A head unit (14/1, 10/2A, 12/27), connectable to an external device, the external device including an actuator (10/1) suitable for operating the head unit, the actuator being fully disposed within the external device, the head unit comprising: a first portion arranged about a main longitudinal axis (20/2A) of said head unit, said first portion having a first rigidity, said first portion including a first connection region (16/2A) reversibly connectable to the external device; a second portion having a second rigidity, smaller than said first rigidity, said second portion comprising a flexible and elastic membrane (30/2A, 13/27), said membrane including: a first protrusion (17/2A, 16/27) extending outwardly from said membrane away from said first portion and including a first extreme point associated with a first virtual tangential plane (15/2A); and a second protrusion (19/2A, 18/27) extending outwardly from said membrane away from said first portion and including a second extreme point associated with a second virtual tangential plane (14/2A); and an intermediate portion (12/2A, 14/27), fixedly attached to said first portion and disposed between said first portion and said second portion, said intermediate portion having a third rigidity, different from said first rigidity and from said second rigidity, wherein a perimeter of said membrane is attached to said intermediate portion along a perimeter of said intermediate portion, wherein said head unit can be operated by the actuator of the external device in a periodic manner with at least one amplitude and at least one frequency, wherein said first and second protrusions are adapted, during operation of said head unit, to engage and apply force to an external surface (40/27) in a graded manner, characterized in that an interior volume (44/27) is formed between said first portion and said second portion, said interior volume being sealed to an environment surrounding the head unit, and wherein, in a rest state of the head unit and during operation of the head unit, said interior volume is filled with a fluid (36/27). a first portion arranged about a main longitudinal axis (20/2A) of said head unit, said first portion having a first rigidity, said first portion including a first connection region (16/2A) reversibly connectable to the external device; a second portion having a second rigidity, smaller than said first rigidity, said second portion comprising a flexible and elastic membrane (30/2A, 13/27), said membrane including: a first protrusion (17/2A, 16/27) extending outwardly from said membrane away from said first portion and including a first extreme point associated with a first virtual tangential plane (15/2A); and a second protrusion (19/2A, 18/27) extending outwardly from said membrane away from said first portion and including a second extreme point associated with a second virtual tangential plane (14/2A); and a first protrusion (17/2A, 16/27) extending outwardly from said membrane away from said first portion and including a first extreme point associated with a first virtual tangential plane (15/2A); and a second protrusion (19/2A, 18/27) extending outwardly from said membrane away from said first portion and including a second extreme point associated with a second virtual tangential plane (14/2A); and an intermediate portion (12/2A, 14/27), fixedly attached to said first portion and disposed between said first portion and said second portion, said intermediate portion having a third rigidity, different from said first rigidity and from said second rigidity, wherein a perimeter of said membrane is attached to said intermediate portion along a perimeter of said intermediate portion, wherein said head unit can be operated by the actuator of the external device in a periodic manner with at least one amplitude and at least one frequency, wherein said first and second protrusions are adapted, during operation of said head unit, to engage and apply force to an external surface (40/27) in a graded manner, characterized in that an interior volume (44/27) is formed between said first portion and said second portion, said interior volume being sealed to an environment surrounding the head unit, and wherein, in a rest state of the head unit and during operation of the head unit, said interior volume is filled with a fluid (36/27).