Table 1.

The most frequently used bonding strength measurement methods and test.

No	Name of the Test	Required Equipment	Measured Value	Complexity	Special Sample Preparation Requirements	Restrictions/Possible Problems	Ref.
1	Manual peeling/ delamination test	Performed by hand	Bond formation (yes/no)	+	-	-Only as a first attempt to verify bond formation. -Results are subjective. -A successful bond might be too strong to peel manually.	[19,20,21,34,35,36,37]
2	Adhesive- assisted tensile strength measurement	Tensile testing machine, adhesive	Tensile strength [Pa]	+	An adhesive must be applied to the samples	-The tensile strength of the adhesive bond must be higher than the quantified bond between PDMS and substrate.	[38]
3	Cylinder-based tensile strength measurement	Tensile testing machine	Tensile strength [Pa]	++	Requires a special molding form for the PDMS.	-The size of the cylinder should be optimized to avoid rupture, and thin substrates are vulnerable to breaking. -Due to the special PDMS geometry, it cannot be used on functional elements or combined with other test methods.	[39,40]
4	Double- substrate-bonding-based tensile strength measurement	Tensile testing machine	Tensile strength [Pa]	++	Two substrates are required to be bonded to the PDMS. If screws are used to fix the sample to the machine, holes must be drilled into the substrate.	-The areas of the two bonds should be precisely controlled, the tensile strength of the bond should be calculated accordingly.	[41,42]
5	String-based tensile strength measurement (Pull test)	Tensile testing machine	Tensile strength [Pa]	+++	The PDMS block and a string should be cured together. A drilled hole is needed in the substrate.	-The bond between the string and PDMS is crucial. The string might be torn out from the PDMS block without detaching the PDMS from the substrate. -The obtained tensile strength might not be directly comparable with other methods.	[43,44,45,46,47]
6	Shear strength measurement	Shear testing machine	Shear strength [Pa](shear strain)	++	The width of the PDMS block should not be wider than the peeling tool.	-Parallel forces should be maintained, PDMS bending is a problem. -The obtained shear strength should be treated with reservations.	[48,49]
7	Lap shear strength measurement	Tensile testing machine	Lap shear strength [Pa]	++	Either two PDMS blocks are bonded to one substrate or two substrates to one PDMS block.	-The sizes of the bonded areas are crucial and should be properly controlled. -The shear strength result has to be calculated accordingly. -The clamping of the PDMS blocks might be difficult.	[50]
8	Peel test	Tensile testing machine, glass slide, adhesive	Peel strength [N/m]	+++	A partially bonded flexible foil, is required. The PDMS is bonded to a fixed support (e.g., a glass slide) with adhesive.	-The pulling angle should be kept 90° during the experiment to ensure normal forces. The movement of the supporting glass slide should be controlled accordingly. -The strength of the adhesive bond should be higher.	[53]
9	T-peel test	Tensile testing machine	Peel strength [N/m]	++	A partially bonded formation is required. A thin foil or a second PDMS substrate can be used.	The forces acting on the bond should be kept normal.	[54,55]
10	Static leakage test (durability test)	Ink, tubes, syringe, microfluidic cell	Time until leakage [h]	+	Microfluidic channels are required inside the PDMS block	-Proper ink leakage detection. -Might be time-consuming for good-quality bonds.	[20,40,42]
11	Dynamic leakage test	Ink, tubes, syringe, microfluidic cell, computer -controlled pump	Max. flow rate before leakage [mL/min]	++	Microfluidic channels are required inside the PDMS block.	Proper ink leakage detection.	[19,35,36,38,43,44,45,46,47,50,56,57]
12	Burst test	Compressed air or fluid, microfluidic cell, syringe pump, high-pressure tank, pressure sensor	Burst pressure [Pa]	++	A specific blister should be formed in the PDMS block.	-The size of the blister is important and should be properly controlled. -PDMS might explode from the substrate at high pressures.	[21,36,37,38,39,40,41,46,47,48,57,59,60,62]