The ornamental design for a device for skintreatment, as shown and described.
FIG. 1 is a perspective view of a device for skin treatment showing my new design;
FIG. 2 is a back view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a top view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the device for skin treatment and form no part of the claimed design.