THE STORY OF SILICON NITRIDE

From Material to Medicine

THE STORY OF SILICON NITRIDE

A Brief Narrative - By Amedica Corporation

Introduction:

At Amedica Corporation, we make and develop silicon nitride ceramics as a platform for diverse applications. This summary was prepared to share our knowledge, passion, and experience with you.

What is silicon nitride (Si

3

N

4

)?

Silicon nitride is a ceramic. It is an inorganic and non-metallic compound, made of silicon and nitrogen, two elements that are vital to life.

1–4 Discovered in 1857, it remained a curiosity until commercial uses began in the 1950’s.

5

Manufacturing costs were reduced in the 1970’s-1980’s, and silicon nitride quickly found a home in many industries.

6 In the 1990’s, 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 and

-pressure furnaces. In a way, these steps are akin to making pottery; the word

“ceramic” comes from the Greek root “kéramos” that refers to pottery.

8

Silicon nitride is a strong and reliable material that resists abrasion, corrosion, and is impervious to most chemicals. It has the highest fracture resistance of any other advanced ceramic.

9 These properties have led to many applications throughout the industrial ecosystem.

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 aircraft.

10 If corrosion, rapid wear, and electric or magnetic fields limit the use of metals, silicon nitride is used as the alternative.

7,

11, 12 Silicon nitride is even present in the bearings of underwater ocean tidal flow meters, where it withstands severe seawater corrosion conditions.

13

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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 gas18 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 >10,000,000 years of space travel to tungstenetched memory chips for spacecraft.

25

Medical Implants:

Amedica Corporation provides 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 >25,000 spine implantations spanning almost 10 years and no reported failures, Amedica markets its products with confidence.

29

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Additionally, silicon nitride can 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 favorable imaging 36 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 oxidize, 40 and allograft bone never fully heals, 41 all of which lead to long-term failures. Today, for example, toxic wear from all-metal hip bearings is a well-known problem; 42 and fretting and electrochemical corrosion in total hips is a genuine concern.

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 oxidize 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 due to 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 while metals appear solid, obscuring visibility of bone anatomy behind the implant. CT scans and MRI scans suffer distortion from metal implants, leading to sub-optimal imaging. In contrast, silicon nitride is easily seen on X-rays, does not block imaging of bone anatomy behind the implant, and its dielectric and non-magnetic nature eliminates distortion in CT and MRI scans.

36

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

55

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Existing Medical Ceramics:

Ceramics such as alumina (Al

2

O

3

) and zirconia (ZrO

2

) have been used in hip and knee replacements because of less wear than metal surfaces.

56–58 Alumina is brittle; it can break suddenly.

59 Zirconia is stronger, but like a chameleon, it can transform after implantation, leading to erratic outcomes.

60 In 2002, zirconia was withdrawn 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 removes oxygen from polyethylene.

66, 67 This remarkable property could support hip and knee replacements beyond two decades of service, something that is only a speculation today.

Scientific & Clinical Data:

Aside from advantages in strength, wear resistance, corrosion resistance, and fracture toughness, 68 there is more to silicon nitride. Below are our recent findings on this novel bioceramic:

Bone Healing : Silicon nitride turns on osteoblasts (bone-forming cells) and suppresses osteoclasts (bone absorbing cells). A manufacturing change called “nitrogen-annealing” results in a near-200% increase in bone formation by cells exposed to silicon nitride.

54 This finding has profound implications.

Cell Proliferation Test

50

40

30

20

10

0

As+sintered

SI

3

N

4

HF-etched

SI

3

N

4

Oxidized

SI

3

N

4

N

2

+annealed

SI

3

N

4

Osteoconductivity Test

250

200

150

100

50

0

As+sintered

SI

3

N

4

HF-etched

SI

3

N

4

Oxidized

SI

3

N

4

N

2

+annealed

SI

3

N

4

+ 40% increase in cell proliferation upon nitrogen annealing

+ 190% increase in osteoconductivity upon nitrogen annealing

Since 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 interbody made of both 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

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Infection Prevention: Bacterial infection of any biomaterial implant 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 shown direct bactericidal effect against oral bacteria.

72

As with cell adhesion, the antibacterial behavior 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 optimize the desired properties for specific implants, is a potent advantage of the material.

35

1.E+07

1.E+06

1.E+05

1.E+04

1.E+03

1.E+02

1.E+01

1.E+00

S epidermidis on Biomaterials

PEEK Titanium Silicone Nitride

1.E+07

1.E+06

1.E+05

1.E+04

1.E+03

1.E+02

1.E+01

1.E+00

PEEK

E coli on Biomaterials

Titanium Silicone Nitride

24 Hours 48 Hours 24 Hours 48 Hours

Silicon nitride demonstrates superior resistance to E coli and S epidermidis biofilm formation relative to other commercial biomaterials.

The Future:

The world is becoming better informed, more mobile, and increasingly demanding, all while living longer than ever before.

Scientists agree that silicon nitride will lead the future of material innovations by addressing the challenges of biomedical implant safety, high-performance, and lifetime durability better than any other material.

73

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

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