CaP-based coatings fabricated by PLD: Comparison
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Pulsed Laser Deposition is an atractive technique used for coating dental and orthopedic implants with various biomaterials, including calcium phosphate-based ones.

  • calcium phosphate-based coatings
  • synthetic and biological hydroxyapatite
  • in vivo assessment
  • mechanical tests
  • bone implants
  • biomedical applications
  • Pulsed Laser Deposition technique

1. Introduction

Bioceramic coatings are currently used for various biomedical applications [1] to modify the implant surface (by increasing its surface roughness), to promote osseointegration. Nowadays, plasma spraying (PS) is the only industrial technology used for coating dental and orthopedic implants with calcium phosphate (CaP)-based materials. In this respect, in the dedicated literature there are numerous and very interesting research works that point out and critically evaluate the physical-chemical and biological characteristics of the structures fabricated by this technique, along with their clinical performances [2][3]. The CaP-based coatings fabricated by conventional thermal PS technique onto medical implants function as an intermediate layer between the tissues and the metallic implants [4]. Despite its wide commercial availability, this technique still has some drawbacks, such as: (i) the synthesized coatings generally consist of several phases (i.e., β-TCP forms at 1200 °C, and it transforms into TTCP at T > 1400 °C); (ii) at higher synthesis temperatures, the negative influence of the mismatch between the thermal expansion coefficients of hydroxyapatite (HA) and tricalcium phosphate and the ones corresponding to Titanium-based alloys (11–15 × 10−6 cm/(cm·K) vs. 8–10 × 10−6 cm/(cm·K), respectively), limit the obtaining of good CaP coatings onto metallic substrates [5]; (iii) it supplies very thick structures with low adherence to the substrate (because the coatings’ tensile stresses have a greater tendency to initiate cracks and cause film delamination [5]); (iv) surface morphology, phase composition, or uniformity of crystallization [6][7] are difficult to be controlled; and (v) it is a line-of-sight method [8]. Therefore, no coating technique can be considered perfect and all these drawbacks gradually supported research efforts focused on the introduction of various, alternative coating techniques to PS (i.e., radio frequency magnetron sputtering, pulsed laser deposition - PLD, electrochemical deposition, etc.) [9][8][10][11][12]. Among these, PLD technique is worth mentioning due to some important advantages over PS method, such as: (i) a much faster surface deposition process; (ii) a stoichiometric transfer of the material’s composition from the target in the synthesized coating; (iii) a better morphological and compositional uniformity; (iv) a lower porosity; (v) precise thickness control; (vi) effectiveness for coating small implants, with complex shapes; (vii) a decreased tendency of the synthesized structures to crack or delaminate; and, very important, (viii) a high adherence to the metallic substrate [9][13]. In the biomedical field, for the fabrication of CaP-based coatings for bone implant applications, one of the most applied plasma-assisted methods is PLD [10]. This technique is able to fabricate dense and extremely adherent films. In this respect, synthetic HA and biological HA (BioHA) coatings obtained by PLD previously demonstrated high adherence to metallic substrates [14]. Moreover, the composition of these coatings is consistent with the one corresponding to the raw (base) materials, along with an improved crystallinity [14]. One should note that in PLD, after the ablation of the target by laser pulses, a plasma plume is generated. When the species existing in the plume reach the surface of the substrate, they may deposit onto the surface and form a film [15]. The number of the deposited species depends both on their density in the plume and their energy. In the case of low energies, the species may not deposit on the substrate surface even if they arrive at the surface [16]. If the substrate temperature is high, the energy of the species can be compensated. Consequently, the number of deposited species onto the substrate surface will increase, along with the density of the droplets. High substrate temperatures also contribute to the atomic diffusion, which, in turn, can determine the appearance of two phenomena: the first one is the atomic rearrangements and crystallization of the film and the second one, the improvement of the film−substrate bonding state [17].

The laser sources appropriate for the ablation of a wide range of materials use wavelengths in the UV domain due to some important advantages over IR and/or visible laser sources, such as (i) a higher penetration depth of the laser beam in the target material and (ii) a higher energy of the photons that allows for a more efficient vaporization of the target [18]. In this respect, the laser sources used for PLD experiments are either excimer lasers (i.e., ArF [19], KrF [20], or XeCl [21], emitting at wavelengths of 193, 248, or 308 nm, respectively) or solid-state lasers (i.e., Nd:YAG [22], emitting at 266 nm). It should be also emphasized that to increase the amount of evaporated material from the ablated target to the detriment of expulsed liquid or solid phases, lasers emitting in a pulsed regime with pulse durations in either the nanoseconds or picoseconds range are generally used [23]. In these regimes, the absorption process takes place more quickly than in the case of thermal diffusion processes. More insights on the PLD and PS techniques are well described elsewhere [24][15][13].

It is important to mention that post-deposition treatments are generally applied to transform CaP phases with lower Ca/P ratio to crystalline HA. Thus, there are two commercially used post-treatments: sintering [24][25][26] and soaking in alkaline solutions [27][28]. These treatments are generally applied for several hours, in the range of 600–800 °C. Their aim is to transform the water trapped in the film during the synthesis process in OH− ions, to stabilize the crystalline structure [29].

There are reports in the dedicated literature on the in vivo testing of CaP-based coatings (especially HA) fabricated by different physical vapor deposition methods, but, to the best of our knowledge, only few of them addressed the PLD technique. Therefore, the in vivo results pertaining to the studies reported in the last two decades on either synthetic or biological-derived CaP coatings only, synthesized by PLD technique, were gathered, compiled and thoroughly discussed.

2. Pulsed Laser Deposition Technique for Bone Implant Applications

Because the biomaterials’ osteoinduction mechanism is not yet entirely understood, one could not precisely answer the question whether if the sole biomaterial or an interaction between the biomaterial and the relevant proteins present in the living system are responsible for the osteoinduction process. Because most of the implants do not possess the capability to induce bone growth, specific material properties are required to activate the osteoinduction process. To begin the differentiation of the undifferentiated inducible osteoprogenitor cells into bone-forming cells, it was suggested that both the chemistry and the geometry of the biomaterial in contact with these cells represent critical factors to be considered [30].

Metallic implants (including Ti) are generally used for various biomedical applications, mainly due to their resistance to corrosion and favorable mechanical characteristics [31]. Because of its bioinert nature, bulk Ti is not capable to form a biochemical bond with the bone, and this biological inactivity often generates a fibrous tissue that surrounds the implanted device [32]. To improve both osseointegration rates and longevity of Ti implants, the deposition of CaP-based coatings onto their surfaces is envisaged. It was therefore demonstrated that implants’ surface functionalization with CaP-based coatings could promote the formation of real bonds with the surrounding bone, due to their proved chemical similarity with natural bone tissue and their high biocompatibility [33]. This process occurs rapidly along the entire surface of the coating, in comparison to the case of simple Ti implants (used as controls in the experiments) [5].

A nowadays growing research interest in the field of biomaterials is related to the use of biological-derived CaP materials as viable, safe, and low-cost alternatives to synthetic CaP-ones [24]. It should be emphasized that, unfortunately, the Earth’s available mineral resources are threatened to become limited in the near future because of the rapid demographic increase and economic growth. The access to sustainable resources is therefore critical. Consequently, this will generate a beneficial economic and environmental impact over the society, allowing for an intelligent use of these renewable resources.

The mechanical properties of CaP-based coatings are responsible with the overall success rate of an implant [34]. Thus, the optimal functioning of an endosseous implant is directly influenced by the biomaterial’s mechanical stability, which can be easily evaluated by extraction tests. To obtain information on the force that occurs between the bone tissue and implanted materials, various experimental study models have been developed, each of them with their own particularities [35][36][37]. In this respect, the investigation of the coating bond strength is typically performed by scratch [38][39][40], pull-off [41], tensile adhesion [42][43][44], or shear strength tests [45], respectively. It is important to emphasize upon that the ISO 13779-2:2008 standard requirement for tensile adhesion strength of CaP-based coatings, used for load-bearing applications, is of 15 MPa [46]. It was reported that, in general, CaP-based coatings synthesized by the PLD technique easily surpass this imposed value [24][25][47]. There are some studies in the literature concentrated on tensile strength measurements [48][49], which, very importantly, can provide a direct measurement of the attachment between the bone and the implant surface, being therefore influenced only by the chemical bonding between those two [50][51][52]. This way, the effects of friction and of mechanical forces introduced by surface roughness can be minimized [53]. In the case of animal trials, the implant’s increased bone retention is considered a clinically relevant indicator of improved stability and capacity of the implants to carry loads without detaching. Unfortunately, this type of information cannot be acquired by histological or SEM investigations, which provide only limited information on the functional performance of an implant. One should note that in none of the studies included in this review, related to mechanical testing of CaP-based coatings, alteration or disruption of the implants were present. In general, the inferred values for all functionalized Ti implants demonstrated significantly improved bone attachment in comparison to Ti ones. Next to the PLD surface functionalization of metallic implants, a longer implantation time period was demonstrated to induce a positive influence on the overall bone bonding strength characteristics of the investigated medical devices. One should note that the fabrication by PLD of novel BioHA implant coatings derived from sustainable and inexpensive CaP-based resources, with improved mechanical properties, correlated with an increased bone fixation in vivo, could stand for a pioneering contribution to the progress of advanced medical devices.

In general, the results of the studies included in this review, obtained using standardized radiography and microradiography [54], computed tomography [55][56][57], histomorphometry [58][59][60][61], and tomodensitometry [71], have confirmed the osseointegration process (pointing to an increase in the osseous density), along with a strong bone–implant connection and no inflammatory process of the soft tissues. The histological investigations, performed with various microscopy techniques [6][17][54][62][58][60][63][55][56][64][57], have indicated also new active bone formation and demonstrated no adverse inflammatory reactions or gaps around the sites of implantation or at the bone–implant interface, for any of the investigated structures. In addition, the bone and the implant were shown to tightly adhere to each other along the full length of the interface. One interesting observation was that the temperature applied during the deposition process seemed to play an important role in the bone growth of the synthesized structures, the osteoblasts in the newly formed bone tissue being shown clear and homogenous [17]. Moreover, the osteointegration rate was demonstrated to be slightly superior in the case of annealed samples, rather than for the non-annealed ones [62]. It seemed also that no matter what the laser source used (i.e., KrF or CO2), between the synthesized coatings, no significant statistical differences in the osteogenesis process were inferred [62]. Significantly higher values of the bone area between the implant surface and the boundaries and bone adherence ratios were inferred at various implantation time periods in the case of functionalized implants in comparison to control ones [56][57]. In this respect, in the case of control samples, a fibrous connective tissue between the metallic implants and the newly formed bone was shown, while in the case of the synthesized coatings, this layer could be observed only seldom [54].

Even though it is generally accepted that CaP-based coatings deposited by PLD improve bone strength and the initial osseointegration rate, the coatings’ properties necessary to achieve an optimum bone response are yet to be determined. This is mainly because of the limited number of in vivo studies available in the dedicated literature. It should be emphasized upon that the in vivo testing should demonstrate stability in biological environment for up to 1 month, which corresponds to the initial healing phase of the implants [65]. The limitations on such experiments can be related to (i) the difficulty to select a suitable animal model in order to properly simulate the actual mechanical loading and unloading conditions in which an implant should function inside a living system; (ii) the need to sacrifice a large number of animals to reach a significant statistical relevance, able to validate the obtained results; (iii) the demands for high costs and long time frame in the case of clinical trials; (iv) the lack of coordination among material scientists and biologists and thus an insufficient understanding of this interdisciplinary subject; and (v) the serious ethical concerns related to the used animals (including also the choice of their correct number), as they might be sometimes subjected to painful procedures or toxic exposures during the experimental trials [66].

Taking into consideration all these aspects, a future important progress of CaP-based materials might be linked to a shift of the focus from osteoconduction to osteoinduction, e.g., by additive manufacturing of scaffolds with complex, controlled three-dimensional porous structures and development of novel ion-substituted CaPs with increased biological activity. Moreover, new strategies, possibly based on self-assembling and/or nanofabrication might be developed for the successful fabrication of load-bearing bone graft substitutes. In the future, the composition, microstructure, and molecular surface chemistry of various types of CaPs might be tailored in such a way to match the specific biological and metabolic requirements of tissues or disease states. The multilayer composite coating systems, fabricated by the PLD technique, should represent also a future trend, able to provide multifunctional properties for the biomedical implants.

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