**3. Conclusions and future perspectives**

Regenerative medicine involves tissue formation and healing in order to restore the functionality of damaged organs or tissues. As tissue repair and regeneration after injury involve the selective recruitment of circulating or resident stem cell populations, stem cell therapy is often employed as one mean to promote tissue regeneration. Its success might, nevertheless, be complicated by strong immune-rejection of transplanted cells or shortage of autologous cell supply. Furthermore, if a scaffold, with or without bioactive agents, is used to administrate the stem cells, poor integration between the scaffold/implant and the host tissue might affect the outcome.

An interesting tissue engineering concept is cell guidance aimed at total *in vivo* tissue engineering without the need of adding bioactive agents or cells. Numerous studies have shown that cellulose itself functions as a chemoattracant and is able to stimulate granulation tissue formation. Uncoated cellulose sponge has been tested in treatment of chronic leg ulcers (Pajarre, unpublished data) and in severe burn injuries (Lagus, unpublished data) in the 1990's with good results. The cellulose sponge adsorbs debris and bacteria from the wound site and attracts inflammatory cells. In these cases, a short-term, powerful inflammatory response is actually necessary. After cleaning the wound bed, the cellulose induces vital granulation tissue formation, and smoothens and prepares the wound bed for successful skin transplantation.

The fascinating property of HA-coated cellulose sponge is its ability to even further amplify the healing mechanisms of the body. The HA-coated cellulose acts as a cell-guiding material, attracting stem cell reserves. The novel finding of haemoglobin expression during wound healing brought into daylight new data concerning blood formation and development of neovascularisation. The clinical relevance of this is the production of more vascularised granulation tissue in the critical early phases of wound healing.

We hypothesise that the cell guiding property of the HA-coated cellulose is due to the combination of silica and calcium phosphate. Preliminary results (Stark et al, unpublished) from our on-going study show that a mineral layer induced by dipping the cellulose sponge in a calcium phosphate solution has not the same beneficial feature on granulation tissue formation (not shown) than the mineral layer induced by the bioactive glass in simulated body fluid. Although there was a somewhat stronger inflammatory reaction when compared to uncoated cellulose it was not as intense as in HA-coated cellulose. Our deduction is that the silica elevates the inflammatory reaction with enhanced level of bioactive agent production that attracts more circulating bone marrow-derived progenitor cells whereas the calcium phosphate layer contributes to hastened stem cell homing to the cellulose sponge.

Due to the cell-guiding property of silica rich HA-coated regenerated oxidised cellulose in combination with the capacity to promote proliferation of richly vascularised connective tissue, this material might have potential in clinical situations when rapid granulation tissue growth is needed as in treatment of poorly healing wounds. The contact with the HAcellulose sponge would be local and temporary, therefore minimizing any possible disadvantages. In addition to safety issues, the manufacturing process of coating cellulose with HA is relatively simple and cheap, and the HA-coated cellulose sponge is easy to handle, form and sterilise.

#### **BOX 2. BIOLOGY OF WOUND HEALING**

96 Cellulose – Medical, Pharmaceutical and Electronic Applications

**3. Conclusions and future perspectives** 

tissue might affect the outcome.

successful skin transplantation.

formation of nitric oxide, which reflects the production of nitric oxide observed in 3-day-old HA-implant, but not at day 10 52, coincides with the strong inflammatory reaction that starts to decline during the second week of implantation 38. The production of haemoglobin during this phase might eliminate the excess nitric oxide and prevent its negative effect on matrix deposition, neovascularisation and apoptosis. In uncoated cellulose implants, iNOS is detected at day 10, which supports the observation of slower

Regenerative medicine involves tissue formation and healing in order to restore the functionality of damaged organs or tissues. As tissue repair and regeneration after injury involve the selective recruitment of circulating or resident stem cell populations, stem cell therapy is often employed as one mean to promote tissue regeneration. Its success might, nevertheless, be complicated by strong immune-rejection of transplanted cells or shortage of autologous cell supply. Furthermore, if a scaffold, with or without bioactive agents, is used to administrate the stem cells, poor integration between the scaffold/implant and the host

An interesting tissue engineering concept is cell guidance aimed at total *in vivo* tissue engineering without the need of adding bioactive agents or cells. Numerous studies have shown that cellulose itself functions as a chemoattracant and is able to stimulate granulation tissue formation. Uncoated cellulose sponge has been tested in treatment of chronic leg ulcers (Pajarre, unpublished data) and in severe burn injuries (Lagus, unpublished data) in the 1990's with good results. The cellulose sponge adsorbs debris and bacteria from the wound site and attracts inflammatory cells. In these cases, a short-term, powerful inflammatory response is actually necessary. After cleaning the wound bed, the cellulose induces vital granulation tissue formation, and smoothens and prepares the wound bed for

The fascinating property of HA-coated cellulose sponge is its ability to even further amplify the healing mechanisms of the body. The HA-coated cellulose acts as a cell-guiding material, attracting stem cell reserves. The novel finding of haemoglobin expression during wound healing brought into daylight new data concerning blood formation and development of neovascularisation. The clinical relevance of this is the production of more vascularised

We hypothesise that the cell guiding property of the HA-coated cellulose is due to the combination of silica and calcium phosphate. Preliminary results (Stark et al, unpublished) from our on-going study show that a mineral layer induced by dipping the cellulose sponge in a calcium phosphate solution has not the same beneficial feature on granulation tissue formation (not shown) than the mineral layer induced by the bioactive glass in simulated body fluid. Although there was a somewhat stronger inflammatory reaction when compared to uncoated cellulose it was not as intense as in HA-coated cellulose. Our deduction is that the silica elevates the inflammatory reaction with enhanced level of bioactive agent production

granulation tissue in the critical early phases of wound healing.

sequence of events in the granulation tissue formation in these uncoated implants

Wound healing is a complex and dynamic process of restoring cellular structures and tissue layers in the body. The physiological and coordinated response to injury is practically similar in all tissues and involves three distinct but overlapping phases that can be divided into inflammation, new tissue formation and remodelling 70. In turn, these three phases comprehend coordinated series of events that includes chemotaxis, phagocytosis, neocollagenesis, collagen degradation, and collagen remodelling. Furthermore, neovascularisation, epithelisation, and the production of new glycosaminoglycans (GAGs) and proteoglycans are vital during wound healing process.

The key initiators of the healing process are the platelets, which within minutes after injury aggregate and form fibrin clot in aim to control bleeding. In addition to their important role in hemostasis, platelets also liberate growth factors that will attract inflammatory and phagocytosing cells. The first cells to arrive are polymorphonuclear leucocytes, i.e. neutrophils that secrete proinflammatory cytokines. Shortly thereafter circulating monocytes will appear, are activated and converted in the tissue to macrophages that kill bacterial pathogens, scavenge tissue debris and destroy remaining neutrophils. Macrophages also mediate the transition from the inflammatory phase to the proliferative phase by releasing a variety of chemotactic agents and growth factors that stimulate cell migration, proliferation and formation of tissue matrix.

The second phase of wound healing is often called the proliferative phase or the granulation tissue formation phase. This stage starts normally two to three days after injury and lasts approximately two to three weeks. During this phase the provisional extracellular matrix is gradually filled with granulation tissue. The phenomenal feature is to diminish the area of tissue loss by contraction and fibroplasia. The infiltrated cells produce a new extracellular matrix, rich in blood vessels, which carry oxygen and nutrients to maintain the metabolic processes. Although new collagen and other extracellular matrix proteins are continuously actively synthesised, the earlier formed fibrin clot is enzymatically degraded. This process allows the proceeding of reepithelisation that is needed to control the growth of the repair tissue and wound closure. The proteolytic activity is also a prerequisite of the neovascularisation.

Cellulose - A Biomaterial with Cell-Guiding Property 99

**Author details** 

Miretta Tommila

study.

**4. References** 

79¸71-76.

197.

Corresponding Author

 \*

Elsivier Academic Press. 864 p.

Surg. childh. intern. 5: 251-255.

derivatives. J. biomed. mat. res. 23: 125-133.

Biomaterials. 28: 5114-5120.

*Hospital, Turku, Finland* 

**Acknowledgement** 

Miretta Tommila, Anne Jokilammi, Risto Penttinen and Erika Ekholm\*

*Department of Medical Biochemistry and Genetics, University of Turku, Turku, Finland* 

*Department of Anaesthesiology, Intensive Care, Emergency and Pain Medicine, Turku University* 

Johanna Holmbom, Christoffer Stark and Ville Peltonen are acknowledged for technical assistance. We thank Bruno Lönnberg, Kurt Lönnqvist and the Cellomeda Ltd. for various celluloses and the Swedish Cultural Foundation for supporting our ongoing cellulose

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Usually by three weeks after injury, new tissue formation starts to decrease, and the emphasis of wound healing process turns to the remodelling and maturation. The main objective of this phase is to achieve maximum tensile strength by reorganisation, degradation and re-synthesis of the extracellular matrix. This final process may last even several years, before the new granulation tissue rich in cells and vascular capillaries has matured into a relatively acellular and avascular scar that lacks appendages, including hair follicles, sebaceous glands, and sweat glands 70.


**Table 1.**
