Planar sensor

1. A planar CO 2 -sensing device comprising:

an electrode with an ion-selective layer,

an electrolyte layer comprising osmotically active species in an amount of 0.8 to 6.0 milliosmol per m 2 of the electrolyte layer area,

wherein the electrolyte layer comprises

at least two hydrophilic osmolarity increasing components having a weight average molecular weight in the range of 80 to 5,000 g/mole, wherein at least one of the at least two osmolarity increasing components has a weight average molecular weight in the range of 800 to 3,000 g/mole,

and wherein the at least two hydrophilic osmolarity increasing components do not add bicarbonate ions or chloride ions to the electrolyte layer and are present in the electrolyte layer in an amount ranging from 0.05 to 1.2 millimol per m 2 of the electrolyte layer area; and

an outer layer, wherein the electrolyte layer is arranged between the ion-selective layer and the outer layer.

2. The sensing device according to claim 1 , wherein the electrolyte layer comprises bicarbonate ions in an amount in the range of 0.1 to 1.0 millimol per m 2 of electrolyte layer area.

3. The sensing device according to claim 1 , wherein the electrolyte layer comprises chloride ions in an amount in the range of 0.05 to 1.0 millimol per m 2 of electrolyte layer area.

4. The sensing device according to claim 1 , wherein the electrolyte layer comprises osmotically active species in an amount of 0.8 to 2.5 milliosmol per m 2 of electrolyte layer area.

5. The sensing device according to claim 4 , wherein the hydrophilic osmolarity increasing components are present in the electrolyte layer in an amount in the range of 0.05 to 0.5 millimol per m 2 of electrolyte layer area.

6. The sensing device according to claim 4 , wherein the electrolyte layer comprises bicarbonate ions in an amount of in the range of 0.1 to 0.5 millimol per m 2 of electrolyte layer area.

7. The sensing device according to claim 4 , wherein the electrolyte layer comprises chloride ions in an amount in the range of 0.05 to 0.5 millimol per m 2 of electrolyte layer area.

8. The sensing device according to claim 1 , wherein at least one of the at least two hydrophilic osmolarity increasing components is selected from the group consisting of oligo(ethylene glycols), poly(ethylene glycols), monosaccharides, disaccharides, oligosaccharides, polysaccharides, and cyclodextrins.

9. The sensing device according to claim 1 , wherein the at least two hydrophilic osmolarity increasing components contribute by more than 8% to the total osmolarity of the electrolyte layer.

10. The sensing device according to claim 1 , wherein the electrolyte layer has a thickness in the range of 3 to 20 μm.

11. The sensing device according to claim 1 , wherein the electrolyte layer has a thickness in the range of 3 to 8 μm.

12. The sensing device according to claim 1 , wherein the electrolyte layer has a thickness in the range of 3 to 20 μm when in equilibrium with a standard human blood sample.

13. The sensing device according to claim 1 , wherein the electrolyte layer has a thickness in the range of 3 to 8 μm when in equilibrium with a standard human blood sample.

14. The sensing device according to claim 1 , wherein the electrolyte layer comprises osmotically active species of an osmolarity of 300 to 350 mOsM.

15. The sensing device according to claim 8 , wherein the at least one of the at least two hydrophilic osmolarity increasing components is poly(ethylene glycol) of a weight average molecular weight of 1500 g/mole.

16. The sensing device according to claim 1 , wherein the electrolyte layer further comprises an additional hydrophilic osmolarity increasing component of a weight average molecular weight of more than 5,000 g/mole.

17. The sensing device according to claim 16 , wherein the additional hydrophilic osmolarity increasing components is selected from the group consisting of poly(vinylpyrrolidone), methylcellulose, hydroxylpropyl methyl cellulose, cellulose and other cellulose derivatives, and agar.

18. The sensing device according to claim 1 , wherein the electrolyte layer further comprises an additional hydrophilic osmolarity increasing component of a weight average molecular weight of more than 10,000 g/mole.

19. The sensing device according to claim 17 , wherein the additional hydrophilic osmolarity increasing components is a hydroxypropyl methyl cellulose.

20. The sensing device according to claim 1 , wherein:

at least one of the at least two hydrophilic osmolarity increasing component is selected from the group consisting of oligo(ethylene glycols), poly(ethylene glycols), monosaccharides, disaccharides, oligosaccharides, polysaccharides, and cyclodextrins;

and wherein the electrolyte layer further comprises at least one additional hydrophilic osmolarity increasing component selected from the group consisting of poly(vinylpyrrolidone), methylcellulose, hydroxylpropyl methyl cellulose, cellulose and other cellulose derivatives, and agar.

21. The sensing device according to claim 1 , wherein:

at least one of the at least two hydrophilic osmolarity increasing components is a poly(ethylene glycol) with a weight average molecular weight of 1,500 g/mole;

and wherein the electrolyte layer further comprises an additional hydrophilic osmolarity increasing component chosen from a hydroxypropyl methyl cellulose.

22. The sensing device according to claim 1 , wherein the at least two hydrophilic osmolarity components are

a poly(ethylene glycol) and

a glycerol.

23. The sensing device according to claim 8 , wherein the at least one of the at least two hydrophilic osmolarity increasing components is an oligo(ethylene glycol) comprising a weight average molecular weight in the range of 800 to 3,000 g/mole.

24. The sensing device according to claim 8 , wherein the at least one of the at least two hydrophilic osmolarity increasing components is a poly(ethylene glycol) comprising a weight average molecular weight in the range of 800 to 3,000 g/mole.