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Journal of Non-Crystalline Solids 354 (2008) 717–721

Thirty-five years of guided tissue engineering

Kaj H. Karlsson *, Leena Hupa

Process Chemistry Centre, Abo Akademi University, 20500 Turku, Finland

Available online 7 November 2007

Abstract

In 1971 Professor Larry L. Hench and his co-workers published a paper on the bonding mechanism of living tissue to glass. Thisstarted a progressively expanding research on materials acting as temporary substrates for reinforcing damaged tissue. The most frequentmaterials are glasses and glass-ceramics with silica content of only 45–55 wt% and roughly equal amounts of alkali and alkaline earth, i.e.glasses that dissolve soon after having guided new tissue growth. The glass is used in form of crushed powder, micro spheres, fibers, etc.and is prepared either by conventional melting or via solgel routes. During the past 35 years glass and glass-ceramics have been used fortissue guiding in filling bone cavities, in treating frontal sinus infections, in repair of damaged suborbital ocular rims, in dentistry fortreating infected bifurcation, etc. In surgery applications are expected for fabrics made from glass fibers alone or in combination withbioactive polymer fibers. The paper is limited to silicate based, high-temperature prepared materials.� 2007 Elsevier B.V. All rights reserved.

PACS: 81.05.Kf

Keywords: Bioglass; Biomaterials

1. Introduction

In 1971 Larry Hench and his colleagues published thefirst report on glass as a bioactive material [1]. It wasintended for bone repair and was chosen in the systemNa2O–CaO–P2O5–SiO2. The composition of the first glassHench made was in weight percent 25% Na2O, 25% CaO,5% P2O5 and 45% SiO2 and noted as Bioglass� 45S5. Laterhe tested some related compositions mainly in attempts toslow down the devitrification rate when hot-working theglass. A summary of these compositions is given in Refs.[2,3]. However, 45S5 is the composition usually meantwhen referring to Bioglass�. It has been an enormous suc-cess and is probably the most investigated bioactive glasscomposition. It is also still the fastest responding glasswhen implanted in the human body, regardless if the sur-rounding tissue is soft or hard. Against Professor Hench’soriginal intentions, in the body the glass is resorbed and theions are used by the body for building new tissue. The suc-

0022-3093/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2007.06.100

* Corresponding author.E-mail address: kaj.karlsson@abo.fi (K.H. Karlsson).

cess of Bioglass� has been nicely summarised in 1991 byProfessor Hench himself [2,4]. The present paper focuseson applications of bioactive silicate materials producedby conventional high-temperature methods and intendedfor guiding bone growth.

2. Bone formation

In trying to understand why the biological system is ableto incorporate glass, Hench broadened the approach anddiscovered that the dissolving ions activate six families ofgenes in old bone cells to form new bone cells. These genesstimulate the cell division and the synthesis of growth fac-tors leading to the new bone cells. These cells not onlyexpand in number but also generate collagen and otherextracellular matrix proteins that mineralize and formnew bone [5,6]. Professor Hench’s approach is thus to tryto activate cells in tissues to repair themselves instead ofreplacing the tissue.

The most abundant noncollagenous protein in bone isosteocalcin. Although the mechanism is not fully clear itis known to influence bone mineralization by binding to

Fig. 1. Hydroxyapatite crystals on Bioglass� 45S5 after (a) 8 h and (b)72 h in simulated body fluid at 37 �C. Newly precipitated crystals formsites for later precipitation and size equalization [27].

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hydroxyapatite (HAP). In addition, it functions in cells bysignalling and recruiting osteoclasts and osteoblasts. It isinteresting to note that the negatively charged protein sur-face coordinates five calcium ions in a spatial orientationthat is complementary to calcium ions in a HAP lattice[7]. It is quite possible that apatite is also initiated in thepresence of bioactive glass. Phosphate and calcium ionsin the body fluid react with Q1-groups in the end of Q2-chains at the surface of a dissolving glass [8]. A similar con-clusion was drawn by Hayakawa et al. in an in vitro studyof apatite formation on silicate glass in simulated bodyfluid (SBF) [9]. The local structural environment of phos-phate ions adsorbed on the glass surface was found to besimilar to that of HAP and cortical bone. Apatite was,however, formed only after four days. Damen and TenCate on the other hand found that polymerized silicic acid(not the monomer!) caused a 60% reduction in the time lagpreceding spontaneous precipitation of HAP [10]. Thus thesilica gel formed on the surface of dissolving glass seems toplay an important role in bone formation, even if the actualbonding is via hydroxyl groups. The spatial distribution inthe gel is also widespread enough to produce micro-sizeapatite crystals.

Recently, Suvorova and Buffat studied the effect of dif-ferent growth conditions on the precipitation of calciumphosphates. They found that with moderate mixing condi-tions HAP after 5 min was the only crystalline phase [11].This encourages the use of solubility product as a simplifiedmodel to describe the conditions for glass to be bioactive.According to a model described by Kardos, body fluid isnot a homogenous liquid, but consists of different typesof cells floating in the blood. Stemcells, for instance, havea semi-permeable wall regulating the calcium concentrationinside the cell. If the concentration rises in the blood, itrises in the cell. When it exceeds the solubility inside thecell, HAP is precipitated and a bone cell is expelled [12].Assuming bone to be in equilibrium with the body fluid,a conditional solubility product of HAP can be calculatedfor the extra cellular fluid. Using this product, the pH ofthe intracellular fluid is found to be 5.3, a value far below9.5, the pH required to precipitate HAP in the stemcell [13].Thus, in order to produce a bonecell, either the calciumconcentration or the pH in the blood must increase. Bothconditions can be reached on the surface of dissolvingglass.

The components of the solubility product was also in themind of Professor Kokubo, when he oxidized the surface ofmetallic titanium in order to get a partially solubleTiO(OH)2 surface. In a simulated body fluid (SBF), the sol-ubility product of HAP is exceeded and it crystallizes ontothe surface of the metal [14].

3. Applications

Examples of use of bioactive glass in tissue repair areinner ear prostheses developed by Hench, coatings on steeland titanium prostheses, bifurcation and periodontal

pocket repair and many others. The glasses Hench usesare Bioglass� 45S5 and improved developments from this.In SBF as well as in the body these glasses rapidly developa surface of micro- or sub-microcrystalline HAP crystals,Fig. 1. When heated, however, 45S5 is apt to liquid–liquidphase separation [15]. This has to some extent limited theproducts that can be made by hot-working the glass.Efforts have therefore been made to develop glass that isless apt to crystallization, but still retaining its bioactivity.In the mid 1980s, Andersson et al. extended the four-com-ponent system to include boric oxide and alumina and useda computer aided experimental planning in order to maxi-mise the information with a minimum number of experi-ments [16]. Their results were expressed by polynomialrelations between compositions and several physical glassproperties as well as response in vivo. The latter relationwas evaluated by measuring the force needed to push-outa conical glass implant after eight weeks in rabbit tibia[17,18]. The glasses which Andersson et al. developedformed a strong bond with bone; in the push-out testsbreakage occurred in bone, not in glass-bone interface,Fig. 2. The first clinical application of a glass from this ser-ies is shown in Fig. 3; in 1987 Professor Allan Aho removed

Fig. 2. Optical micrograph of fracture in bone after push-out of a glasscone implanted for 8 weeks in rabbit tibia. The composition of the glass isin wt% 46.0 SiO2, 26.0 Na2O, 25.0 CaO, 1.0 Al2O3, 2.0 B2O3 [18].

Fig. 4. Optical micrograph of flame sprayed spheres and fibers made frombioactive glass. Diameter of spheres is about 250 lm, fiber thickness 10–15 lm.

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a cancerous tumor from the tibia of a patient and filled thecavity with crushed glass. The bone healed in less than twoyears as seen in Fig. 3(e) and (f). The operation was fol-lowed up for 13 more years and no complications werenoted [19].

All bioactive glasses developed by mid 1990s crystallizedtoo easily to enable hot-working by traditional productionmethods. To overcome this, Brink added in 1997 potassiumand magnesium oxides to glasses in the phase fieldNa2O Æ 2CaO Æ 3SiO2(ss) [20]. Three years later Ylanenextended the compositions into the wollastonite field [21].The onset of b-wollastonite crystallization for a glass withthe composition in wt% 6 Na2O, 11 K2O, 5 MgO, 22 CaO,1 B2O3, 2 P2O5 and 53 SiO2 is about 800 �C, or highenough to enable hot-working like sintering, flame spray-ing of crushed glass, fiber drawing, etc. as shown inFig. 4. It would be preferable to get a-wollastonite, whichdissolves easily in body fluid, but if diopside is formed,the b-phase seems to prevail. To prevent this, forming tem-peratures should be kept low and heating times short.Within dentistry some clinical tests have successfully beenperformed with this glass.

Fig. 3. X-ray images of cancerous tumor in human tibia. (a) Front view, (b) sifront views two years later of the cured bone [19]. Courtesy Prof. Allan Aho.

Ylanen also changed the test piece used in the in vivo

tests by sintering narrow fractions of glass spheres to formporous, conical implants [21–23]. This enabled ingrowthsof tissue into the implant. The porosity had two beneficialeffects. The active surface area increased and whenimplanted through cortex, capillary forces filled the poros-ities with bone marrow. In contact with glass, marrowtransformed within eight weeks to bone, cf. the stained areain Fig. 5(a). In a porous titanium implant used as reference,marrow was transformed only in the cortex area, Fig. 5(b)[21]. The experiment seems to show that titanium is osteo-conductive, while glass is osteopromotive [25].

One of the critical factors for bone ingrowths in animplant is said to be the size of interconnecting pores.However, Itala et al. showed that laser drilled holesthrough a titanium plate were filled with new bone tissuewithin 12 weeks. The smallest holes were 50 lm in diameterand still filled with living bone [25]. Porous implants werealso studied in vivo in a rabbit knee as well as in a dog tibia[22,24]. When implanted in the rabbit knee the implantextended from the surface of the cartilage through the sub-chondral bone and into the marrow space. Capillary forcesfilled the porosities with marrow. After eight weeks, the

de view, (c) tumor removed, (d) cavity filled with glass, (e) and (f) side and

Fig. 5. Optical micrographs of van Gieson stained (a) porous glassimplant after 6 weeks in tibia of rabbit, (b) porous titanium implant after12 weeks in tibia of rabbit [21].

Fig. 6. Fabric made with polylactide warp and glass fibers as weft [31].

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part of the implant in contact with joint liquid was coveredwith a hyaline-like cartilage, while the part extended deeperdown was filled with new bone [22]. The possibility torepair worn joints instead of replacing them with metalimplants is an interesting possibility which still remainsto be investigated.

The use of bioactive glass for long-term load-bearingapplications is restricted because of its brittleness. One pos-sibility to overcome this drawback is to crystallize the glassto obtain a glass-ceramic. Professor Kokubo and his co-workers developed a glass-ceramic containing apatite andwollastonite in a glass matrix [26]. The apatite crystals formsites for bone growth; the long wollastonite crystals rein-force the glass. This AW-ceramic has found wide applica-tions in bone repair, particularly in Japan. The long-termimplications of wollastonite crystals are, however, stillnot fully investigated.

The physical properties as well as the bioactive responsein vitro of 30 glasses in the system Na2O–K2O–MgO–CaO–B2O3–P2O5–SiO2 have recently been measured [27]. The

compositions were chosen from both the 1:2:3 and the b-wollastonite fields as well as from the region where thesefields meet. A set of glasses in the border region betweenthe1:2:3 and wollastonite fields showed interesting proper-ties [28]. In the border region, the onset of surface crystal-lization for the 1:2:3(ss)-crystals is about 100C higher thanfor Bioglass� 45S5, and the onset for wollastonite againabout 200C higher. In these glasses, the properties showa combination of the high bioactivity of the 1:2:3(ss) crys-tals and the reinforcement achieved by the wollastonitecrystals. When heat treated, dolomite crystals may formand grow to an unfavorable size. This can, however, be reg-ulated by suitable control of time and temperature. Glassesboth within the primary fields and on the border have beentested for medical purposes in the form of for instance,crushed powder, solid blocks, fabric alone or in combina-tion with polylactide fibers, fibers pressed to waffles, etc.[29]. All glasses so far tested in vivo were vitreous and didnot contain any crystals. Further, it is interesting to notethat some of these glasses also have an antibacterial effect[30].

Fig. 6 shows a fabric made with warp from polylactideand weft from a bioactive glass with the composition inwt% 6 Na2O, 11 K2O, 5 MgO, 22 CaO, 1 B2O3, 2 P2O5

and 53 SiO2 [31]. This type of fabric is expected to get sev-eral applications in surgery, in particular as temporary sup-port in repair of broken bones.

Several other applications of bioactive glasses may bementioned, like repair of damaged eye shelf [32], treatmentof infected frontal sinus [33], repair of bifurcation damageand filling of periodontal pocket [34] and lift of maxillarysinus floor to get sufficient bone for implantation of tita-nium roots [35].

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4. Summary

Over the past 35 years the bioactivity of glass has beenapproached in principally two ways, by investigating thebiochemical interaction between glass and body or by usingthe effect of dissolved ions to develop new compositions.The former requires deep insight in the interaction betweenglass and genes. The latter is more concentrated on study-ing the effect of dissolved ions on the formation ofhydroxyapatite in the body. Both approaches aim todevelop biodegradable implants which, although they needtime for healing, do not require later removing.

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