Femtosecond lasers: powerful tools for biological hard tissue investigations
The use of femtosecond lasers is also gaining ground in dentistry. The introduction of lasers for the removal of hard dental tissue dates back to the mid-1990s. Initial efforts for their use in a clinical setting were focused on developing inexpensive hand-held devices with a high ablation speed. However, recent enamel and dentine ablation studies show that the use of conventional laser sources with nano- or microsecond pulse widths leads to cracking and uncontrolled material removal. The poor absorption of laser radiation by tooth material also requires high laser power, causing excessive heat deposition in the tooth being treated and thermal effects in the surrounding dental tissue. Recent reports however, show that no collateral thermal damage is observed with the use of subpicosecond laser pulses.3
Ultrashort pulses provide efficient, fast and localized energy deposition, low deformation and ablation thresholds, as well as small molecular fragmentation levels. This, plus the high accuracy and reproducibility of femtosecond lasers has increased their potential as low-cost tools for a wide variety of surgical applications. Our current research aims to investigate laser ablation processes using biological samples such as tooth and bone as target materials. By studying the process of laser ablation, we seek to qualitatively improve the biomedical applications of femtosecond lasers.
Experimental set upWe examined the ablation process of biological samples by varying parameters such as pulse duration, laser wavelength, and laser fluence.4 In our approach, we combine ultrashort laser ablation to time-of-flight mass spectroscopy (TOF-MS). After removal of macroscopic amounts of matter from the surface of the material, TOF-MS is then used for the detection and trace analysis of the atoms and molecules ablated or sputtered from the samples. This material consists chiefly of polypeptides and proteins.4
All experiments were carried out under ultrahigh vaccum (UHV) conditions.5 The UHV chamber was equipped with a reflectron TOF mass spectrometer. We used the femtosecond laser system shown in Figure 2 to deliver ≈30fs-pulses at a laser intensity of 1015W/cm2 to ablate material from tooth and bones samples.
Mass spectrometry is a powerful tool for the analysis of molecular species on a solid surface.6 It has also proven most useful in physicochemical studies of biological laser-induced photo-decomposition products.7 Since we knew that the main components of our tooth and bone samples were collagen fibers8 and hydroxyapatite (Ca10(PO4)6(OH)2),4 we expected to observe their dissociation products in the measured mass spectra of the ablated species.
Figure 3 shows the TOF-MS spectrum of the material ablated from a bone sample. Laser irradiation breaks the hydroxyapatite OH- bond, releasing H+ ions and Ca ions. The spectrum shows a peak at m/z=40, attributed to Ca.4 The presence of residues from the collagen polypeptide chain is also confirmed by the presence of CnHm peaks (see Figure 3, inset).
Figure 4 shows the spectrum obtained for the material ablated from a tooth sample. As was the case for the bone mass spectrum, a Ca ion peak is also observed, but with much higher intense (see Figure 4, inset). Peaks assigned to CnHm collagen fragments are also observed at m/z=104, 113, 127, 130, and 155.4
ConclusionOur work demonstrates the sensitivity of TOF-MS for the identification of the biomolecular species present in laser-ablated material from hard tissues. Performing femtosecond laser ablation at 800nm altered the chemical composition of the ablated tissue and yielded the highest number of characteristic ions. The results obtained on our tooth sample are especially interesting for dental surgery applications. They show that ultrashort pulses can be used as a minimally invasive technique to ablate dental hard tissue with minimal thermal damage to the surrounding tissues. Our preliminary results also confirm earlier reports showing that it is possible to process dental hard tissue with high efficiency and we are accordingly planning further reproducibility studies.3
Albena Daskalova is currently a research associate at the Institute of Electronics of the Bulgarian Academy of Sciences. She obtained her MS at the Vienna University of Technology. From 2000 to 2003, she worked as a research associate at the Vienna University of Technology while completing her thesis, obtaining her PhD in 2003. Her research interests are focused on ultrafast laser-matter interactions.
