PDMS Bonding Technologies for Microfluidic Applications: A Review
Abstract
:1. Introduction
2. Bond-Strength Testing Methods
2.1. Manual Peeling/Delamination Test
2.2. Tensile Strength Measurements
2.3. Shear Strength Measurement
2.4. Peel Test
2.5. Leakage Test
2.6. Burst Test
3. Bonding Methods
3.1. Surface Activation by Oxygen Plasma Treatment
3.2. Surface Activation by Corona Treatment
3.3. Surface Activation by UV/Ozone Treatment
3.4. Chemical Gluing
3.5. Adhesive-Based Gluing
4. Bonding Strategies for Various Substrates
4.1. PDMS
4.2. Glass (Silicon)
4.3. PMMA
4.4. PC
4.5. PS
4.6. PET
4.7. PI
4.8. Other Polymer Substrates
4.9. Metals
5. Conclusions
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Abbreviations
References
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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] |
Substrate Type | Bonding Technology (see Table 3) | Test Method (see Table 1) | Test Results | Ref. | |
---|---|---|---|---|---|
Value | Parameter/Condition | ||||
PDMS | Corona treatment | 11 | >60 µL/min | max. tested flow rate, ~1500 × min−1 internal volume | [19] |
12 | 290 kPa | average burst pressure (air), variation: 227–380 kPa | [60] | ||
O2 plasma | 5 | 91 kPa | tensile strength | [45] | |
9 | 190 N/m | peel strength, for optimized plasma power | [55] | ||
11 | 10 mL/min | max. flow rate, ~30,000 × min−1 internal volume | [56] | ||
12 | 300 kPa | average burst pressure (air), variation: 180–515 kPa | [60] | ||
12 | 400 kPa | burst pressure (N2), with optimized parameters (range: 69–400 kPa) | [59] | ||
12 | 105–180 kPa | burst pressure (air) | [62] | ||
Air plasma | 9 | 400 N/m | max. peel strength, with optimized parameters | [54] | |
O2 plasma + APTES / GPTMS surface treatment | 5 | 184 kPa | tensile strength | [45] | |
11 | 5 mL/min | no leakage was observed, 2000 × min−1 internal volume | [45] | ||
UV illumination | 12 | 87–95 kPa | burst pressure (air) | [62] | |
DMPMS pre- polymer gluing | 11 | 330–500 µL/min | max. flow rate, ~6600 × min−1, depending on the preparation | [35] | |
PDMS pre- polymer gluing | 12 | 671 kPa | average burst pressure (air), variation: 545–690 kPa | [60] | |
12 | >500 kPa | burst pressure (air) | [62] | ||
Glass | Corona discharge | 7 | 94 kPa | max. lap shear strength | [50] |
O2 plasma | 6 | 125–245 kPa | max. shear stress, for bonding areas between 10–90 mm2 | [48] | |
6 | 70–113 kPa | max. shear stress | [49] | ||
11 | 4 mL/min | max. flow rate before leakage ~12000 × min−1 internal volume | [56] | ||
12 | 185–270 kPa | burst pressure (air) | [62] | ||
12 | 190–610 kPa | burst pressure (ink), for bonding areas between 10–90 mm2 | [48] | ||
12 | 510 kPa | burst pressure (N2), with optimized parameters (range: 276–510 kPa) | [59] | ||
UV illumination | 12 | 270–314 kPa | burst pressure (air) | [62] | |
DMPMS pre- polymer gluing | 11 | 500 µL/min | max. flow rate, ~6600× min−1, with optimized preparation | [35] | |
PDMS pre- polymer gluing | 12 | >500 kPa | burst pressure (air) | [62] | |
Parylene coated silicon | PDMS pre- polymer gluing | 3 | 400 kPa | tensile strength | [39] |
12 | 36 kPa | burst pressure (water) | [39] | ||
Plasma treatment with N2 and SF6 mixture | 3 | 1.4 MPa | bond tensile strength with optimized parameters | [39] | |
12 | 145 kPa | burst pressure (water) | [39] | ||
Gold-coated glass | UV illumination + MPTMS surface treatment | 7 | 120 kPa | max. lap shear strength | [50] |
PMMA | PDMS pre- polymer gluing | 4 | 15 kPa | tensile strength | [42] |
Corona discharge | 5 | 336 kPa | tensile strength | [43] | |
Corona discharge + APTES surface treatment + epoxy term. PDMS linker | 5 | 306 kPa | tensile strength | [47] | |
11 | 45 mL/min | no leakage for 3000× min−1 internal volume | [47] | ||
12 | 586 kPa | burst pressure (air) | [47] | ||
O2 plasma + APTES surface treatment | 2 | 385 kPa | tensile strength | [38] | |
4 | 1.6 MPa | max. tensile strength, with optimized APTES functionalization | [41] | ||
11 | 60 mL/min | no leakage at 24,000 × min−1 internal volume | [38] | ||
12 | 100 kPa | delamination pressure in water, >500 kPa in air | [37] | ||
12 | 260 kPa | average burst pressure (water), with optimized APTES functionalization | [41] | ||
O2 plasma + APTES surface treatment + Corona discharge | 4 | 2.5 MPa | max. tensile strength, with optimized APTES functionalization | [41] | |
12 | 300 kPa | average burst pressure (water), with optimized APTES functionalization | [41] | ||
O2 plasma + APTES/TESPSA surface treatment | 5 | 259 kPa | tensile strength | [46] | |
11 | 30 mL/min | no leakage at 3000× min−1 internal volume | [46] | ||
12 | 345 kPa | burst pressure (air) | [46] | ||
O2 plasma + APTES/GPTMS surface treatment | 5 | 180 kPa | tensile strength | [44] | |
11 | 30 mL/min | no leakage at 2000× min−1 internal volume | [44] | ||
12 | 510–538 kPa | burst pressure (air) | [44] | ||
PC | Corona discharge + MPTMS surface treatment | 5 | 511 kPa | tensile strength | [43] |
Corona discharge + APTES surface treatment + epoxy terminated PDMS linker | 5 | 220 kPa | tensile strength | [47] | |
12 | 620 kPa | burst pressure (air) | [47] | ||
O2 plasma + APTES surface treatment | 2 | 430–490 kPa | tensile strength | [38] | |
11 | 60 mL/min | no leakage at 24,000 × min−1 internal volume | [38] | ||
12 | >228 kPa | burst pressure (water) | [58] | ||
12 | 100 kPa | delamination pressure in water, >500 kPa in air | [37] | ||
O2 plasma + APTES/TESPSA surface treatment | 5 | 477 kPa | tensile strength | [46] | |
11 | 30 mL/min | no leakage at 3000× min−1 internal volume | [46] | ||
12 | 413 kPa | burst pressure (air) | [46] | ||
O2 plasma + APTES / GPTMS surface treatment | 5 | 178 kPa | tensile strength | [44] | |
11 | 30 mL/min | no leakage at 2000× min−1 internal volume | [44] | ||
12 | 524–579 kPa | burst pressure (air) | [44] | ||
PS | Corona discharge + APTES surface treatment + epoxy terminated PDMS linker | 5 | 476 kPa | tensile strength | [47] |
12 | 620 kPa | burst pressure (air) | [47] | ||
Air plasma | 10 | - | no leakage with 1 M HCl for 3 days, 1 M NaOH for 1 week, water for 1 month | [36] | |
11 | 15 mL/min | no leakage at 9000× min−1 internal volume | [36] | ||
12 | >500 kPa | burst pressure (air) | [36] | ||
O2 plasma + APTES surface treatment | 2 | 388 kPa | tensile strength | [38] | |
11 | 60 mL/min | no leakage at 24,000 × min−1 internal volume | [38] | ||
3 | 12 kPa | tensile strength | [40] | ||
O2 plasma + APTES / TESPSA surface treatment | 5 | 520 kPa | tensile strength | [46] | |
11 | 30 mL/min | no leakage at 3000× min−1 internal volume | [46] | ||
12 | 448 kPa | burst pressure (air) | [46] | ||
O2 plasma + epoxy adhesive | 11 | 500 µL/min | no leakage at 1000× min−1 internal volume | [57] | |
12 | >414 kPa | burst pressure (air) | [57] | ||
PET | Corona discharge + APTES surface treatment + epoxy terminated PDMS linker | 5 | 189 kPa | tensile strength | [47] |
Corona discharge + MPTMS surface treatment | 5 | 476 kPa | tensile strength | [43] | |
O2 plasma + APTES / GPTMS surface treatment | 1 | - | successful bonding | [45] | |
12 | 579 kPa | burst pressure (air) | [44] | ||
O2 plasma + APTES/TESPSA surface treatment | 5 | 458 kPa | tensile strength | [46] | |
11 | 30 mL/min | no leakage at 3000× min−1 internal volume | [46] | ||
12 | 379 kPa | burst pressure (air) | [46] | ||
KOH activation + MPTMS surface treatment + O2 plasma | 1 | - | successful bonding— no delamination with optimized parameters | [20] | |
10 | - | no leakage during 1 month storage | [20] | ||
PI | epoxy adhesive | 8 | 1.7 N/m | peel strength | [53] |
UV/ozone + silicone adhesive | 8 | 72 N/m | peel strength | [53] | |
UV/ozone treatment + APTES/GPTMS surface treatment | 8 | 2.7 N/m | peel strength | [53] | |
UV/ozone treatment + MPTMS/GPTMS surface treatment | 8 | 200 N/m | peel strength | [53] | |
UV/ozone treatment + MPTMS by liquid deposition + epoxy adhesive | 8 | 470 N/m | peel strength | [53] | |
KOH activation + MPTMS surface treatment + O2 plasma | 1 | - | successful bonding— no delamination with optimized parameters | [20] | |
10 | - | no leakage for 1-month storage | [20] | ||
PP | Air plasma | 10 | - | no leakage with 1 M HCl for 3 days, 1 M NaOH for 1 week, water for 1 month | [36] |
11 | 15 mL/min | no leakage at 9000× min−1 internal volume | [36] | ||
12 | >500 kPa | burst pressure (air) | [36] | ||
COC | Air plasma | 12 | >500 kPa | >500 kPa burst pressure (air) | [36] |
11 | 15 mL/min | no leakage at 9000× min−1 internal volume | [36] | ||
10 | - | no leakage with 1 M HCl for 3 days, 1 M NaOH for 1 week, water for 1 month | [36] | ||
O2 plasma | 1 | - | the bond was not permanent | [21] | |
12 | 150 kPa | burst pressure (air) | [21] | ||
O2 plasma + APTES surface treatment | 1 | - | strong bond | [21] | |
12 | 380 kPa | burst pressure (air) | [21] | ||
2 | 432 kPa | tensile strength | [38] | ||
11 | 60 mL/min | no leakage at 24,000 × min−1 internal volume | [38] | ||
O2 plasma + GPTMS surface treatment | 1 | - | strong bond | [21] | |
O2 plasma + APTES/GPTMS surface treatment | 1 | - | stronger bond than with only APTES or GPTMS | [21] | |
12 | >800 kPa | burst pressure (air), after 6 months ≥ 700 kPa | [21] | ||
ABS | O2 plasma + APTES surface treatment | 12 | 100 kPa | delamination pressure in water, >500 kPa in air | [37] |
PEN | KOH activation + MPTMS surface treatment + O2 plasma | 1 | - | successful bonding— no delamination with optimized parameters | [20] |
10 | - | no leakage for 1-month storage | [20] | ||
PVC | Corona discharge + MPTMS surface treatment | 5 | 467 kPa | tensile strength | [43] |
Substrate Type | Molding Form and Fabrication Technology | PDMS Curing | Surface Activation Parameters | Chemical Gluing Parameters | Adhesives | Treated Side | Bonding Conditions | Ref. | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
R [[–] | T [°C] | t [min] | Type | t [s] | P [W] | p [mTorr] | f [sccm] | Type | Conc. [%] | T [°C] | t [min] | PDMS | Substrate | T [°C] | t [min] | p [MPa] | ||||
PDMS | NA | 1:10 | 60 | 60 | corona | 30–40 | - | - | - | - | - | - | - | - | activation | activation | RT | 30–60 | - | [19] |
PEN, PET, PI | PDMS: one-step photolithography and deep reactive ion etching (DRIE) plastics: - | 1:10 | 90 | 60 | O2 plasma (PDMS)3M KOH activation + O2 plasma (plastics) | 20 10 | 0.4/cm2 0.4/cm2 | 100 100 | - | MPTMS | 5.5 (m) | RT | 120 | - | activation | KOH activation + MPTMS + plasma treatment | - | - | - | [20] |
COC | PDMS: -substrate: hot embossing | 1:10 | 80 | 60 | O2 plasma | 20 | 500 | 8 | - | APTES GPTMS | 5 (a) 5 (a) | 50 50 | 60 60 | - | activation +GPTMS | activation +APTES | 80 | 120 | - | [21] |
PDMS, glass | PDMS: SU-8 soft-lithography substrate: - | 1:10 | 75 | 60 | - | - | - | - | - | - | - | - | - | pre-polymer gluing with DMPMS (7 µm, spin coated) | pre-polymer glue | pre-polymer glue | 70 | 240 | 0.02 | [35] |
COC, PP, PS | PDMS: soft-lithography, glass mold fabricated by microlithography plastic: - | 1:10 | 80 | 60 | air plasma | 30 | 150 (COC) 250 (PP) 150 (PS) | 75 | 14 | - | - | - | - | - | activation | activation | - | - | - | [36] |
ABS, PC, PMMA | PDMS: one-step photolithography and deep reactive ion etching (DRIE) plastics: - | 1:10 | 60 | 60 | O2 plasma | 60–300 | 50 | 1500 | - | APTES | 5 (m) | 60 | 20 | - | activation | activation + APTES | 80 | 60 | - | [37] |
COC, PC, PMMA, PS | PDMS: SU-8 soft-lithography plastics: - | 1:10 | 80 | 30 | O2 plasma | 60 | 60 | - | - | APTES | 1 (a) | RT | 20 | - | activation + APTES | activation | RT | 60 | - | [38] |
parylene C on glass | PDMS: SU-8 soft-lithography substrate: - | 1:10 | RT | 720 | mixed plasma | 60 | 300 | 5 | 50 (N2) 30 (SF6) | - | - | - | - | pre-polymer gluing with PDMS | pre-polymer glue + plasma treatment | pre-polymer glue + plasma treatment | RT | 720 | - | [39] |
PS | PDMS: SU-8 soft-lithography plastic: - | 1:13 | 65 | 120 | O2 plasma | 60 | 30 | 320 | - | APTES | 1 (a) | RT | 20 | - | activation | activation + APTES | 65 | 60 | - | [40] |
PMMA | PDMS: SU-8 soft-lithography plastic: - | 1:10 | 65 | 240 | O2 plasma corona | 60 60 | 200 - | - - | 50 - | APTES | 5 (a) | 85 | 1 | - | activation (corona) | activation (O2) + APTES + corona | 65 | 120 | - | [41] |
PET, PI, PP, PS, PVC, metals | PDMS: -substrates: - | - | - | - | corona | 120 | - | - | - | MPTMS | 2 (a) | RT | 1-5 | - | activation | MPTMS + corona treatment | RT | 10 | - | [43] |
PC, PET, PI, PMMA | PDMS: SU-8 soft-lithography plastics: hot embossing | 1:10 | 80 | 30 | O2 plasma | 60 | 50-60 | - | - | APTES GPTES | 1 (a) 1 (a) | RT RT | 20 20 | - | activation + APTES | activation + GPTES | RT | 60 | - | [44] |
PDMS, PET | PDMS: SU-8 soft-lithography plastic: - | 1:10 | 80 | 30 | O2 plasma | 60 | - | - | - | APTES GPTMS | 1 (a) 1 (a) | RT RT | 20 20 | - | activation + APTES | activation + GPTMS | RT | 60 | - | [45] |
PC, PET, PMMA, PS | PDMS: -plastics: CNS milling, engraving | 1:10 | 80 | 120 | O2 plasma | 60 | - | - | - | APTES TESPSA | 1 (a) 1 (a) | RT RT | 30 30 | - | activation + TESPSA | activation + APTES | RT | - | - | [46] |
PC, PET, PMMA, PS | PDMS: SU-8 soft-lithography plastics: CNC milling, engraving | 1:10 | 80 | 60 | corona | 60 | - | - | - | APTES | 5 (a) | 80 | 20 | monoglycidyl ether terminated, low-molecular-weight PDMS was added at 80 °C for 4 h | activation | activation + APTES + adhesive | 25 | 15 | 0.1 | [47] |
glass | PDMS: SU-8 soft-lithography substrate: - | 1:10 | 60 + 100 | 45 + 135 | O2 plasma | 35 (PDMS) 120 (glass) | 10.5 (PDMS) 18 (glass) | - - | 60 (glass) 40 (PDMS) | - | - | - | - | - | activation | activation | - | - | - | [48] |
glass | - | 1:10 | 100 | 60 | O2 plasma | 300 | 200 | - | 60 | - | - | - | - | - | activation | activation | 75 | 300 | 0.5 | [49] |
gold on glass | PDMS: SU-8 soft-lithography substrate: - | 1:10 | 70 | 60 | UV/ozone | 300 | - | - | - | MPTMS | 2 (e) | RT | 60 | - | activation + MPTMS | cleaning | 60 | 60 | - | [50] |
glass | PDMS: SU-8 soft-lithography substrate: - | 1:10 | 70 | 60 | corona | 120 | - | - | - | - | - | - | - | - | activation | activation | 80 | 720 | - | [50] |
PI | - | 1:10 | 60 | 120 | UV/ozone | 600 | - | - | - | MPTMS GPTMS | 1 (m) 1 (m) | RT RT | 60 60 | epoxy adhesive (LePage Gel epoxy adhesive) —optionally | activation + MPTMS | activation + GPTMS or MPTMS + epoxy | RT | 720 | 0.03 | [53] |
PDMS | - | 1:10 | 200 | 8 | air plasma | 50 | 18 | 200 | - | - | - | - | - | - | activation | activation | RT | 5 | 0.03 | [54] |
PDMS | - | 1:10 | 80 | 60 | O2 plasma | 300 | 300 | atm | 15 | - | - | - | - | - | activation | activation | 160 | 20 | 1.4 | [55] |
PDMS, glass | PDMS: SU-8 soft-lithography substrate: - | 1:10 | 65 | 60 | O2 plasma | 12 | 150 | - | - | - | - | - | - | - | activation | activation | - | - | - | [56] |
PS | PDMS: SU-8 soft-lithography plastic: - | 1:10 | 60 | 180 | O2 plasma | 30 (PDMS) 75 (PS) | 18 | 45 | 100 | - | - | - | - | silicone adhesive (PrimeCoat, 1 µm) epoxy adhesive (Epoxy 301-2, 3-4 µm) | activation + adhesives | activation | 60 | 180 | - | [57] |
glass | PDMS: SU-8 soft-lithography | - | - | - | O2 plasma | 20 | 20 | 1000 | - | - | - | - | - | - | activation | activation | - | - | - | [59] |
PDMS | mold fabricated by Xurography | 1:10 | 60 | 720 | corona | 30 | - | - | - | - | - | - | - | - | activation | activation | - | - | - | [60] |
PDMS | mold fabricated by Xurography | 1:10 | 60 | 720 | O2 plasma | 20 | 20 | 700 | - | - | - | - | - | - | activation | activation | - | - | - | [60] |
COC, PET, PMMA, PS, glass, metals | PDMS: SU-8 soft-lithography plastics: - | 1:10 | 80 | 90 | O2 plasma | 60 | 25 | - | - | APTES | 1 (a) | RT | 20 | epoxy adhesive (NOA74), deposited by spin coating (6000 rpm 1 min), cured with UV lamp (20 mW/cm2) | activation + APTES + NOA74 epoxy glue | - | - | 90 | - | [61] |
PDMS, glass | PDMS: Aluminium mold, CNC milled substrate: - | 1:10 | 70 | 180 | O2 plasma UV/ozone | 180–300 180–300 | - - | - - | - | - | - | - | - | - | activation | activation | 80 | 15 | - | [62] |
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Borók, A.; Laboda, K.; Bonyár, A. PDMS Bonding Technologies for Microfluidic Applications: A Review. Biosensors 2021, 11, 292. https://doi.org/10.3390/bios11080292
Borók A, Laboda K, Bonyár A. PDMS Bonding Technologies for Microfluidic Applications: A Review. Biosensors. 2021; 11(8):292. https://doi.org/10.3390/bios11080292
Chicago/Turabian StyleBorók, Alexandra, Kristóf Laboda, and Attila Bonyár. 2021. "PDMS Bonding Technologies for Microfluidic Applications: A Review" Biosensors 11, no. 8: 292. https://doi.org/10.3390/bios11080292
APA StyleBorók, A., Laboda, K., & Bonyár, A. (2021). PDMS Bonding Technologies for Microfluidic Applications: A Review. Biosensors, 11(8), 292. https://doi.org/10.3390/bios11080292