absorbance and catalytic properties of SBF, this fluid is commonly used to estimate the biocompatibility

To treat the titanium substrates, a range of low to high average laser powers from 6 to 12W was used, while frequency and scanning speeds were kept constant in all cases. The effec‐ tive number of pulses for all results was kept constant to be 25 pulses for frequency of 100

desired pattern is irradiated across the surface of the selected material.

implant surfaces [30–35].

358 360High Energy and Short Pulse Lasers

**3. Effects of laser power**

level of materials used in implant productions.

kHz [36, 37]. **Table 1** introduces the parameters used in more detail.

**2.1. Laser system**

Once treated, there is often more available exposed surface area of the material because of the increase in surface roughness, which, in return, enhances the apatite‐inducing ability and cell adhesion rate of that material. **Figure 1** displays the variation in the treated substrates using the stated powers.

**Figure 1.** Surface of treated titanium (A) 6 W, (B) 8 W, (C) 10 W, (D) 12 W.

Although the number of pulses delivered to each of these substrates is the same, as indicat‐ ed in **Table 1**, the images shown in **Figure 1** clearly indicate different surface topography after laser treatment. With an increase in power, the energy delivered to the surface of the materi‐ al increases as well, hence, more ablation and topography changes occur. Substrates treated with a power of 12 W have a larger surface affected by laser irradiation as opposed to 6 W.

During laser irradiation, a plasma plume with radial surface tension is formed surrounding the irradiated area due to the high temperature gradient caused by laser energy transfer. Immediately following irradiation, a high temperature difference exists between the surface of the material and the generated plasma plume. Thus, a high cooling rate results, which causes rapid re‐solidification of the ablated material. Consequently, the tension within the laser plume causes shooting of the molten titanium to outside of the ablated zones [37]. The micro re‐ solidified particles observed outside the crater in **Figure 1** have been formed due to the same reason.

Using 3D microscopy, the surface profile of the treated substrates can be compared; this is introduced in **Figure 2**. This study used the Zeta‐20 optical profiler to scan the surface of the samples in order to obtain the surface topography results across each sample.

**Figure 2.** Surface profiles of Ti (A) 6 W, (B) 8 W, (C) 10 W, (D) 12 W.

As shown, an increase in laser power clearly affects the surface topography profile of titanium substrates. Different laser powers create different surface irregularities across the surface of the treated titanium substrates.

#### **3.2. Surface temperature analysis**

Different power levels deliver various amounts of energy to the surface of the substrate, and, therefore, increase the surface temperature of the titanium. This increase in the surface temperature could affect the surface structure, topographic properties, and oxidation of the titanium substrates. Oxidation is an important factor when considering bioactivity of a material. With higher oxidation, a higher surface energy is present, which results in more interaction between the implant and the body. Therefore, generating larger amounts of titanium oxide across the surface substrates consequently increases the biocompatibility of the treated work pieces. However, after a certain power threshold, the energy delivered to the surface of the titanium increases the surface temperature beyond the oxidation limit. Therefore, the material develops less titanium oxide as the surface undergoes larger ablation [32, 34, 38].

In order to calculate the surface temperature in this chapter, a theoretical calculation has been conducted according to the previous published results [25, 36, 39]; in this method, we assumed that the laser energy is absorbed in a much thinner layer compared to the penetration depth of the heat wave. Finally, the average surface temperature of the Ti substrate can be ob‐ tained as fully discussed in [25, 39]:

$$T\_n = 2\alpha \frac{\left[1 - \frac{2}{3}\alpha\right]}{(1 + \alpha^2)} \frac{T\_n}{(1 - \alpha)} \left[1 + \frac{\alpha^n - \alpha}{n(1 - \alpha)}\right] \tag{1}$$

where *T*m is the maximum temperature calculated at the end of first pulse, *n* is the pulse numbers, and α is the constant ratio for the previous maximum and the following minimum temperatures and equal to α = (*t*p/*t*pp)1/2, where *t*pp is the pulse interval and equal to *t*pp *=* 1*/f* (*f* is pulse repetition rate) and *t*p is pulse duration. The analytical results obtained in this study are associated with possible errors due to multiple assumptions made throughout the study.

Upon completion of the analytical analysis, the temperature profile for both cases with 8 and 10 W powers, and a pulse number range of 2 pulses to 50 pulses is obtained.

The trend of average surface temperature of the treated titanium substrates is shown in **Figure 3**.

**Figure 3.** Maximum average surface temperature in each power.

causes shooting of the molten titanium to outside of the ablated zones [37]. The micro re‐ solidified particles observed outside the crater in **Figure 1** have been formed due to the same

Using 3D microscopy, the surface profile of the treated substrates can be compared; this is introduced in **Figure 2**. This study used the Zeta‐20 optical profiler to scan the surface of the

As shown, an increase in laser power clearly affects the surface topography profile of titanium substrates. Different laser powers create different surface irregularities across the surface of

Different power levels deliver various amounts of energy to the surface of the substrate, and, therefore, increase the surface temperature of the titanium. This increase in the surface temperature could affect the surface structure, topographic properties, and oxidation of the titanium substrates. Oxidation is an important factor when considering bioactivity of a material. With higher oxidation, a higher surface energy is present, which results in more interaction between the implant and the body. Therefore, generating larger amounts of titanium oxide across the surface substrates consequently increases the biocompatibility of the treated work pieces. However, after a certain power threshold, the energy delivered to the surface of the titanium increases the surface temperature beyond the oxidation limit. Therefore, the material develops less titanium oxide as the surface undergoes larger ablation [32, 34, 38].

samples in order to obtain the surface topography results across each sample.

**Figure 2.** Surface profiles of Ti (A) 6 W, (B) 8 W, (C) 10 W, (D) 12 W.

the treated titanium substrates.

**3.2. Surface temperature analysis**

reason.

360 362High Energy and Short Pulse Lasers

As evident, with 12 W of power, the maximum average surface temperature of the titanium exceeds its evaporation point, and therefore less concentration of titanium oxide is generat‐ ed along the surface of the substrates. Also, more ablation occurs, which agrees with the observations. Having a larger concentration of titanium oxide across the surface results in better cell interactions. To further investigate the oxidation of treated substrates, energy dispersive X‐ray (EDX) analysis was conducted to detect the amount of oxygen available across the surface of substrates treated using the stated powers.

#### **Figure 4** indicates the results.

**Figure 4.** EDX analysis of the irradiated Ti at 6, 8, 10, and 12 W.

Comparing the trace of oxygen across the surface of all four substrates, it is observed that with an increase in power, the oxygen concentration increases slightly as well, until a power of 12 W is reached, which shows slightly a lower trace of oxygen. This is in agreement with the average temperature results observed in **Figure 3**. Considering the results shown in **Figures 4** and **5**, it is expected that powers of 8 and 10 W would result in a better biocompatibility compared to othertwo powers. This conclusion is assessed through the use of simulated bodily fluids (SBF).

**Figure 5.** SBF‐soaked substrates. (A) 8 W, (B) 10 W.

#### **3.3. Biocompatibility assessment**

**Figure 4** indicates the results.

362 364High Energy and Short Pulse Lasers

**Figure 4.** EDX analysis of the irradiated Ti at 6, 8, 10, and 12 W.

**Figure 5.** SBF‐soaked substrates. (A) 8 W, (B) 10 W.

fluids (SBF).

Comparing the trace of oxygen across the surface of all four substrates, it is observed that with an increase in power, the oxygen concentration increases slightly as well, until a power of 12 W is reached, which shows slightly a lower trace of oxygen. This is in agreement with the average temperature results observed in **Figure 3**. Considering the results shown in **Figures 4** and **5**, it is expected that powers of 8 and 10 W would result in a better biocompatibility compared to othertwo powers. This conclusion is assessed through the use of simulated bodily

To determine how the differences in surface topography affect the biocompatibility of titanium, the treated substrates using powers of 8 and 10 W were immersed in SBF for 7 days. **Figure 5** shows the scanning electron microscopy (SEM) photographs of the substrates after the completion of the assessment.

In **Figure 5**, the white layers across the induced line patterns indicate the bone‐like apatite deposition on the surface of the titanium. These microscopy images show that generally with an increase in power, apatite‐inducing ability across the surface of treated titanium increases as well. Looking closely into **Figure 5**, the EDX results for 10 W curve indicate a larger amount of apatite deposition compared to the 8 W curve. This is further observed in the EDX analy‐ sis conducted on these titanium substrates shown in **Figure 6**. Detecting larger amount of oxygen, calcium, and phosphorous elements indicates a higher apatite‐inducing ability forthe substrates.

**Figure 6.** EDX analysis at 8 and 10 W.

As shown, the apatite‐inducing ability of substrates treated with a power of 10 W is higher than the substrates treated with 8 W. This is in agreement with results observed with oxida‐ tion and SEM photography analysis.

Overall, the biocompatibility was highest for substrates treated with 10 W. This power provides adequate surface topography and the energy delivered is within the oxidation temperature range, hence creating a desirable environment for cell attachment to take place.
