Current Situation and Challenges in Vitreous Substitutes

REVIEW www.mbs-jou nal.de Cu en Si ua ion and Challenges in Vi eous Subs i u es C is ina Mondelo-Ga cía, En ique Bandín-Vila , Lau a Ga cía-Quin anilla, Ana Cas o-Balado, E a M. del Amo, Ma ía Gil-Ma ínez, Ma ía José Blanco-Teijei o, Miguel González-Ba cia, I ene Za a-Fe o, Anxo Fe nández-Fe ei o,* and F ancisco J. O e o-Espina * Vi eo- e inal diso de s cons i u e a significan po ion o ea able ocula diseases. These pa hologies o en equi e i eo- e inal su ge y and, as a consequence, he use o i eous subs i u es. Nowadays, he i eous subs i u es ha a e used in clinical p ac ice a e mainly di ided in o gases (ai , SF6,C 2F6,C 3F8) and liquids (pe fluo oca bon liquids, silicone oils, and hea y silicone oils). The e a e specific ad an ages and d awbacks o each o hese, which de e mine hei clinical indica ions. Howe e , de eloping he ideal bioma e ial o i eous subs i u ion con inues o be one o he mos impo an challenges in oph halmology, and a mul idisciplina y app oach is equi ed. In his sense, ecen esea ch has ocused on he de elopmen o biocompa ible, biodeg adable, and injec able hyd ogels (na u al, syn he ic, and sma ), which also ac as medium and long- e m in e nal amponade agen s. This comp ehensi e e iew aims o co e he main cha ac e is ics and indica ions o use o he ex ensi e ange o i eous subs i u es ha a e cu en ly used in clinical p ac ice, be o e going on o desc ibe he hyd ogels ha ha e been de eloped ecen ly and which ha e eme ged as p omising bioma e ials o i eous subs i u ion. C. Mondelo-Ga cía, E. Bandín-Vila , L. Ga cía-Quin anilla, A. Cas o-Balado, M. González-Ba cia, I. Za a-Fe o, A. Fe nández-Fe ei o Pha macy Depa men Uni e si y Clinical Hospi al o San iago de Compos ela (SERGAS) San iago de Compos ela 15706, Spain E-mail: [email p o ec ed] C. Mondelo-Ga cía, E. Bandín-Vila , L. Ga cía-Quin anilla, A. Cas o-Balado, M. González-Ba cia, I. Za a-Fe o, A. Fe nández-Fe ei o Pha macology G oup Heal h Resea ch Ins i u e o San iago de Compos ela (FIDIS) San iago de Compos ela 15706, Spain The ORCID iden ifica ion numbe (s) o he au ho (s) o his a icle can be ound unde h ps://doi.o g/10.1002/mabi.202100066 © 2021 The Au ho s. Mac omolecula Bioscience published by Wiley-VCH GmbH. This is an open access a icle unde he e ms o he C ea i e Commons A ibu ion-NonComme cial License, which pe mi s use, dis ibu ion and ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly ci ed and is no used o comme cial pu poses. DOI: 10.1002/mabi.202100066 1. In oduc ion The i eous humou is a anspa en gel p esen be ween he lens and he e ina. I has a olume o a ound 4 mL and occupies 80% o he eye olume.[1,2] I weighs a ound 4 g and con ains app oxima ely 99% wa e , only adhe ing o ocula s uc u es in he ol- lowing pa s: he macula, he op ic ne e disc, and he an e io bo de o he a ea su - ounding he e ina.[2] Vi eo- e inal diso de s cons i u e a sig- nifican po ion o ea able ocula dis- eases. The i eous humou o en becomes dys unc ional due o opacifica ion, lique- ac ion o physical collapse, as a esul o inflamma o y diseases, de elopmen al abno mali ies, i eous hemo hage, u- mo s, diabe es, o degene a i e p ocesses. In addi ion, i eous damage can also be caused by in aocula o eign bodies o auma.[3] The i eous humou de- e mines he cla i y o ision meaning E. M. del Amo School o Pha macy, Facul y o Heal h Sciences Uni e si y o Eas e n Finland Kuopio 70211, Finland M. Gil-Ma ínez, M. J. Blanco-Teijei o Oph halmology Depa men Uni e si y Clinical Hospi al o San iago de Compos ela (SERGAS) San iago de Compos ela 15706, Spain F. J. O e o-Espina Depa men o Pha macology, Pha macy and Pha maceu ical Technology, Facul y o Pha macy Uni e si y o San iago de Compos ela (USC) San iago de Compos ela 15782, Spain E-mail: [email p o ec ed] Mac omol. Biosci. 2021, 2100066 2100066 (1 o 15) © 2021 The Au ho s. Mac omolecula Bioscience published by Wiley-VCH GmbH www.ad ancedsciencenews.com www.mbs-jou nal.de he e o e ha , i no ea ed p ope ly, hese diso de s can cause blindness. Vi eous subs i u es a e c ucial adjunc s du ing i eo- e inal (VR) su ge y o e inal diseases.[2,4,5] Nowadays, he mos common agen s ha a e used as i e- ous subs i u es in clinical p ac ice boas ce ain ad an ages, in- cluding chemical ine ness and op ical cla i y, none heless, he e a e also many limi a ions o hei use.[2] In his sense, he de- elopmen and cha ac e iza ion o new i eous subs i u es ha e played an impo an ole in i eo- e inal su ge y. Despi e he ac ha conside able effo s ha e been made in de eloping new bio- ma e ials o i eous subs i u ion in sea ch o he pe ec al e - na i e ha is able o o e come he disad an ages p esen ed by he cu en ly a ailable subs ances, u he esea ch mus be pe - o med. In his ega d, de eloping he ideal bioma e ial o i - eous subs i u ion con inues o be one o he mos significan challenges in oph halmology. Gi en he complexi y o his ma - e , a mul idisciplina y app oach, which in ol es oph halmic su - geons, pha macis s, chemis s, and expe s in bioma e ials will be necessa y i we wan o o e come his p oblem. Ou aim is o discuss he main cha ac e is ics and applica ions o he wide a ie y o i eous subs i u es ha a e cu en ly used in clinical p ac ice, be o e going on o add ess he de elopmen o new hyd ogels ha ha e been p esen ed as p omising al e na- i es o he op imiza ion o i eous subs i u ion. 2. Composi ion and Func ions o he Vi eous The i eous is a gela inous s uc u e ha is composed o 98% wa e .[3,4] I p o ec s he adjacen s uc u es and issues om au- mas, as well as pe mi ing he ci cula ion o nu ien s and so- lu es, and con olling he oxygen ension wi hin he eye. I helps o main ain he shape o he ocula globe, as well as keeping he c ys alline lens and he e ina in hei place.[3,6] The main compo- nen s o he i eous humou , as well as he majo cha ac e is ics o he a o emen ioned componen s a e ou lined below. 2.1. P o eins P o eins ep esen a small pe cen age o he o e all con en o he i eous componen s. The majo i y o he soluble p o eins a e albumin (40%), wi h he o he impo an componen s includ- ing immunoglobulins and i on-binding p o eins such as ans- e in, which helps o educe i on oxici y in he case o small hemo hages.[1,6,7] Wi h ega ds o insoluble p o eins, collagens a e he mos abundan . The e a e diffe en ypes o p o eins which play a majo ole in he i eous s uc u e.[4,8] 2.2. Glycosaminoglycans (GAGs) GAGs, which a e ex acellula polysaccha ides a e a key compo- nen o he i eous s uc u e, and hese a e mainly di ided in o h ee ypes—hyalu onic acid, hepa an sul a e, and chond oi in sul a e.[1] 2.2.1. Hyalu onic Acid (HA) HA is a majo componen o he i eous, o ming 3D s uc u es wi h collagen. The ac ha i does no con ain sul a e makes i dis inguishable om he o he GAGs, and his also means ha i does no a ach o p o eins o o m a p o eoglycan.[1] The highes concen a ions o hyalu onan molecules a e ound in he pos e io i eous co ex.[6,9–11] The HA p epa a ions consis o molecules wi h g ea e a iabili y in e ms o hyd odynamic size, and hese a e a ele an componen in de e mining i e- ous iscosi y.[12] 2.2.2. Chond oi in Sul a e (CS) CS is a sul a ed GAG, which consis s o a chain o al e na ing sug- a s (N-ace ylgalac osamine and glucu onic acid). I cons i u es a majo componen o he ex acellula ma ix and is also p esen in he i eous, appea ing in he o m o he p o eoglycans e si- can and ype IX collagen.[4,11] CS is used o p ese e he in eg i y o he i eous and p o ide esis ance agains comp ession.[1] 2.2.3. Hepa an Sul a e (HS) HS is a enewable p o eoglycan, which ensu es ha he e is ad- equa e spacing be ween he collagen fib ils, none heless, small amoun s o HS a e p esen in he i eous.[13] I also enhances he egula ion o angiogenesis and blood coagula ion, as well as main aining i eo- e inal adhesion.[4] 2.3. Glucose, Lac ic Acid, and An ioxidan s Due o he impo an ole ha he i eous plays in he cellu- la me abolism o ocula issues, i s componen s include se e al subs ances ha ac as sub ac s o me abolism, such as glucose and lac ic acid, which a e necessa y o suppo he me abolism in he su ounding issues.[4,6] In addi ion, he i eous ac s as a ese oi o glucose o he cilia y body.[14] On he o he hand, asco bic acid is a c ucial an ioxidan o lens and e inal me abolism, in pa icula i is used as a me abolic buffe in po assium homeos asis. Fu he mo e, i may also inhibi neo ascula iza ion and inc ease he p oli e a ion o hyalocy es.[15–17] 2.4. Cells and Enzymes The e a e h ee ypes o cells ha a e ound in he i eous body: hyalocy es, fib ocy es/fib oblas s, and mac ophages.[6,18,19] The main unc ions o hese cells a e ela ed o he c ea ion, egula ion, and deg ada ion o he i eous ma ix. Se e al en- zymes ha e been isola ed, which include hyalu onidase, se ine p o eases, and enin-angio ensin-con e ing enzyme.[20–22] 3. Vi eous Subs i u es in Clinical P ac ice Vi eous subs i u es mus ha e physical and biological p op- e ies ha make hem sui able o use in clinical p ac ice. In e ms o hei physical p ope ies, ideal i eous subs i u es will be hyd ophilic and insoluble in wa e , easy o manipula e du - ing su ge y, clea and anspa en o acili a e isualiza ion, and Mac omol. Biosci. 2021, 2100066 2100066 (2 o 15) © 2021 The Au ho s. Mac omolecula Bioscience published by Wiley-VCH GmbH www.ad ancedsciencenews.com www.mbs-jou nal.de Figu e 1. Gas injec ion in pneummo e inopexy. A) Example o a supe io de achmen and injec ion wi h he pa ien lying in he supine posi ion ( ace-up). B) The pa ien is placed in an up igh posi ion and he bubble amponades he b eak. C ea ed wi h BioRende .com. hey will also ha e a e ac i e index ha is simila o he hu- man i eous. In addi ion, hese mus emain s able when in- jec ed h ough a small sy inge, as well as ensu ing adequa e su ace ension in he a emp o seal he e inal b eak.[2,23] On he o he hand, and in e ms o hei biological p ope ies, he ideal i eous subs i u es will be biodeg adable and biocompa i- ble, biologically and chemically ine , and hey will no block he aqueous d ainage. Fu he mo e, hey mus be non oxic o e inal issues.[6,23] An ideal i eous subs i u e does no ye exis ; all o hem o - e ad an ages and d awbacks, which may o may no make hem sui able depending on he clinical si ua ion. Those cu en ly used in clinical p ac ice a e di ided in o diffe en ca ego ies based on diffe en p ope ies. Specifically, hese a e di ided in o gases (ai , SF6,C 3F8,andC 2F6) and liquids (sal solu ion, silicon oils (SO), pe fluo oca bon liquids, and semi fluo ina ed alkanes). The cha ac e is ics o each indi idual g oup, as well as hei clin- ical indica ions ha e been ou lined in de ail in he ollowing sec ions. 3.1. Gases Ohm was he fi s o desc ibe in aocula gas use in 1911.[24] Se - e al indica ions o i s use we e desc ibed, wi h he fi s o hese being i s use as an in e nal amponade.[25] I also p o es use ul in un olding and olding he e ina, as well as in imp o ing i s pos - ope a i e isualiza ion and eplacing he globe olume o p e- en fluid mo emen in o e inal b eaks. Gas is also used in se - e al echniques: pneummo e inopexy (Figu e 1); he pa s plana i ec omy o e inal de achmen (RD), wi h and wi hou scle- al buckle, o fla en he e ina; sub e inal blood displacemen in macula hemo hages; and pos i ec omy liquid–gas exchange in p e iously i ec omized eyes.[26] Pos ope a i e ca e is e y impo an in hese pa ien s. P one ( ace-down) posi ioning mus commence as soon as possible a e he i ec omy p ocedu e has been pe o med. This is c ucial in p e en ing any shi o he ea ached e ina. Mo eo e , i dec eases he con ac be ween he pos e io su ace o he lens wi h he gas bubble in phakic pa ien s ( he bubble is si ua ed owa ds he e ina); he e o e, educing he isk o ca a ac o ma ion. Posi ioning may also a y depending on he loca ion o he e inal b eak, usually lying on he opposi e side o he b eak. The mos widely used in aocula gases a e sul u hexafluo ide (SF6) and pe fluo op opane (C3F8). These a e non oxic and ine gases ha a e insoluble in he aqueous humou , and hey boas a lowe wa e solubili y han ni ogen he e o e allowing hem o expand.[27] The decision as o which one o hese gases is o be used will be based on he amponade du a ion and he su geon’s p e e ences, aking in o conside a ion he ype and loca ion o he e inal b eak.[28] I is gene ally accep ed ha SF6can be used in uncomplica ed p ima y RD. Howe e , i a final amponade ac ion is necessa y, based on he esul s om “The Silicone S udy”, in which he esul s in pa ien s wi h RD and p oli e a i e i eo e inopa hy (PVR), he pa s plana i ec omy wi h ei he C3F8gas o SO am- ponades we e a o able.[29] Howe e , he clea ad an age o using gases is ha i is no necessa y o emo e hem. The e a e h ee s ages in he abso p ion o he in aocula gas: expansion o he gas when injec ed, ni ogen equilib ium, and dissolu ion.[30] Gases may be abso bed by diffusion ac oss he e ina in o he blood s eam, o hey may be dissol ed in o he aqueous humou , and emo ed h ough he an e io chambe .[31] The abso p ion o hese gases demons a es fi s -o de pha ma- cokine ics, wi h small olumes las ing in he in aocula ca i y o days o weeks depending on he gas used.[30] Knowledge o he kine ics o hese gases, as de ailed in Table 1, comes mos ly om animal models and small s udies in which indi ec measu emen s ha e been aken in humans.[30–35] These s udies may no be ep esen a i e o cu en p ac ice, especially wi h ega ds o he du a ion o he ai bubble in he pos e io chambe o he eye, which is pe cei ed o be longe in clinical se ings han in heo e ical ones.[36,37] The hal -li e o in aocula gases is sho e in aphakic eyes han in phakic eyes, due o he in- c eased con ec ion in he i eous ca i y.[38] Following he i ec- omy p ocedu e, con ec ion cu en s appea in he aphakic eye, which accele a e he abso p ion a e. In phakic eyes wi h no mal i eous, he e a e much ewe con ec ion cu en s; he e o e, he expansion and abso p ion o long-ac ing gases is slowe .[30] 3.1.1. Ai Room ai amponade applied using non-expansible gas begins o sh ink immedia ely a e injec ion as i dissol es in he i eous.[28] This offe s an ad an age o e o he long-las ing gases as i equi es sho e p one posi ioning, allowing o a as e e- co e y o ision and less ad e se effec s.[41] The hal -li e o a oom ai amponade was 1.6 d in phakic eyes[30] wi h a longe i y o 5 d, bu ecen clinical imp essions ha e sugges ed ha ai emains in he i eous ca i y o a longe pe iod o ime. In a ecen e ospec i e coho s udy, a hal -li e o 3.3 d wi h a longe i y o 11.4 d was de e mined.[37] 3.1.2. O he Gases: SF6,C 2F6,C 3F8 Sul u hexafluo ide (SF6) and pe fluo op opane (C3F8) a e he mos commonly used in aocula gases in clinical p ac ice, com- pa ed o hexafluo oe hane (C2F6) which is much less equen ly used.[36] Mac omol. Biosci. 2021, 2100066 2100066 (3 o 15) © 2021 The Au ho s. Mac omolecula Bioscience published by Wiley-VCH GmbH www.ad ancedsciencenews.com www.mbs-jou nal.de Table 1. Pha macokine ics o in aocula gases. Gases Nonexpansile concen a ion Du a ion o maximal expanded olume [h] Hal -li e [d] Du a ion o ai bubble [d] Indica ions Rabbi s Humans Rabbi s Humans Animals Humans Ai – Nonexpansible – 1.6a) 0.9b) 5–6 10.7– 11.4c) - Simple cases in which a sho du a ion is equi ed SF6(sul u ic hexafluo ide) 20% 24–48 21 – 2.6a) 2.4b) 8–11 18a),c) - RD (Re inal de achmen ) wi h supe io b eaks - RD wi h in e io b eaks - Fla RD in he case o me iculous i eous dissec ion. - Following macula hole su ge y C2F6(hexafluo oe hane) 16% 72 27 2 – 16 34.5a),c) No app o ed by FDA C3F8(pe fluo op opane) 12% 72–96 30 6 5.7a) 4.5b) 4.3c) 28 67.7a),c) - RD and mul iple b eaks - RD wi h supe io gian ea - RD wi h p oli e a i e i eo e inopa hy (PVR) - Failed p io RD su ge y - Pe sis en sub e inal fluid - Following macula hole su ge y - Pneuma ic displacemen o sub e inal blood clo Re . [28] [33,39] [36] [31] [30] [32,34] [36,37,40] [25] a) Re e ing o phakic eyes; b) Re e ing o aphakic eyes; c) Re e ing o pseudo-phakic eyes. Gi en ha hese gases boas a lowe wa e solubili y han ni- ogen, hey end o expand o a leas wice he olume o he gas injec ed, as a esul , nonexpansile o minimally expansile mix- u e o gas is p e e able in o de o p e en ad e se effec s such as in aocula p essu e ele a ion (IOP).[28] Wa e solubili y a ies depending on he ca bon chain leng h. The longe he ca bon chain, he lowe he solubili y in wa e ; he e o e, esul ing in a longe in aocula longe i y:[42] A mix- u e o 10% C3F8had a hal -li e o 5.7 d in phakic and 4.3 d in pseudophakic eyes.[30] A mix u e o 20% SF6had a hal -li e o 2.8 d in phakic eyes. The hal -li e o C2F6was no measu ed; how- e e , a longe i y o 34 d was de e mined in he i eous ca i y.[36] Ce ain pos ope a i e complica ions ha e been epo ed ollowing he use o in aocula gas, howe e , mos o hese can be p e en ed by aking g ea e ca e when unde aking he su gical p ocedu e. Fo example, gas could go unde he e ina al hough his is p e en able, o gas could become en apped a he injec ion si e. One o he mos equen complica ions is he o ma ion o ca a ac s due o he gas coming in o con ac wi h he c ys- alline lens.[43] Raised in aocula p essu e may occu , bu his ends o happen on he fi s pos ope a i e day, and i s cause has been a ibu ed o he expansion o he bubble o o an o e - filled eye. On he con a y, hypo ony may occu i he e is any gas leakage om he scle o omies. O he complica ions include he p esence o gas in he an e io chambe , seconda y co neal decompensa ion,[44] which is mo e equen in aphakic pa ien s, and non-in ac pos e io capsule. In pseudophakic pa ien s, in- aocula lens cap u e may occu due o i being pushed o wa d in o he an e io chambe .[45,46] 3.2. Liquids Diffe en liquid i eous subs i u es a e used in clinical p ac ice. The main g oups and hei majo cha ac e is ics a e depic ed in Table 2. 3.2.1. Sal Solu ions Sal solu ions ha e simila cha ac e is ics o aqueous humou in e ms o hei densi y, e ac i e index and anspa ency.[3] In he clinical se ing, hese a e used on a empo a y basis du ing he exchange wi h ai o liquids as hei low su ace ension means ha hese do no ha e amponade p ope ies.[4] 3.2.2. Silicon Oil Silicon oil (SO) is a polyme ized siloxane wi h o ganic side chains. I belongs o he class o syn he ic o ganosilicon com- pounds and is a epe i ion o he –[R2Si–O]– g oup in which R is he o ganic side chain.[47] Specifically, SOs used as i eous sub- s i u es a e polyme s o polydime hylsiloxane (PDMS). In con as o silicone ubbe , polyme chains a e sho e , and gi en he lack o chemical c oss-linking be ween hem, hese p esen in a liquid o m. They a e hyd ophobic subs ances wi h a specific g a i y, which is sligh ly lowe han wa e , and a e- ac i e index ha is highe han ha o he i eous.[48] These a e a ailable in diffe en iscosi ies, which is measu ed in cen is- okes (cS ), and which anges om 1000 o 5000 cS in clinical p ac ice. Mac omol. Biosci. 2021, 2100066 2100066 (4 o 15) © 2021 The Au ho s. Mac omolecula Bioscience published by Wiley-VCH GmbH www.ad ancedsciencenews.com www.mbs-jou nal.de Table 2. Physical p ope ies o liquid i eous subs i u es and clinical indica ions. P oduc Specific g a i y [g cm−3] Viscosi y [cS ] Re ac i e index Indica ions A. SALT SOLUTIONS BSS 1 Tempo a y eplacemen du ing ai /oil exchange B. SILICON OILS SO 1000 cS 0.97 1000 1.4 Complex RD associa ed wi h PVR Gian ea , RD wi h PVR T auma ic RD wi h PVR Recu en RD wi h b eaks in ol ing he lowe quad an s RD associa ed wi h se e e p oli e a i e diabe ic e inopa hy RD associa ed wi h macula hole in pa hologic myopia Pedia ic RD RD associa ed wi h i al e ini is Pos auma ic endoph halmi is SO 2000 cS 0.97 2000 1.4 SO 5000 cS 0.97 5000 1.4 C. PERFLUOROCARBON LIQUIDS Pe fluo ooc ane (C8F18) 1.76 0.69 1.27 P ima y Rhegma ogenous RD Complica ed RD wi h PVR Gian ea RD wi h PVR RD associa ed wi h disc coloboma, Disloca ed lens, Sup acho oidal hemo hage Pe fluo odecalin (C10F18) 1.94 2.7 1.33 Pe fluo ope hyd ophenan h ene (C14F24) 2.03 8.03 1.31 Oc afluo op opane (C3F8) 1.6 0.465 1.22 i .SEMIFLUORINATED ALKANES Pe fluo ohexyloc ane (F6H8) 1.35 3.44 1.387 D. SFA-SO combina ions (a) Double fillings (b) Hea y Silicon Oils Densi on-68 1.06 1387 1.39 RD associa ed wi h in e io ea s and PVR Oxane HD 1.02 3300 1.4 HWS 46–3000 1.12 2903 1.37 Since he 1960s, SO ha e been used as sho and long- e m i - eous subs i u es because o hei anspa ency, low su ace en- sion, buoyancy, and low oxici y. The ole o SO in clinical p ac ice was defined by “The Silicone S udy”,[49,50] a mul icen e p ospec- i e andomized clinical ial ha compa ed he effec o SO o long-ac ing in aocula gases (SF6and C3F8) in he managemen o complex RDs associa ed wi h se e e PVR. Globally, SO was demons a ed o be mo e effec i e han SF6, and equally as e - ec i e as C3F8in ea aching he e ina.[49] In addi ion, SO and C3F8p oduced e y simila esul s in e ms o he imp o emen o isual unc ion and he low complica ion a es. Fu he mo e, he oph halmologis ‘s p e e ence o he need o he pa ien o ake a fligh soon a e he in e en ion could be easons o using SO.[1] The use o SO in gian ea s wi hou PVR is s ill being deba ed. In his sense, good ana omic success has been epo ed wi h SO and gases. Gene ally speaking, SO is he mos used agen in Eu ope, while in he Uni ed S a es some oph halmologis s s ill ha e a p e e ence o in aocula gas.[51] SO amponade ends o be adminis e ed a he p ima y i - ec omy o ac ion RD associa ed wi h se e e p oli e a i e di- abe ic e inopa hy.[52] Howe e , o da e, no clinical ials ha e adequa ely e alua ed i s efficacy in his use. In addi ion, wi h ega ds o he ea men o RDs in i al e ini is, SO offe s long- e m in e nal amponade, he e o e dec easing he isk o e-de achmen .[53,54] Rega ding he pedia ic popula ion, he main indica ions o he use o SO amponade a e RDs associa ed wi h e inopa hy o p ema u i y, auma, congeni al anomaly, and myopia. In he case o se e e auma isms, SO in e nal amponade may help o fla en he e ina and p e en hemo hage, which would inc ease he isk o PVR.[55] Finally, i has been a gued ha SOs ha e ce ain an imic obial ac i i y, which is why hey a e usually used as a amponade in pos auma ic endoph halmi is cases.[56] Wi h ega ds o he disad an ages o SOs, hese include he need o op ical adjus men s o be made due o he diffe en e- ac i e index when compa ed o he na u al i eous body, and he less effec i e na u e o he use o he amponade in ea ing in e io e ina b eaks due o i s low specific g a i y.[57,58] Fu he mo e, se ious complica ions such as e inal oxici y,[59] op ical neu opa hy,[60] o glaucoma [61] ha e been epo ed wi h he use o SO, some o which a e ela ed o he emulsifica ion o SO, especially in long- e m use. This emulsifica ion leads he o iginal SO bubble o b eak down in o smalle d ople s, esul ing in e inal inflamma ion by inducing a mac ophagic esponse.[62] SOs mus be emo ed as soon as hey ha e ulfilled hei pu pose, and when i is es ablished ha u he e en ion could inc ease he isk o complica ions (Figu e 2). This emo al is gene ally ecommended wi hin a six-mon h pe iod a e he in e en ion. Mac omol. Biosci. 2021, 2100066 2100066 (5 o 15) © 2021 The Au ho s. Mac omolecula Bioscience published by Wiley-VCH GmbH www.ad ancedsciencenews.com www.mbs-jou nal.de Figu e 2. Silicone oil (SO) emo al h ough ac i e suc ion wi h machine assis ance. C ea ed wi h BioRende .com. 3.2.3. Pe fluo oca bon Liquids Pe fluo oca bon liquids (PFCLs) a e fluo ine-subs i u ed hyd o- ca bons, which a e clea , colo less and odo less liquids. These syn he ic compounds a e cha ac e ized by a high specific g a - i y, be ween 1.7 and 2.1 g cm−3, wice as high as wa e , and hei e ac i e index is simila o ha o he i eous humou .[63] They a e insoluble in wa e and poo ly soluble in SOs. The mos commonly used PFCLs in clinical p ac ice a e pe fluo odecalin (C10F18), pe fluo ope hyd ophenan h ene (C14F24), pe fluo ooc- ane (C8F18 and oc afluo op opane (C3F8).[4] His o ically, hese we e fi s in es iga ed as blood subs i u es due o hei ex en- si e capaci y o anspo ing and eleasing oxygen and ca bon dioxide.[64] In he clinical se ing, he use o PFCLs has imp o ed he i- sual ou comes and he ana omic success a e in PVR su gical p ocedu es.[65–68] PFCLs p o ide he bes cu en ly a ailable in e - nal amponade du ing memb ane dissec ion. Simila ly, he use o PFCLs in ea ing gian - ea RD has imp o ed he ana omic success a e by o e 90%.[66] The PFCLs allow o he eposi ion- ing o he olded flap, enabling di ec PFCL-SO exchange in o de o p e en he pos e io flap om slipping. PFCLs also offe se e al ad an ages in he ea men o au- ma ic RD, including he s abiliza ion o he e ina du ing he i ec omy p ocedu e, he displacemen o p e e inal, sub e inal, and sup acho oidal blood, he elimina ion o inca ce a ed i e- ous o e ina, and he main enance o a anspa en medium o isualiza ion du ing su ge y.[69] The use o PFCLs in o he oph halmic pa hologies has al eady been p o ed, o example in cases o RD associa ed wi h diabe ic e inopa hy,[70] de achmen associa ed wi h disc coloboma,[71] de- achmen om e inopa hy o p ema u i y,[72] i ec omy o en- doph halmi is, displacemen o submacula hemo hage du ing su gical d ainage, and he excision o sub e inal memb anes.[73] Figu e 3. Pe fluo oca bon liquid (PFCL) – silicone oil (SO) exchange. SO is filled p og essi ely supe io ly, while PFCL is ex uded h ough he flu e needle placed wi hin he PFCL bubble. C ea ed wi h BioRende .com. Wi h ega ds o he sa e y o PFCLs, in ecen yea s, se e al cases o e ino oxici y caused by pe fluo ooc ane ha e been e- po ed wo ldwide. In his ega d, i is necessa y o s ic p o o- cols o be es ablished o de e mine he cy o oxici y o in aocula medical de ices in o de o ensu e he adequa e quali y o hese p oduc s.[74] Nowadays, hei use is limi ed o he in aope a i e se ing because o hei long- e m oxici y, and as a esul hey ha e been exchanged wi h SO (Figu e 3) o ano he long- e m i eous subs i u es.[75–77] 3.2.4. Semifluo ina ed Alkanes Semifluo ina ed alkanes (SFAs) we e iden ified in he 2000 s as an al e na i e o PFCLs gi en he p esump ion ha he la - e could cause e inal oxici y due o hei high specific densi y. Mo eo e , SFAs main ain p ope ies such as ine ness, biocom- pa ibili y, in e ace ensions, e c., con aining bo h pe fluo oca - bon and hyd oca bon segmen s. They ha e lowe densi ies com- pa ed o PFCLs, anging om 1.1 o 1.7 g cm−3, and hey a e soluble in PFCLs, hyd oca bons and SO. They also ha e e y low su ace and in e ace ensions.[76] The sho e he pe fluo oalkyl chains and/o he longe he alkyl chain, he mo e oxic he semifluo ina ed alkanes a e. Im- pu i ies con aining –CHF g oups mus also be aken in o ac- coun , gi en ha hyd ogen fluo ide g oups can be elimina ed in he p esence o nucleophilic bases, esul ing in oxic alkenes.[76] SFAs we e ini ially used as SO sol en , and la e as em- po a y endo amponades when i was obse ed ha SO was ineffec i e.[78] The mos common p oblems ela ed o he use o SFAs a e ca a ac s and emulsifica ion. Nowadays, SFAs end o be mixed wi h SO. Mac omol. Biosci. 2021, 2100066 2100066 (6 o 15) © 2021 The Au ho s. Mac omolecula Bioscience published by Wiley-VCH GmbH www.ad ancedsciencenews.com www.mbs-jou nal.de 3.2.5. Silicon Oils and Semifluo ina ed Alkanes Combina ion The combined use o SO and SFAs amponade agen s has been widely s udied, wi h he idea o b inging oge he he high is- cosi y o SO and he high specific g a i y o SFAs. This mix u e gene a es i eous subs i u es ha boas good amponade p op- e ies and minimal emulsifica ion.[79] Depending on he p opo ion o SO and SFAs included in he mix u e, i is possible o ob ain homogeneous solu ions (hea y silicone oils) o sepa a ed solu ions (double fillings).[80] Hea y silicone oils—Hea y silicone oils (HSO) a e homoge- neous solu ions ha a e hea ie han wa e and ha a e o med by combining SO and SFAs. In clinical p ac ice, hese a e used o ea ing complica ed RDs, especially hose which in ol e in- e io PVR. The e a e h ee p e ab ica ed HSOs cu en ly on he ma ke : Densi on 68, Oxane HD, and HWS 46–3000. Densi on 68 (Fluo on Co, Ulm, Ge many) is a mix u e o 30.5% SFA F6H8(pe fluo ohexyloc ane) wi h 69.5% SO, 5000 cS .[78] By adding SO, he iscosi y o F6H8inc eases om 2.5 o almos 1400 mPa s, educing i s dispe sion endency, which is belie ed o cause he p oblems de i ed om he long- e m use o F6H8. I has a specific g a i y o 1.06 g cm−3and a e ac i e index o 1.387. Oxane HD (Bausch & Lomb, Toulouse, F ance) is a mix u e o 88.1% Oxane 5700, a 5000 mPa s SO, wi h 11.9% RMN3, a pa - ially fluo ina ed olefin. I has a sligh ly supe io specific g a - i y and e ac i e index han wa e , 1.02 g cm−3and 1.4, espec- i ely. I is he mos iscous, 3800 mPa s, and leas hea y HSO o he h ee p oduc s. Because o i s lowe specific g a i y, sligh ly highe e-de achmen a es we e ound when using his HSO.[81] The las comme cialized HSO is called HWS 46–3000, which is a mix u e o 45% ul apu ified SO 100 000 ( iscosi y 97 100 mPa s and specific g a i y 0.977 g cm−3) and 55% pe fluo obu yl- hexane (F4H6) a semifluo ina ed alkane ( iscosi y 1.28 mPa s and specific g a i y o 1.254 g cm−3). The esul ing solu ion (specific g a i y o 1.105 g cm−3and a iscosi y o 3109 mPa s) is homo- geneous and s able in he p esence o wa e , ai o PFCLs. I is he hea ies and mos iscous mix u e o he h ee. In he pilo s udy by Rizzo e al., high success and low complica ion a es we e achie ed when HWS 46–3000 was used as a long- e m am- ponade (1–3 mon hs), e en hough, due o i s highe iscosi y, handling his subs ance may be mo e difficul , e.g., when emo - ing i .[75] Densi on 68, Oxane and HWS 46–3000 ha e shown p omising esul s in he ea men o RDs associa ed wi h in e io ea s.[82] Howe e , in a p ospec i e, mul icen e ed, andomized con olled ial (HSO S udy) ha compa ed he effec o hea y amponade (Densi on 68) and con en ional SO in eyes wi h in e io and pos- e io PVR g ade C o abo e, esea che s concluded ha he e we e no significan benefi s o using hea y amponade ins ead o con en ional SOs in hese cases.[29] Double fillings—Double fillings a e he e ogeneous solu ions in which he SFA sinks and he SO floa s due o i s specific g a i y, meaning ha hey a e able o p o ide supe io and in e- io amponades simul aneously. The mos commonly used SFA is pe fluo ohexyloc ane (F6H8). The amoun o F6H8is much g ea e han he amoun ha can be dissol ed by SO; he e o e, he op pa o he bubble consis s o SO sa u a ed wi h dissol ed F6H8, whe eas he bo om pa o he bubble is pu e F6H8.The Table 3. Ad an ages and limi a ions o he i eous subs i u es used in clinical p ac ice. Ad an ages Limi a ions Gases - No need o emo al - Non- oxic -Ine - Expansile - P one posi ioning a e i ec omy - Expansile gases can p oduce in aocula p essu e ele a ion (IOP) Silicone Oils - T anspa ency - Low su ace ension - Long- e m in e nal amponade - Low oxici y - Mus be emo ed wi hin a 6-mon h pe iod - Op ical adjus men s may be equi ed - Less effec i e amponade o he in e io e ina due o i s low specific g a i y Pe fluo oca bon liquids - Clea , colo less and odo less - Simila e ac i e index o i eous humou - S abiliza ion o he e ina du ing i ec omy - Limi ed o in aope a i e se ing - Long- e m oxici y - Mus be eplacedwi hSO Semifluo ina ed alkanes - Less e ino oxici y han PFCLs due o i s lowe specific densi y -SolubleinPFCLsand SOs. - Emulsifica ion - Ca a ac s Hea y Silicone Oils - Good anspa ency, high densi y and iscosi y - Good amponade p ope ies - Less endency o dispe se - Difficul o handle due o i s iscosi y - Mus be emo ed wi hin a 2-mon h pe iod. mos commonly epo ed combina ion is F6H8mixed wi h 1000 cS SO in a 3:7 p opo ion.[83,84] A combined in e nal amponade o F6H8and SO may be use ul o ea ing complica ed RD wi h b eaks in ol ing he e ina’s lowe quad an s.[85] In o de o cla i y all o he ad an ages and limi a ions o he i eous subs i u es ha a e cu en ly used in clinical p ac ice, hese cha ac e is ics ha e been included in Table 3. 4. Influence o Vi eous Subs i u ion on Pha macokine ics o In a i eal D ugs When he i eous is subs i u ed wi h SO, 80% o he i eous ca - i y will be filled wi h he amponade, while he es will be eplen- ished wi h aqueous humou and some i eous emnan s may be p esen . In he case o ine gas subs i u es, he p opo ion ha is o be efilled wi h he aqueous humou will inc ease as he gas disappea s, and he ime i akes o o ally disappea will diffe de- pending on he ype o gas (Table 1). A ificial subs i u es ha a e cu en ly used in clinical p ac ice diffe in e ms o hei i eous composi ion, and his may affec he pha macokine ics o in a - i eally injec ed d ugs.[86] Mos o hese d ugs a e wa e -soluble and will only dissol e in he i eous aqueous phase. To he bes o ou knowledge, e y ew in a i eal pha macokine ic s udies ha e been conduc ed on amponade animal eye models[87,88] in Mac omol. Biosci. 2021, 2100066 2100066 (7 o 15) © 2021 The Au ho s. Mac omolecula Bioscience published by Wiley-VCH GmbH www.ad ancedsciencenews.com www.mbs-jou nal.de which SO was used as he i eous subs i uen . In his sense, Xu e . al in es iga ed he be acizumab injec ion (1.25 mg/0.05 mL) in a abbi model, obse ing ha silicone ac ed as a empo a y depo o con olled d ug deli e y, delaying d ug dis ibu ion in o he emaining i eous aqueous phase, whe e he d ug dissol ed, be o e being u he dis ibu ed in o he su ounding issues. The e o e, i is an icipa ed ha he d ug concen a ion- ime p o- files will change be ween he na i e and filled i eous. Howe e , gi en he lack o in e nal con ol he in a i eal clea ance o hal - li e diffe ences be ween SO- i eous and he con ol heal hy eye we e no de e mined in hose s udies.[87,88] Fo e hical easons i is no possible o pe o m hese ypes o pha macokine ic s udies in pa ien s. Ne e heless, a ew s udies ha e e alua ed he sa e y and efficacy o ea men e sus no- ea men in SO-filled eyes wi h an i i als[89] o be acizumab injec ions,[90,91] obse ing pos- i i e clinical ou comes in he ea ed pa ien s. Se e al o he case epo s ha e shown ha he Ozu dex implan (dexame hasone 0.7 mg loaded in a biodeg adable sus ained- elease in a i eal implan (Alle gan Inc., I ine, CA)) appea s o be ole a ed by and beneficial o pa ien s wi h SO amponade.[92–94] Howe e , i mus be no ed ha he epo ed clinical s udies a e based on a limi ed numbe o pa ien s. The d ug elease om he implan will de- pend on he phase in which said implan is loca ed, ha is o say he aqueous phase o he SO/gas phase, as his elease will only be possible in a medium in which he d ug can be dissol ed. In he case o ac ing-gas amponade, he elease may also be depen- den on he iming o he gas disappea ing. Mo eo e , he d ug elease om he implan may be delayed in he filled-eye, and longe d ug le els may be main ained, none heless, u he in- es iga ions on an animal models a e equi ed.[95] O e all, he e is s ill a lack o quan i a i e da a on he effec o he i eous subs i uen s on in a i eal pha macokine ics. Fu - he pha macokine ic s udies mus be conduc ed in o de o cla - i y hei effec on he d ug concen a ions ollowing in a i eal adminis a ion, o bo h d ug solu ions and implan s. 5. Expe imen al Vi eous Subs i u es: Hyd ogels The e a e s ill nume ous incon eniences and limi a ions o he use o cu en ly a ailable i eous subs i u es in clinical p ac ice. Consequen ly, he sea ch o new bioma e ials ha can be used o achie e he ideal i eous subs i u e s ill con inues. P e ious esea ch a emp ed o p oduce i eous subs i u es ha boas ed simila physiological p ope ies and molecula s uc u e o he i eous body. The limi s o his app oach included he oxici y o he compounds and hei incapaci y o p o ide sufficien in- e nal amponade o i eous eplacemen su ge y.[96] In o de o o e come hese d awbacks, ecen esea ch has ocused on de- eloping biocompa ible, biodeg adable, and injec able hyd ogels (na u al, syn he ic, and sma ), which will also ac as medium and long- e m in e nal amponade agen s.[2] Hyd ogels do no ha e o be emo ed a e a ce ain pe iod o ime, he e o e o e coming one o he main incon eniences ela ed o his p ocedu e. In ad- di ion, depending on he ypes o polyme s used o hei syn he- sis, some o hei p ope ies can be op imized. Specifically, hei iscosi y, po osi y, good mechanical s eng h, and he possibili y o d ug encapsula ion make hese ad an ageous o hei clinical use in pa ien s. Main hyd ogels which ha e been de eloped as i - eous subs i u es in ecen yea s ha e been ou lined in Table 4. 5.1. Na u al Hyd ogels The use o HA and collagen as i eous subs i u es has been e al- ua ed due o hei g ea biocompa ibili y and gi en ha hese a e he main componen s o he i eous. Howe e , hey ha e a poo amponade effec and a limi ed e en ion ime in i o compa - ing o he esul s p oduced by syn he ic and sma hyd ogels, due o he molecules endency owa ds deg ada ion and hei low iscosi y.[23,97] To inc ease e en ion ime, HA has been c oss-linked h ough UV and dihyd azide, esul ing in biocompa ible hyd ogels ha p esen good anspa ency, iscosi y, and amponade effec hanks o hei hyd ophilic p ope ies, none heless, hese ma e- ials s ill p esen ela i ely sho - e m s abili y.[99,103] In addi ion, c oss-linked hyalu ona e o mula ions and combina ions o HA wi h o he polyme s, such as mic obial anionic polysaccha ide gellan ha e also been es ed. Howe e , due o he ins abili y o he physical c osslinks, hese combina ions a e no a ailable o long- e m use.[97,98,100,101] Aiming o imp o e his ea u e, Raia e al. syn hesized silk and HA composi e hyd ogels by c oss-linking he y osine esidues na i e o silk fib oin and y amine-conjuga ed HA. In his sense, he composi e silk-HA hyd ogel e ained he a o able p ope ies o each o he polyme s. Consequen ly, he be e con ol o he wa e con en in he composi e ma ix exe ed by HA and he slow p o eoly ic deg ada ion o he silk esul ed in longe s abili y and du abili y.[125] Addi ionally, Uesugi e al. used a na u al polyme , which was no based on collagen o HA as a i eous subs i u e. Specifi- cally, hey epo ed he use o PanaceaGel SPG-178 (0.1%), a sel - assembling gel, he main componen o which is 13 amino acid syn he ic pep ide. This gel can be injec ed h ough a 27-gauge needle and i s e ac i e index, isible ligh ansmission a e, and heological p ope ies a e simila o hose o human i eous. In addi ion, hey ca ied ou a h ee-mon h in i o s udy in abbi s in which good biocompa ibili y and no oxici y we e obse ed.[102] Ne e heless, apid deg ada ion emains a majo p oblem o his ype o subs i u es, as bioma e ials end o deg ade and change hei physicomechanical p ope ies in a sho pe iod o ime. This is a conside able d awback gi en ha he ideal i e- ous subs i u es mus be s able o long pe iods o ime, p e e ably o e h ee mon hs.[2,6] 5.2. Syn he ic Hyd ogels Polyme ic hyd ogels a e he nex s ep owa ds p oducing he ideal i eous subs i u e. These ma e ials a e ne wo ks o c oss- linked hyd ophilic polyme chains wi h ex ensi e swelling, ab- so bing se e al imes hei own weigh in wa e .[126,127] They ha e a good le el o anspa ency, biocompa ibili y, and p esen is- coelas ic p ope ies ha a e simila o he i eous body, imi a ing i s bio unc ionali y.[128] Poly(1- inyl-2-py olidone) (PVP) was he fi s syn he ic poly- me o be es ed as a po en ial i eous subs i u e. The mos commonly epo ed ad e se effec s we e i eous opacifica ion and inflamma ion eac ion, esul ing in ea ly PVP deg ada- ion due o phagocy osis.[129] In addi ion, 1- inyl-2-py olidone (VP) monome was polyme ized wi h di inyl glycol (DVG) as a Mac omol. Biosci. 2021, 2100066 2100066 (8 o 15) © 2021 The Au ho s. Mac omolecula Bioscience published by Wiley-VCH GmbH www.ad ancedsciencenews.com www.mbs-jou nal.de Table 4. Hyd ogels de eloped as i eous subs i u es and hei main cha ac e is ics. Hyd ogels Polyme con en [%] Re ac i e index Ligh ansmi ance [%] In i o s udies Re e ence Na u al polyme s Gellan and hyalu onic acid 1 85–95 no [97] Me hac yla ed gellan gum 1 no [98] Hyalu onic acid 1 1.338 abbi s [99] Hyalu onic acid 3 1.341 abbi s [100] Hyalu onic acid 1–2.2 1.32–1.34 abbi s [101] Pep ide gel 0.10 1.3339 96.7 abbi s [102] Hyalu onic acid 1 1.32–1.33 abbi s [103] Hyalu onic acid 1 1.336 75–91 no [23] Syn he ic polyme s Poly inyl alcohol me hac yla e 9 no [104] Poly inyl alcohol 7 macaques [105] Poly inyl alcohol 4 85 no [106] Poly(e hylene glycol) 5 1.339 abbi s [107] Poly inyl alcohol 5 no [108] Poly inyl alcohol 1–7 1.3361 93 abbi s [109] Poly inyl alcohol 4 1.3420 no [110] Ac ylic acid and ac ylamide 1.25 –1.75 no [111] Poly N-ac yloyl glycinamide-polyca boxybe aine ac ylamide 1.60 1.3354 93.2 abbi s [112] Sma hyd ogels WTG-127 89.3 abbi s [113] Poly(e hylene glycol) 25 1.353 >90 no [114] Poly(e hylene glycol) 10 1.3325 abbi s [115] Sul obe aine me hac ylamide and ac yloyl cys amine monome s 5 >90 abbi s [116] Gellan and poly(me hac ylamide-co-me hac yla e) 0.65–1.29 1.3351–1.3372 87.6–94 abbi s [117] Me hac ylic acid, me hylac ylamide, and bisme hac yloylcys amine 1–1.4 no [118] Poly(e hylene glycol) 0.4–0.7 abbi s [119] Polyme hac ylamide and poly-me hac yla e 0.9–1.8 1.3345–1.3348 >95 no [120] Hyd oxyp opyl chi osan and algina e dialdehyde 1–3 1.3348 >80 abbi s [121] Poly(e hylene glycol), poly(p opylene glycol), and poly(𝜖-cap olac one) 3–12 1.339–1.344 abbi s [122] Poly(e hylene glycol) me hac yla e and poly(e hylene glycol) diac yla e 0.75–5.7 1.3350–1.3359 >90 no [123] Gellan and poly(me hac ylamide-co-me hac yla e-co- bis(me hac yloyl)cys amine) 1.3355–1.3370 >83 abbi s [124] c oss-linking agen in o de o ob ain a anspa en hyd ogel wi h a simila densi y and iscosi y o he i eous body.[129] Finally, VP was also co-polyme ized wi h 2-hyd oxye hyl me hac yla e (HEMA) using diallyl e he (DAE) as a c oss-linking agen , e- sul ing in a clea and anspa en gel wi h mechanical p ope ies close o hose o he i eous, howe e , he main incon enience was ha he elas ic p ope ies we e educed o e en los when injec ed.[130] Polyac ylamide (PAA) has been syn hesized by he polyme - iza ion o ac ylamide, a oxic and ca cinogenic subs ance, wi h a disulfide c oss-linking agen . Howe e , his polyme iza ion p o- cess highly imp o es i s biocompa ibili y. PAA p esen s good bio- compa ibili y and long- e m s abili y, as well as offe ing a simila iscosi y and densi y o he i eous. Wi h ega ds o ad e se eac- ions, se e e ocula inflamma ion and i eous opacifica ion ha e been epo ed.[131] Poly(2-hyd oxye hylac yla e) (PHEA) p esen ed e y good physical p ope ies; howe e , due o he eme gence o inflamma- o y eac ions, as well as ca a ac and glaucoma his subs ance is no longe being in es iga ed.[4,6] All o he a o emen ioned polyme s p esen ed complica ions ela ed o inflamma ion and oxici y. As a esul , o he polyme s such as poly(glyce yl me hac yla e) (PGMA) and hyd oxyp opyl me hylcellulose (HPMC) we e in es iga ed; howe e , hese did no each he clinical s udy s age due o hei sho deg ada ion ime.[132,133] Mac omol. Biosci. 2021, 2100066 2100066 (9 o 15) © 2021 The Au ho s. Mac omolecula Bioscience published by Wiley-VCH GmbH