the hydrophilic material that is used , in the present invention , as the material of the insert may be of known type . the ability to produce shapes of a hydrophilic material in a stressed condition , which regain their unstressed shape on hydration , and suitable hydrophilic materials , are described , for example , in gb - a - 1566552 and u . s . pat . no . 4 , 565 , 722 . for example , a cylindrical piece of suitable material can be stressed to produce a longer , tubular piece ; on hydration , the cylindrical shape is obtained again , but larger . suitable hydrophilic materials are known to those skilled in the art . the present invention relies on the application of such known technology , by the inclusion of a pre - stressed hydrophilic insert in a hypodermic syringe , e . g . in the body of the syringe or in the needle , such that the insert retains its stressed shape until it is hydrated , immediately after the first use of the syringe . a particularly preferred material is a copolymer of acrylonitrile and vinylpyrrolidone . this has a high tear strength and is therefore resistant to being ruptured on pre - stressing . a hypodermic syringe generally comprises a body ( e . g . of a plastics material ) and a piston defining a chamber for liquid to be injected , and a hypodermic needle of say , steel . for use in the invention , the body , the needle or an intermediate connecting part may be adapted to define a second chamber , that is out of contact with the injectable liquid before use , but through which the liquid passes in use . by suitable choice of material and its shape , the insert will relax to its unstressed state after a reasonable delay that may be , for example , from 15 seconds to 2 or 3 minutes . the shape of the material when unstressed will be chosen so that it completely blocks the second chamber . it is an important characteristic of the invention that this blockage resists any subsequent attempt to dehydrate the insert , because although the hydrophilic insert may shrink if dehydrated , the shape change is irreversible . as suggested above , a particularly preferred embodiment of the invention comprises a hypodermic syringe that is , in effect , self - blunting . in such an embodiment , the needle point may be formed separately from the main , elongate portion of the needle , the hydrophilic insert providing a bridge between these two parts . alternatively , the hydrophilic insert may itself provide the needle point . in each case , hydration of the insert causes deformation of the point with respect to the syringe , thereby both blocking the syringe and blunting the needle . however , the present invention also provides blocking or blunting , either independently of the other , if desired . fig1 a shows a hypodermic syringe which comprises , as conventional components , a chamber 1 and a piston 2 to which is attached ( not to scale ) a needle 3 having a point 4 ( it will be understood that this invention includes alternative embodiments of such components , or equivalents thereof ). the other drawings show only part of the needle . fig1 shows a hydrophilic insert 5 within a further chamber 6 formed in the needle 3 . fig1 a shows the insert , in its dry state after forming ; fig1 b shows the same insert in its hydrated state . the &# 34 ; dry after forming &# 34 ; shape is essentially tubular , defining a path for injectable liquid therethrough . in the wet , cylindrical state , this path is closed . in the embodiment shown in fig1 the hydrophilic insert is provided in a chamber within the hypodermic needle . as an alternative , the insert may be provided within a plastics member to which the needle is attached . the plastics member is adapted for association with a syringe body , in conventional manner . fig2 shows a needle 7 and a hydrophilic insert 8 . in this case , the needle is shaped to receive the insert in its dry state . these two parts may be fixed together , e . g . by means of adhesive . the insert 8 is itself shaped as a needle point which is deformed on hydration . fig3 shows an alternative embodiment to fig2 in which the needle point is covered by a thin metallic layer 9 which is supported mechanically by the dry hydrophilic insert . the layer 9 may be either a preformed member or deposited on the insert by a conventional technique such as printing or vacuum - coating . it is a feature of this embodiment that the shape and degree of cover afforded by the metallic coating can be used as a control on the rate of liquid uptake by the hydrophilic insert , and therefore on the rate of blockage and blunting . the pointed insert 8 may be hydrated on contact with blood . the metallised coating 9 may be sufficiently thin that it fractures when the insert is hydrated . a further alternative is that the coating 4 is any soluble or water - permeable material . fig4 a and 4b show an embodiment similar to that of fig3 but in which the needle point is a pre - formed member 10 , e . g . of the same material as the main needle member 11 , the two being linked by a hydrophilic insert 12 . stress imposed on the insert 12 , which is released by hydration , leads to retraction of the member 10 , from the position shown in fig4 a to the position in fig4 b . the hypodermic syringe is thus both blocked and blunted .
in a hypodermic syringe of the type comprising a chamber for liquid to be injected and a needle having a point for injection of the liquid , the syringe additionally includes , in contact with the liquid flow path through the syringe , a pre - stressed hydrophilic insert that , on hydration by contact with liquid , expands to block the flow path . the syringe is thus self - blocking ; it can also be self - blunting , thus prevent reuse .