The story of silicon nitride

Amedica Corporation makes and develops silicon nitride ceramics. This summary was prepared to share their knowledge and passion of an incredibly versatile material that has myriad applications in medicine.

What is silicon nitride (Si₃N₄)?

Silicon nitride is a ceramic. It is an inorganic and non-metallic compound of silicon and nitrogen, two elements that are vital to life [1–4]. Discovered in 1857, silicon nitride remained a curiosity until commercial use began in the 1950s [5].

Manufacturing costs were reduced in the 1970s–80s, and silicon nitride quickly found a home in many industries [6]. In the 1990s, naturally occurring silicon nitride was discovered in meteorite stardust, suggesting intergalactic origins from the beginning of time [7].

Properties

To make silicon nitride, refined material powders are mixed into a slurry, from which desired shapes are crafted, and finished in high-temperature pressure furnaces. These steps are similar to making pottery; the word ‘ceramic’ comes from the Greek root kéramos, which refers to pottery [8].

Silicon nitride is a tough and strong material that resists abrasion, corrosion and most chemicals. It has the highest fracture resistance of any advanced ceramic [9]. These properties have led to many applications throughout industry.

Industrial uses

Human life is touched almost daily by silicon nitride. It is used in high-end bearings for gas and diesel engines, wind turbines, motorsports equipment, bicycles, rollerblades, skateboards, computer disk drives, machine tools, dental hand-pieces and flap-actuators in aircrafts [10]. If corrosion, rapid wear and electric or magnetic fields limit the use of metals, silicon nitride is used instead [7,11,12]. Silicon nitride is even used in the bearings of underwater ocean tidal flow meters, where it withstands severe seawater corrosion conditions [13].

Due to its extreme strength, hardness and resistance to chemical and thermal factors [11,12,14–16], silicon nitride is commonly used in high-speed cutting tools, and to break up rocks during oil fracking [17]. Its heat resistance has led to uses in the valve trains of gas [18] and diesel engines [11], rotors and stators in gas-turbines [19,20], automotive turbochargers [21] and rocket nozzles and thrusters [22]. Few materials can survive these extreme conditions.

Outer space

Silicon nitride has powered human dreams into space. It is in the cryogenic pump bearings of NASA space shuttles [23], the thrusters of the Japanese space probe Akatsuki [24], and provides a lifespan of over 10 million years of space travel to tungsten-etched memory chips for spacecraft [25].

Medical implants

Amedica Corporation makes spinal implants of medical-grade silicon nitride. These implants can be dense, porous, or even a combination that mimics the cortical-cancellous nature of living bone [26,27]. Silicon nitride is extremely biocompatible and bioactive, has bacterial resistance and shows superb bone affinity [28]. With more than 25,000 spine implantations and an eight-year track record, the material has yet to fail [29].

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Silicon nitride can also be polished to provide an exceptionally smooth and wear-resistant surface for articulating applications, such as bearings for hip and knee replacements [30–32]. It is truly the ideal biomaterial, that not only meets, but exceeds all human implant requirements, such as:

material phase stability [33];

wear resistance [33,34];

strength and fracture toughness [9];

hydrophilicity [35];

favourable imaging [36]; and

bacterial resistance [37,69,72].

Evolution of biomaterials

In days past, wood, leather, pig bladders, glass and ivory were used to repair broken hips, and treat hip arthritis [38]. Today, metals, bone grafts and polymers are used to rebuild human bodies, and help maintain function into old age.

All biomaterials degrade in the wet, warm, saline environment of the human body – metals fret and corrode [39], plastics oxidise [40] and allograft bone never fully heals [41], all of which lead to long-term failures. For example, toxic wear from all-metal hip bearings is a well-known problem [42]; and fretting, crevice and electrochemical corrosion in total hips is an emerging failure mode [43].

Silicon nitride can address these concerns. Not only is its wear extremely low [44], but the minimal wear particles are soluble and resorb in the body [45]. Silicon nitride is also chemically resistant, hard, stiff and has a high dielectric constant, all of which discourage fretting and corrosion [46].

Plastic (polyethylene) bearings in artificial hip and knee joints oxidise over time, leading to strategies such as cross-linking [47] and vitamin E doping [48] to slow down this process. Silicon nitride’s unique surface chemistry actually absorbs oxygen from polyethylene [44,49], thus limiting polyethylene oxidation in hip and knee replacements.

Bone grafts present significant limitations by way of harvesting morbidity, lack of bioactivity, and concerns about disease transmission [41]. Even synthetic bone fillers are made mostly from a material called hydroxyapatite, which has an affinity for bone but is still very brittle [50–52]. Silicon nitride bone scaffolds and bone-fusion devices [53] provide superior and reliable mechanical strength, that can be engineered to result in bone healing similar to hydroxyapatite [54].

On X-ray images, plastic implants are invisible, whereas metals appear solid, obscuring visibility of bone anatomy behind the implant. CAT scans and MRI scans suffer distortion from metal implants, leading to sub-optimal imaging. In contrast, silicon nitride is easily seen on X-rays; it does not block imaging of bone anatomy behind the implant, and its dielectric and non-magnetic nature eliminates distortion in CAT and MRI scans [36].

In summary, silicon nitride has the right combination of strength, toughness, wear, biocompatibility, bioactivity, bone integration, structural stability, corrosion resistance and easier imaging, all of which are desirable in medical implants [55].

Existing medical ceramics

Ceramics such as alumina (Al2O3) and zirconia (ZrO2) have been used in hip and knee replacements because of lower wear rates than metal surfaces [56–58]. Alumina is brittle and can break suddenly [59]; zirconia is stronger, but can transform after implantation, leading to erratic outcomes [60]. Zirconia was withdrawn in 2002, in the wake of failures from uncontrolled material transformation [61].

Today, a mix of alumina and zirconia, called zirconia-toughened alumina (ZTA) is a popular ceramic used in hip and knee implants [62]. ZTA is an engineering compromise between the alumina and zirconia [63]; however, ZTA recovered during repeat surgery shows that it too can change its composition in the body, and reduce its surface mechanical integrity [64].

In the body, alumina and ZTA both release oxygen ions, which can degrade polyethylene bearings [49,65]. Silicon nitride is unique in that it is a non-oxide ceramic, which means not only is it stronger and tougher than alumina and ZTA [9], it also absorbs oxygen away from polyethylene [66,67]. This remarkable property could take hip and knee replacements into the third and fourth decades of service, something that is only a speculation today.

Scientific and clinical data

Besides advantages in strength, wear, corrosion resistance and fracture toughness [68], there is more to silicon nitride. Here are Amedica’s recent findings on this novel bioceramic.

Bone healing

Silicon nitride turns on osteoblasts and suppresses osteoclasts. A manufacturing change called ‘nitrogen-annealing’ results in a near-200 per cent increase in bone formation by cells exposed to silicon nitride [54]. This finding has profound implications.

From tribal bone-setting in ancient cultures, to modern fracture fixation, surgeons have yet to alter the biology of bone healing. Nitrogen-annealed silicon nitride could accelerate bone healing, fusion and implant ingrowth. Cells adhere preferentially to silicon nitride over polymer or metal [69]. Cell adhesion promotes tissue development, and enhances the bioactivity of materials. Cell adhesion to silicon nitride is a function of pH, chemical and ionic changes at the material surface.

Composite devices

In a clinical trial, a spine spacer made of solid and porous silicon nitride fused the cervical spine without added cells or bone fillers [70]. Composite devices based on porous silicon nitride herald a new class of reconstructive implants [27,71].

Infection prevention

Bacterial infection of any biomaterial implants is a serious risk. Solutions have included material coatings, surface texturing, antibiotic treatments and other enhancements to confer bacterial resistance. Silicon nitride offers an easy solution; not only is it is inherently resistant to bacteria and biofilm formation [37,69], recent studies have proven direct bactericidal effect against oral bacteria [72].

As with cell adhesion, the antibacterial behaviour of silicon nitride relates to its complex surface phenomena – invoking chemistry, surface pH, texture and electrical charge properties [35]. The surface modulation of silicon nitride to optimise the desired properties of implants is a potent advantage of the material [35].

The future

The world is becoming better informed, more active and more demanding, and people are living longer than ever before. Scientists agree that silicon nitride will lead the future of material innovations. It comes closest to addressing the challenges of biomedical implant safety, high-performance and lifetime durability [73].

With proven success in industry and medical applications, and new research showing yet more beneficial attributes, silicon nitride continues to prove itself. With increased awareness, this advanced biomaterial technology will be found increasingly across reconstructive surgery and many other medical fields.

References

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