Table 3. Data extraction of studies using interlayers.

S.no.	Author and year	Journal	Country	Material Characteristics	Properties evaluated	Results
1.	Okazaki 1998⁵³	Dental Materials Journal	Japan	Metal-ceramic restoration of palladium alloy with low-fusing ceramic. Intermediate layers used were gold intermediate layer consisting of pure gold paste (Au), gold-Pd paste (70: 30 wt/wt) and gold-porcelain paste (70: 30 wt/wt).	Bending test	The bonding between metal and porcelain was strengthened due to the mechanical grip created and the varied compositions found within the gold intermediate layer.
2.	Henriques 2012⁵⁴	Journal of the Mechanical Behaviour of Biomedical Materials	Portugal	Metal-ceramic of CoCrMo alloy with porcelain. Intermediate layers consisted of 50% metal-50% porcelain vol%.	Nano hardness	The study demonstrated improved performance of the new functionally graded restorations compared to conventional PFM restorations under fatigue testing conditions.
3.	Huang 2007⁵⁵	Journal of Materials Science: Materials in Medicine	USA Canada	Functionally graded layer between dental ceramic and cement layer. Ten layered (0.1 mm of each layer) of 10 μm thickness were used.	Stress distribution using FEA with critical crack length	Bioinspired functionally graded material layer alleviated stress within dental crown restoration structures. Finite element simulations indicate a substantial reduction in stress and an increase in the critical crack length through this approach.
4.	Niu 2009⁵⁶	Journal of the Mechanical Behaviour of Biomedical Materials	USA	Functionally graded layers between the top zirconia ceramic layer and dentin-like PMC foundation. Ten layered FGM of 10 μm thickness were used.	Stress distribution using FEA, Hertzian contact experiments, and slow crack growth rate analysis	Loading rate dependence of the critical loads is due to the combined effects of rate-dependent Young’s modulus (of the substrate layer) and slow crack growth in the top zirconia layer.
5.	Rahbar 2011⁵⁷	Fatigue and Fracture of Engineering Materials & Structures	USA	Functionally graded layer between porcelain veneered zirconia core and dentin foundation. Ten layered of 100-150 μm thickness (100 μ thick in 2D in horizontal and vertical gradations) were used.	Stress distribution using FEA, Hertzian contact experiments and critical crack length determination	The use of bioinspired functionally graded structures reduces high stress concentrations in the top ceramic layer of crown structures. Experimental evidence highlights a significant stress decrease (around 32%) and improved pop-in loads. Additionally, these graded layers enhance critical crack size.
6.	Du 2013⁵⁸	Acta Biomaterialia	USA Hong Kong Nigeria	Functionally graded nanocomposite layer was sandwiched between hard ceramic top layer and soft composite substrate. A 10-layered structure with the Young’s modulus increasing linearly from 18 GPa in the foundation to 205 GPa in the top ceramic layer.	Nanoindentation, Hertzian contact with slow crack growth model	Functionally graded material (FGM) structures effectively alleviate stresses in the top zirconia ceramic crowns. Significantly greater stress reductions were noted in the ceramic crowns when the moduli gradients within the FGM were more pronounced near the ceramic layer.
7.	Cui 2014⁵⁹	Dental Materials Journal	China	Functionally graded layer was sandwiched between bilayer ceramic (veneering layer and core ceramic layer) and cement with a dentin-like PMC foundation. Ten layered FGM of 0.01 mm each(layer 1 to layer 10 from ceramic core to cement) was used.	Stress distribution using FEA	The bio-inspired functionally graded material (FGM) layer notably decreased stress concentration at the interface between the crown and cement. This reduction in stress was more prominent with a higher elastic modulus in the ceramic core.
8.	Henriques 2015⁶⁰	Clencia & Technologia dos Materials	Brazil Portugal Finland	Composite interlayer (50% metal + 50% ceramic vol%) interface was used between metal and ceramic restoration.	Thermomechanical fatigue, Thermal residual stress, Shear bond strength between interlayers and metal-ceramic	There was a substantial enhancement in bond strength by 240%, improved resistance against thermomechanical fatigue, and a notable reduction in thermal residual stress.
9.	Henriques 2015⁶¹	Journal of the Mechanical Behaviour of Biomedical Materials	Brazil Portugal Finland	Composite interlayer (50% metal + 50% ceramic vol%) interface was used between metal and ceramic restoration.	Residual and thermal stress, stress distribution using FEA and tensile stress	The introduction of a 50% metal–50% ceramic interlayer and the functionally graded material (FGM) interlayer led to a noteworthy reduction in thermal residual stress and improved stress profiles. Maximum stress magnitudes decreased by 10%, while the FGM interlayer showcased a more substantial reduction of 20% in maximum stress. Additionally, there was a 19% reduction in tensile stress observed.
10.	Henriques 2017⁶²	Materials Science and Engineering C	Brazil Portugal USA	Homogenous and graded interlayers between the veneer and core all-ceramic restorations with varying thicknesses and compositions were used. Interlayer composition consisted of feldspar-based porcelain and zirconia/ alumina mixture having constant composition and fractions of porcelain (10%,30%, 50%, 70%, 90%).	Thermal residual stress	The interlayer’s composition and thickness impact residual thermal stresses. An optimal composition of 30–50% porcelain balances stresses within the framework and interfaces. Increased interlayer thickness leads to stress reduction. Graded one displayed the best thermal stress distribution, reducing stress within the framework and at the interface simultaneously.
11.	Askari 2018⁶³	Journal of the Mechanical Behaviour of Biomedical Materials	Denmark Portugal	Intermediate layer composed from a mixture of porcelain and zirconia with continuous change in volume fraction of porcelain through the thickness sandwiched between top and bottom layers of monolithic porcelain and zirconia, respectively.	Thermal residual stress	A particle swarm optimization algorithm was created to minimize stresses resulting from cooling down after processing. An analytical model, employing laminate theory, estimated the thermal residual stresses (TRS) in graded dental prostheses. Additionally, Hsueh’s model was used to predict bending stresses in graded zirconia systems.
12.	Fabris 2016⁶⁴	Ceramics international	Brazil USA	Continuous and step-wise gradations from zirconia and alumina core to porcelain veneer were used.	Thermal residual stress, Bending stress distribution	Alumina-porcelain discs exhibit lower residual stress compared to zirconia-porcelain discs. A uniform interlayer can reduce thermal stress by approximately 40% compared to bilayered systems. Thinner interlayers tend to result in lower thermal stresses.
13.	Pentadeol 2019⁶⁵	American Journal of Dentistry	Italy	Bioinspired crown with decreasing elastic modulus (90-30 GPa) and crowns with increasing elastic modulus (from 30-90 GPa) were used with controls of rigid and flexible crowns (30 GPa). 10 layered with a gradual transition of elastic modulus (18-205 GPa) from internal to the external surface of the restoration was used.	Stress dixstribution using FEA	Bioinspired crowns showed acceptable stress distribution
14.	Sait 2023⁶⁶	Journal of the Mechanical Behaviour of Biomedical Materials	Turkey	Porcelain fused to metal mandibular for a 1st molar restoration was used. A phase-field model was used for the smooth transition from one phase to another.	Phase field model analysis and stress distribution using FEA	The FGM crown experiences lower stresses from material property mismatches. Additionally, it demonstrates increased resilience against the separation of the porcelain top layer.