hydrogels made from the crosslinked peg polymeric structures of the invention can be used in drug delivery systems and for wound dressings . wound dressings could be used internally to provide dressings that degrade within the body over time . the hydrogels of the invention could be usefully applied in drug delivery systems to burns to apply polymer conjugated therapeutic agents to burns . drug delivery systems can be prepared in which the rate of hydrolysis of the hydrogel is controlled to provide controlled release of drug components . by “ drug ” is meant : any substance intended for the diagnosis , cure , mitigation , treatment , or prevention of disease in humans and other animals , or to otherwise enhance physical or mental well being . the invention could be used for delivery of biologically active substances generally that have some activity or function in a living organism or in a substance taken from a living organism . the terms “ group ,” “ functional group ,” “ moiety ,” “ active moiety ,” “ reactive site ,” and “ radical ” are all somewhat synonymous in the chemical arts and are used in the art and herein to refer to distinct , definable portions or units of a molecule and to units that perform some function or activity and are reactive with other molecules or portions of molecules . the term “ linkage ” is used to refer to groups that normally are formed as the result of a chemical reaction and typically are covalent linkages . hydrolytically stable linkages means that the linkages are stable in water and do not react with water at useful phs for an extended period of time , potentially indefinitely . hydrolytically unstable linkages are those that react with water , typically causing degradation of a hydrogel and release of substances trapped within the matrix . the linkage is said to be subject to hydrolysis and to be hydrolyzable . the time it takes to degrade the crosslinked polymeric structure is referred to as the rate of hydrolysis and is usually measured in terms of its half life . the skilled artisan should recognize that when reference is made to a z moiety reacting with a y moiety , that additional reagents or steps may be employed according to commonly accepted chemical procedures and standards to achieve the desired linkage was the case may be . there are many possible routes , too numerous to mention here , that could be taken and that should be readily apparent to the skilled artisan . for example , one of skill in the art can be expected to understand that when an alcohol and a carboxylic acid are reacted , the acid typically is converted to another form , the acid chloride , prior to reaction with alcohol . several examples are demonstrated in the examples below . it should also be recognized that related branched nonpeptidic polymers that do not have hydrolytically unstable linkages can be used instead of the branched peg polymer as an ingredient in the preparation of the hydrogels of the invention . these other branched polymers include poly ( vinyl alcohol ) (“ pva ”); other poly ( alkylen oxides ) such as poly ( propylene glycol ) (“ ppg ”) and the like ; and poly ( oxyethylated polyols ) such as poly ( oxyethylated glycerol ), poly ( oxyethylated sorbitol ), and poly ( oxyethylated glucose ), and the like . the polymers can be homopolymers or random or block copolymers and terpolymers based on the monomers of the above polymers , straight chain or branched , or substituted or unsubstituted similar to mpeg and other capped , monofunctional pegs having al single active site available for attachment to la linker . specific examples of suitable additional polymers include poly ( oxazoline ), poly ( acryloylmorpholine ) (“ pacm ”) as described in published italian pat . application mi - 92 - a - 0002616 filed nov . 17 , 1992 , and poly ( vinylpyrrolidone ) (“ pvp ”). pvp and poly ( oxazoline ) are well known polymers in the art and their preparation and use in the syntheses described with branched peg should be readily apparent to the skilled artisan . the following examples illustrate preparation of pegs having hydrolytically unstable linkages in the polymer backbone and their use in preparing degradable hydrogels for the release of peg and biomolecule conjugates . pegs having hydrolytically unstable linkages and their preparation are also described in a copending patent application u . s . ser . no . 08 / 928 , 049 entitled degradable poly ( ethylene glycol ) hydrogels with controlled half - life and precursors therefor , which was filed on sep . 12 , 1997 and claims priority from provisional application serial no . 60 / 026 , 066 , which was filed on sep . 13 , 1996 , the contents of which relating to the preparation of pegs having hydrolytically unstable linkages in the polymer backbone are incorporated by reference in their entirety . synthesis of peg derivatives having hydrolytically unstable backbone linkages and terminal nhs active carbonates ( nhs — ooco — peg — w — peg — ocoo — nhs ) in a 100 ml round - bottom flask , benzyloxy - peg carboxymethyl acid 3400 ( 3 . 4 g , 1 mmol , shearwater polymers , huntsville , ala .) in toluene was azeotropically distilled for two hours and then cooled to room temperature . a solution of thionyl chloride ( 2m , 4 ml , 8 mmole , aldrich ) in methylene ride was injected and the mixture was stirred under n 2 overnight . the solvent was condensed by rotary evaporation and the syrup was dried in vacuo for about four hours over p 2 o 5 powder . to the residue was added anhydrous methylene chloride ( 5 ml ) and azeotropically dried benzyloxy - peg 3400 ( 2 . 55 g , 0 . 75 mmol ) in toluene ( 20 ml ). after the benzyloxy - peg acyl chloride was dissolved , freshly distilled triethylamine ( 0 . 6 ml ) was added . the mixture was stirred overnight , the triethylamine salt filtered off , and the product collected by precipitation with ethyl ether . it was further purified by dissolving in water and extracting with methylene chloride . the organic phase was dried over anhydrous sodium sulfate , condensed under vacuum , and precipitated into ethyl ether . the precipitate was dried in vacuo . hplc ( gpc ) of the product showed that 100 % of benzyloxy - peg had been converted into the peg ester and about 15 wt . % benzyloxy - peg acid remained . the mixture was chromatographically purified on an ion - exchange column ( deae sepharose fast flow , pharmacia ) to remove the benzyloxy - peg acid . 100 % pure α - benzyloxy - ω - benzyloxy peg ester 6800 ( 2 g , 0 . 59 mmole end group ) in 1 , 4 - dioxane ( 20 ml ) was hydrogenolyzed with h 2 ( 2 atm pressure ) and pd / c ( 1 g , 10 % pd ) overnight . the catalyst was removed by filtration and the product precipitated into ethyl after most of the solvent was removed on a rotary evaporator . α - hydroxy - ω - hydroxy peg ester 6800 was collected by filtration and dried in vacuo . yield : 1 . 5 gram ( 75 %). α - hydroxy - ω - hydroxy peg ester 6800 ( 1 . 5 g , 0 . 44 mmole end group ) was a zeotropically dried with 100 ml of acetronitrile and cooled to room temperature . to this solution was added disuccimidyl carbonate ( dsc ) ( 0 . 88 mmole , fluka ) and pyridine ( 0 . 1 ml ), and the solution was stirred at room temperature overnight . the solvent was removed under vacuum and the syrup was dried in vacuo . the product was dissolved in 35 ml of dry methylene chloride , the insoluble solid was removed by filtration , and the filtrate washed with ph 4 . 5 sodium chloride saturated acetate buffer . the organic phase was dried over anhydrous sodium sulfate , condensed under vacuum , and precipitated into ethyl ether . the precipitate was dried over p 2 o 5 in vacuo . yield : 1 . 4 g ( 93 %). nmr ( dmso - d 6 ): ( 1 ) product from benzyloxy - peg propionic acid : δ3 . 5 ( br m , peg ), 2 . 55 ( t , — och 2 c h 2 coopeg —), 4 . 13 ( t , — peg — cooc h 2 ch 2 o —), 4 . 45 ( t , — pegoch 2 c h 2 oco — nhs ), 2 . 80b [ s , nhs , 4h ]; ( 2 ) product from benzyloxy - peg carboxymethyl acid : δ3 . 5 ( br m , peg ), 4 . 14 ( s , — oc h 2 coopeg —), 4 . 18 ( t , — och 2 cooc h 2 ch 2 —), 4 . 45 ( t , — pego — ch 2 c h 2 oconhs ), 2 . 81 [ s , nhs , 4h ]. synthesis of peg derivatives having hydrolytically unstable backbone linkages and terminal nhs active esters ( nhs — ooc —( ch 2 ) n — o — peg — o —( ch 2 ) n — co 2 — peg — o 2 c —( ch 2 ) n — o — peg — o —( ch 2 ) n — coonhs ) in a 100 ml round - bottom flask , difunctional peg 2000 ( 2 g , 1 mmol , shearwater polymers ) and difunctional peg acid 2000 ( 4 g , 2 mmole , shearwater polymers ) were azeotropically distilled with 70 ml of toluene under n 2 . after two hours , the solution was cooled to room temperature and stannous 2 - ethylhexanoate ( 200 mg , sigma chemical ) was added . the solution was then refluxed under n 2 for 24 hours . the solvent was then condensed under vacuum and the syrup precipitated into 100 ml of ether . the product was collected by filtration , dried under vacuum , and dissolved in a sodium acetate buffer solution at ph 5 . 0 . the slightly milky solution was centrifuged and the upper clear solution was extracted three times with methylene chloride . the organic phase was dried over anhydrous sodium sulfate , filtered , condensed under vacuum , and precipitated into ether . the product was collected by filtration and dried under vacuum . hplc : 70 % product , 15 % di - acid reactant and 15 % monoacid . the mixture was further purified by ion exchange chromatography and gel permeation chromatography . yield 3 g ( 50 %). 1 h nmr ( dmso - d 6 ): ( 1 ) product from peg carboxymethyl acid : δ6 3 . 5 ( br m , peg ), 4 . 15 ( s , — oc h 2 cooch 2 —), 4 . 18 ( t , — och 2 cooc h 2 ch 2 —), 3 . 98 ( s , — peg — oc h2 cooh ); ( 2 ) product from peg propionic acid : δ3 . 5 ( br m , peg ), 2 . 55 ( t , — pegoch 2 c h 2 cooch 2 —), 4 . 13 ( t , — och 2 ch 2 cooc h 2 ch 2 —), 2 . 43 ( t , — pegoch 2 c h 2 cooh ) in a round - bottom , flask , the difunctional acid having weak linkages ( obtained from previous step ) 3 g . approx . 1 mmole end group ) and n - hydroxysuccinimide ( nhs ) ( 126 mg , 1 . 05 mmole ) were dissolved in 50 ml of dry methylene chloride . to this solution was added dicyclohexylcarbodiimide . ( 240 mg , 1 . 15 mmole ) in 5 ml dry methylene chloride . the mixture was stirred under n 2 overnight . the solvent was condensed and the syrup was redissolved in 15 ml of anhydrous toluene . the insoluble salt was removed by filtration and the filtrate was precipitated into 200 ml of dry ethyl ether . the precipitate was collected by filtration and dried in vacuo . yield 2 . 7 g ( 90 %). 1 h nmr ( dmso - d 6 ): δ3 . 5 ( br m , peg ), 2 . 8 ( s , nhs , 4h ) 4 . 6 ( s , — peg — o — c h 2 — coonhs ) or 2 . 85 ( t , — peg — o — ch 2 c h 2 — coonhs ). to precisely measure the hydrolysis kinetics of the ester linkages , water - soluble , non - crosslinked mpeg — o —( ch 2 ) n — coo — pegm was synthesized as in example 2 . hydrolysis was carried out in buffer solutions ( 0 . 1 m ) at different phs and temperatures , and followed by hplc - gpc ( ultrahydrogel ® 250 , waters ). the half - lives of the ester bonds are listed in table 1 . in a test tube , i6o mg ( 14 . 7 μmole ) of difunctional peg active carbonate 6800 ( nhs — ooco — peg — w — peg — ocoonhs , prepared in example 1 ) was dissolved in 0 . 75 ml of buffer ( 0 . 1m phosphate , ph 7 ). to the solution were added 0 . 15 ml of 8 - arm - peg - amine boon ( 250 mg / ml ) and 0 . 1 ml of fitc - bsa ( 10 mg / ml ). after rapid shaking , it was allowed to sit and a gel formed in a few minutes . a suitable buffer ph range was found to be 5 . 5 to 8 . preparation of a hydrolytically unstable peg hydrogel from branched peg amine , model protein , and peg derivatives having hydrolytically unstable backbone linkages and terminal nhs active esters ( nhs — ooc —( ch 2 ) n — o — peg — o —( ch 2 ) n — co 2 — peg — o 2 c —( ch 2 ) n — o — peg — o —( ch 2 ) n — coonhs ) 100 mg ( approx . 16 . 6 μmole ) difunctional peg active ester ( nhs — ooc —( ch 2 ) n — o — peg — o —( ch 2 ) n — co 2 — peg — o 2 c —( ch 2 ) n — o — peg — o —( ch 2 ) n — coonhs , prepared in example 2 ) was dissolved in 0 . 75 ml of buffer ( 0 . 1m phosphate , ph 7 ). to the solution were added 0 . 166 ml of 8 - arm - peg - amine 10000 ( 250 mg / ml ) and 0 . 1 ml of fitc - bsa ( 10 mg / ml ). after rapid shaking , it was 1 allowed to sit and a gel formed in a few minutes . a suitable buffer ph range was found to be 5 . 5 to 8 . all protein - loaded hydrogel disks were weighed and their diameters : measured before release studies . then each gel disk was immersed , at time t = 0 , in phosphate buffer ( 0 . 1 m , ph 7 . 0 ). the amount of the buffer was more than 50 times that of the wet gel weight . the solution was maintained at 37 ° c ., and gently shaken . at a predetermined time , a small amount of buffer solution was removed for protein concentration determination and then put back after measurement . the protein concentration was determined by uv measurement at 495 nm . fig1 shows some release profiles of peg - fitc - bsa from the hydrogels in units plotted against time in days of the fraction of moles at time t divided by the moles at infinity , which is defined as the completion of degradation of the hydrogel . the invention has been described in particular exemplified embodiments . however , the foregoing description is not intended to limit the invention to the exemplified embodiments , and the skilled artisan should recognize that variations can be made within the scope of the invention as described in the foregoing specification . the invention includes all alternatives , modifications , and equivalents that may be included within the true spirit and scope of the invention as defined by the appended claims .
a degradable peg hydrogel is described that , upon hydrolysis , releases conjugates of substantially non - peptidic polymers and biologically active molecules . for example , peg and protein conjugates can be released in vivo from the hydrogels for therapeutic application .