Additive manufacturing stands as a revolutionary processing technology, facilitating the creation of ceramics with unique geometries that, particularly for piezoelectrics, unlock novel functionalities. Each additive manufacturing technique possesses distinct advantages and limitations concerning shape flexibility, surface quality, and cost. Furthermore, these methods necessitate entirely different ceramic feedstocks, demanding optimization for each composition and ceramic powder. This presentation will cover lead-free piezoelectric ceramics crafted through three diverse additive manufacturing techniques: Direct Ink Writing (robocasting), Digital Light Processing, and Freeform Injection Molding. For Direct Ink Writing, both BaTiO3 and K0.5Na0.5NbO3 were produced. BaTiO3 originated from a "hybrid" sol-gel + colloidal ink, while K0.5Na0.5NbO3 emerged from a colloidal cellulose-based binder. This method allowed the rapid printing of tall structures and spanning lattices. Shifting focus to Digital Light Processing, dense K0.5Na0.5NbO3 samples were prepared using two distinct ceramic powders synthesized through either solid-state synthesis or spray pyrolysis. The characteristics of the powder, such as particle size distribution and agglomeration state, significantly influenced the attainable solid loading in the photopolymerizable vat and its rheology. The use of spray pyrolysis-made powder enabled the creation of vats with 40 vol% solid loading, showcasing the desired shear-thinning behavior. This facilitated the printing of K0.5Na0.5NbO3 samples that subsequently sintered to approximately 93% of theoretical density. Finally, for Freeform Injection Molding, custom-made additively manufactured water-soluble polymer molds were employed for injecting a thermoplastic-based feedstock of BaTiO3 and Bi0.5Na0.5TiO3-BaTiO3 materials. The molds were dissolved before binder burnout and sintering, resulting in dense ceramics shaped as double helix or focusing discs. These ceramics exhibited high surface quality and demonstrated the anticipated piezo- and ferroelectric performance.

(K0.5Na0.5)NbO3 (KNN) ceramics were the first studied in Europe as lead-free industrial replacement of lead titanate zirconate (PZT) piezoceramics [J. Eur. Ceram. Soc., 25(12), 2701 (2005)]. Since then, an extended effort was paid for the optimization of the properties of this material, which were first investigated in the 50s [Phys. Rev., 96(3), 581 (1954)] and are tested nowadays in a variety of devices. Here, efficient and non-contaminant attrition ball milling [J. Eur. Ceram. Soc., 44(5), 2944 (2024)] during 3h of the oxides and carbonates was made prior to the air synthesis from 450ºC to 850ºC, for 2h. A second attrition ball milling after calcination gives place to a majoritarian population of submicron grain size particles. Ceramics were obtained by pressureless sintering in air at 1050 ºC and 1100 ºC, also for 2h. Coarse grained ceramics were obtained from powders calcined at 700ºC/2h. Fine grain and homogeneous ceramics were obtained after air synthesis at 800 and 850ºC/2h. The enhanced sinterability was attributed to the enhanced mass transfer kinetics during sintering owing to the reactivity of the calcined and attrition milled powder. An automatic iterative method for the analysis of the impedance curves at the electrically induced resonances was applied for the determination of the ceramic characteristics as ultrasonic transducers [Sensors, 21 (12), 4107 (2021)].The piezoelectric activity of the ceramics reached the values of d33=115 pC/N, d31= -22 pC/N, kp= 20 % and kt= 40 %. Comparison with recent literature shows that this are excellent values for ceramics obtained by this straight and sustainable route.

The human body relies on electric signalling for all cellular communication, and the application of exogenous electric fields has been shown to improve would healing, particularly in bones. The advent of lead-free of piezoelectrics provides an opportunity to utilise electromechanically active biomaterials that interface with the human body, electrically communicating with cells and promoting healing. Human bone itself is piezoelectric, using electrical signals to guide bone repair, but contemporary orthopaedic implants lack this inherent electrical stimulation. A piezoelectric implant material could provide both the structural support and electrical signalling necessary to stimulate healing and bone growth. However, the performance of piezoelectric implant materials in biological conditions is not fully understood.

Piezoceramics are well suited to orthopaedic applications due to their bulk mechanical properties, and (K,Na)NbO3 (KNN) is a potential candidate considering its broadly non-toxic constituent ions. Herein we control compositional changes in KNN to improve functional properties and reproducibility in solid-state processing.

Local inhomogeneities in ion distribution frequently arise during KNN processing, which then leads to the formation of hygroscopic polyniobate phases. These hygroscopic phases decrease stability in aqueous environments, and thus preclude the use of KNN as an orthopaedic implant. This work presents two approaches for reducing or eliminating these phases: (i) inducing a 2 mol% alkali excess, or (ii) sintering stoichiometric KNN in a hybrid atmosphere. Dimensional stability and electrical properties of these modified KNN materials were maintained over a 14-day in vitro study. Both KNN compositions maintained significant polarisation (up to d₃₃ = 48 pC/N; compare with human bone: 0.7-7 pC/N) with open porosity (up to 24.4%). Pore and interconnect sizes were found to be larger than 10 mm, which is advantageous for tissue infiltration and osseointegration of bone tissue scaffolds. Biocompatibility was established with human osteoblast cell cultures, which exhibited equal or better performance than conventional titanium implant materials.
With demonstrated functional stability and cellular compatibility in vitro, KNN-based bioceramics show promise as orthopaedic implant materials.