An assembly (101; 102; 103; 104; 201) for forming an injection device (151, 152, 153, 154) for administering a fluid to a subject, the assembly comprising: - a foot part having a first contact surface adapted to be placed on a skin of the subject, the foot part having a tubular shape for receiving a body (20; 60); - the body (20; 60) comprising at least one needle fixedly mounted to the body (20; 60), and comprising a channel in fluid communication with the at least one needle for delivering the fluid to be administered to the subject, characterized in that the body (20; 60) is movably mounted to the foot part for allowing movement of the body (20; 60) from a first position in which the needle is in a retracted position not extending out of the first contact surface and not accessible from the outside of the assembly, to a second position in which the needle extends out of said first contact surface by a predefined distance for limiting a penetration depth of the needle; - the assembly further comprises first friction means (21,22,23,24) for inhibiting movement of the body (20; 60) relative to the foot part when the body (20; 60) is in the first position, until a predefined static friction force is overcome, and for causing or allowing a sudden acceleration of the body (20; 60) towards the foot part for increasing a speed of the needle for increasing chance of penetration; - the assembly further comprises second friction means (21,22) for creating a dynamic friction between the foot part and the body (20; 60) when the body (20; 60) is moving towards the foot part for maintaining contact with the skin, the predefined dynamic friction being smaller than the predefined static friction. - a foot part having a first contact surface adapted to be placed on a skin of the subject, the foot part having a tubular shape for receiving a body (20; 60); - the body (20; 60) comprising at least one needle fixedly mounted to the body (20; 60), and comprising a channel in fluid communication with the at least one needle for delivering the fluid to be administered to the subject, characterized in that the body (20; 60) is movably mounted to the foot part for allowing movement of the body (20; 60) from a first position in which the needle is in a retracted position not extending out of the first contact surface and not accessible from the outside of the assembly, to a second position in which the needle extends out of said first contact surface by a predefined distance for limiting a penetration depth of the needle; - the assembly further comprises first friction means (21,22,23,24) for inhibiting movement of the body (20; 60) relative to the foot part when the body (20; 60) is in the first position, until a predefined static friction force is overcome, and for causing or allowing a sudden acceleration of the body (20; 60) towards the foot part for increasing a speed of the needle for increasing chance of penetration; - the assembly further comprises second friction means (21,22) for creating a dynamic friction between the foot part and the body (20; 60) when the body (20; 60) is moving towards the foot part for maintaining contact with the skin, the predefined dynamic friction being smaller than the predefined static friction.