referring to the drawings , fig1 shows a toothbrush 1 having a head 2 integrally formed with a handle 3 via a neck . the head 2 is provided with bristles 4 made of nylon . the head 2 and handle 3 are made of a resilient transparent material such as polymethyl methacrylate and define a cavity 5 extending substantially along the entire length of the brush 1 . the cavity 5 is filled with a piezochromic material , e . g . a liquid crystal cholesterol ester . a predetermined pressure ( suitable for brushing teeth ) applied to the bristles 4 , or a predetermined flexure of the handle 3 , will be transmitted to , and thereby cause an optical response ( such as a change of colour ) of , the piezochromic material . the cavity 5 may , instead of extending substantially the entire length of the brush 1 , be more localised , for example , it may be confined to a neck region 6 joining the handle 3 to the head 2 . fig2 and 3 show respectively a side and plan view of the head 12 of a second embodiment of the invention to a larger scale . bristles 14 are interleaved with plates 17 made of piezochromic material liquid crystal cholesterol ester . pressure on the brush head 12 in the direction of the arrow a causes flexure of the bristles 14 and the plates 17 in the way shown , and this deformation is transmitted to the plates 17 which will cause an optical response such as a change in colour in the piezochromic material should the correct brushing pressure be applied . fig4 shows a side view of a third embodiment of the invention , and shows a brush head 22 provided with bristles 24 . the base of each bristle 24 is attached to a flexible membrane 28 which contains the piezochromic material liquid crystal cholesterol ester . pressure applied to the bristles 24 in the direction of the arrow b is transmitted to the membrane 28 , and causes it to deform . deformation of the membrane 28 stresses the piezochromic material contained within it , causing an optical response in the material such as a change of colour on the application of suitable pressure . fig5 shows a modified version of the brush head of fig4 in which piezochromic material is contained in a sac 29 . pressure on the bristles 24 is transmitted to the sac 29 via flexible membrane 28 causing it to deform , thereby causing an optical response in the piezochromic material . in this modified brush head , the membrane 28 does not contain piezochromic material . indeed , in a further modification , the membrane is not required , in which case pressure on the bristles 24 is transmitted directly to the sac 29 . fig6 shows a side view of a fourth embodiment of the invention , in which a handle 33 and a neck 36 of the brush 31 are pivotally connected at 40 . a portion 41 ( indicated in dotted lines ) of the neck 36 extends beyond the pivot 40 into a cavity 35 formed in the handle 33 . the neck 36 is integral with the head 32 of the brush 31 . in this embodiment at least the handle 33 is formed of a transparent plastics material such as polymethyl methacrylate . pressure applied to the bristles 34 of the brush 31 in the direction of the arrow c , whilst the handle 33 is being held firmly , causes the handle and the head 32 to rotate relative to each other about the pivot 40 in the directions of the arrows d 1 and d 2 . rotation is impeded as the neck portion 41 comes into contact with a pad 42 containing a piezochromic material . continued mechanical pressure on the head 32 of the brush 31 is transmitted to the pad 42 causing an optical response in the piezochromic material . fig7 illustrates the role of the integrated circuit in certain embodiments of the invention . the integrated circuit will receive input from the pressure sensor and by way of a simple logic gate open or close the circuit to the indicator . for example , when brushing pressure is greater than zero the logic gate allows current to flow to the liquid crystal , which may be located in a cell as commonly found in calculator screens , and so provide a signal . as pressure is increased current may be allowed to flow to more cells indicating to the user that pressure is increasing . when pressure reaches an unacceptable level the logic gate closes the circuit to the indicator and the signal is withdrawn . it will be apparent that modifications could be made to the embodiments described above . in particular , the piezochromic material could be replaced by any material ( such as a mechanochromic or thermochromic material ) that alters its visual appearance in response to a mechanical strain . for example , a liquid crystal that changes colour when subjected to a mechanical strain could be used . alternatively , it is also envisaged that polarised materials will be used , with pieces of polarised material being configured such that their planes of polarisation are at 90 ° c . to each other . one of the pieces of material is fixed , and the other is able to move on the application of excessive pressure ( for example by being attached to a mechanical type embodiment as described in conjunction with fig6 above ) such that a different orientation of polarisation planes is achieved , and thereby a colour change is observed . otherwise , also envisaged is an embodiment of sheet form polarisors which are orientated parallel to each other , and which are separated by an arrangement such as a coiled spring and / or a helical arrangement , whereby the two polarisors are caused to rotate relative to each other when they are brought closer to or further apart from each other in response to changes in brushing pressure . as a result , the polarisors adopt a different configuration relative to each other , and a colour change is observed .
a toothbrush incorporates an indicator comprising a pressure indicating material which provides an optical signal without the provision of a power supply and wherein the signal is provided when the correct brushing pressure is being applied . preferably , the pressure sensitive material is a piezochromic material , such as a liquid crystal cholesterol ester .